This is nasm.info, produced by makeinfo version 4.0 from nasmdoc.texi.

INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* NASM: (nasm).                The Netwide Assembler for x86.
END-INFO-DIR-ENTRY

   This file documents NASM, the Netwide Assembler: an assembler
targetting the Intel x86 series of processors, with portable source.

   Copyright 1997 Simon Tatham

   All rights reserved. This document is redistributable under the
licence given in the file "Licence" distributed in the NASM archive.


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   This file documents NASM, the Netwide Assembler: an assembler
targetting the Intel x86 series of processors, with portable source.

* Menu:

* Chapter 1:: Introduction
* Chapter 2:: Running NASM
* Chapter 3:: The NASM Language
* Chapter 4:: The NASM Preprocessor
* Chapter 5:: Assembler Directives
* Chapter 6:: Output Formats
* Chapter 7:: Writing 16-bit Code (DOS, Windows 3/3.1)
* Chapter 8:: Writing 32-bit Code (Unix, Win32, DJGPP)
* Chapter 9:: Mixing 16 and 32 Bit Code
* Chapter 10:: Troubleshooting
* Appendix A:: Intel x86 Instruction Reference
* Index::


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Chapter 1: Introduction
***********************

* Menu:

* Section 1.1:: What Is NASM?
* Section 1.2:: Contact Information
* Section 1.3:: Installation


File: nasm.info,  Node: Section 1.1,  Next: Section 1.1.1,  Prev: Chapter 1,  Up: Chapter 1

1.1. What Is NASM?
******************

   The Netwide Assembler, NASM, is an 80x86 assembler designed for
portability and modularity. It supports a range of object file formats,
including Linux `a.out' and ELF, NetBSD/FreeBSD, COFF, Microsoft 16-bit
OBJ and Win32.  It will also output plain binary files. Its syntax is
designed to be simple and easy to understand, similar to Intel's but
less complex. It supports Pentium, P6 and MMX opcodes, and has macro
capability.

* Menu:

* Section 1.1.1:: Why Yet Another Assembler?
* Section 1.1.2:: Licence Conditions


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1.1.1. Why Yet Another Assembler?
*********************************

   The Netwide Assembler grew out of an idea on `comp.lang.asm.x86' (or
possibly `alt.lang.asm' - I forget which), which was essentially that
there didn't seem to be a good free x86-series assembler around, and
that maybe someone ought to write one.

   * `a86' is good, but not free, and in particular you don't get any
     32- bit capability until you pay. It's DOS only, too.

   * `gas' is free, and ports over DOS and Unix, but it's not very good,
     since it's designed to be a back end to `gcc', which always feeds
     it correct code. So its error checking is minimal. Also, its
     syntax is horrible, from the point of view of anyone trying to
     actually _write_ anything in it. Plus you can't write 16-bit code
     in it (properly).

   * `as86' is Linux-specific, and (my version at least) doesn't seem to
     have much (or any) documentation.

   * MASM isn't very good, and it's expensive, and it runs only under
     DOS.

   * TASM is better, but still strives for MASM compatibility, which
     means millions of directives and tons of red tape. And its syntax
     is essentially MASM's, with the contradictions and quirks that
     entails (although it sorts out some of those by means of Ideal
     mode). It's expensive too. And it's DOS-only.

   So here, for your coding pleasure, is NASM. At present it's still in
prototype stage - we don't promise that it can outperform any of these
assemblers. But please, _please_ send us bug reports, fixes, helpful
information, and anything else you can get your hands on (and thanks to
the many people who've done this already! You all know who you are),
and we'll improve it out of all recognition. Again.


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1.1.2. Licence Conditions
*************************

   Please see the file `Licence', supplied as part of any NASM
distribution archive, for the licence conditions under which you may use
NASM.


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1.2. Contact Information
************************

   The current version of NASM (since 0.98) are maintained by H. Peter
Anvin, `hpa@zytor.com'. If you want to report a bug, please read *Note
Section 10.2:: first.

   NASM has a WWW page at `http://www.cryogen.com/Nasm'.

   The original authors are e-mailable as `jules@earthcorp.com' and
`anakin@pobox.com'.

   New releases of NASM are uploaded to `ftp.kernel.org',
`sunsite.unc.edu', `ftp.simtel.net' and `ftp.coast.net'.  Announcements
are posted to `comp.lang.asm.x86', `alt.lang.asm',
`comp.os.linux.announce' and `comp.archives.msdos.announce' (the last
one is done automagically by uploading to `ftp.simtel.net').

   If you don't have Usenet access, or would rather be informed by
e-mail when new releases come out, you can subscribe to the
`nasm-announce' email list by sending an email containing the line
`subscribe nasm-announce' to `majordomo@linux.kernel.org'.

   If you want information about NASM beta releases, please subscribe
to the `nasm-beta' email list by sending an email containing the line
`subscribe nasm-beta' to `majordomo@linux.kernel.org'.


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1.3. Installation
*****************

* Menu:

* Section 1.3.1:: Installing NASM under MS-DOS or Windows
* Section 1.3.2:: Installing NASM under Unix


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1.3.1. Installing NASM under MS-DOS or Windows
**********************************************

   Once you've obtained the DOS archive for NASM, `nasmXXX.zip' (where
`XXX' denotes the version number of NASM contained in the archive),
unpack it into its own directory (for example `c:\nasm').

   The archive will contain four executable files: the NASM executable
files `nasm.exe' and `nasmw.exe', and the NDISASM executable files
`ndisasm.exe' and `ndisasmw.exe'. In each case, the file whose name
ends in `w' is a Win32 executable, designed to run under Windows 95 or
Windows NT Intel, and the other one is a 16-bit DOS executable.

   The only file NASM needs to run is its own executable, so copy (at
least) one of `nasm.exe' and `nasmw.exe' to a directory on your PATH, or
alternatively edit `autoexec.bat' to add the `nasm' directory to your
`PATH'. (If you're only installing the Win32 version, you may wish to
rename it to `nasm.exe'.)

   That's it - NASM is installed. You don't need the `nasm' directory to
be present to run NASM (unless you've added it to your `PATH'), so you
can delete it if you need to save space; however, you may want to keep
the documentation or test programs.

   If you've downloaded the DOS source archive, `nasmXXXs.zip', the
`nasm' directory will also contain the full NASM source code, and a
selection of Makefiles you can (hopefully) use to rebuild your copy of
NASM from scratch. The file `Readme' lists the various Makefiles and
which compilers they work with.

   Note that the source files `insnsa.c', `insnsd.c', `insnsi.h' and
`insnsn.c' are automatically generated from the master instruction
table `insns.dat' by a Perl script; the file `macros.c' is generated
from `standard.mac' by another Perl script. Although the NASM 0.98
distribution includes these generated files, you will need to rebuild
them (and hence, will need a Perl interpreter) if you change
`insns.dat', `standard.mac' or the documentation. It is possible future
source distributions may not include these files at all.  Ports of Perl
for a variety of platforms, including DOS and Windows, are available
from www.cpan.org.


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1.3.2. Installing NASM under Unix
*********************************

   Once you've obtained the Unix source archive for NASM,
`nasm-X.XX.tar.gz' (where `X.XX' denotes the version number of NASM
contained in the archive), unpack it into a directory such as
`/usr/local/src'. The archive, when unpacked, will create its own
subdirectory `nasm-X.XX'.

   NASM is an auto-configuring package: once you've unpacked it, `cd' to
the directory it's been unpacked into and type `./configure'. This
shell script will find the best C compiler to use for building NASM and
set up Makefiles accordingly.

   Once NASM has auto-configured, you can type `make' to build the
`nasm' and `ndisasm' binaries, and then `make install' to install them
in `/usr/local/bin' and install the man pages `nasm.1' and `ndisasm.1'
in `/usr/local/man/man1'.  Alternatively, you can give options such as
`--prefix' to the `configure' script (see the file `INSTALL' for more
details), or install the programs yourself.

   NASM also comes with a set of utilities for handling the RDOFF custom
object-file format, which are in the `rdoff' subdirectory of the NASM
archive. You can build these with `make rdf' and install them with
`make rdf_install', if you want them.

   If NASM fails to auto-configure, you may still be able to make it
compile by using the fall-back Unix makefile `Makefile.unx'. Copy or
rename that file to `Makefile' and try typing `make'. There is also a
`Makefile.unx' file in the `rdoff' subdirectory.


File: nasm.info,  Node: Chapter 2,  Next: Section 2.1,  Prev: Section 1.3.2,  Up: Top

Chapter 2: Running NASM
***********************

* Menu:

* Section 2.1:: NASM Command-Line Syntax
* Section 2.2:: Quick Start for MASM Users


File: nasm.info,  Node: Section 2.1,  Next: Section 2.1.1,  Prev: Chapter 2,  Up: Chapter 2

2.1. NASM Command-Line Syntax
*****************************

   To assemble a file, you issue a command of the form

     nasm -f <format> <filename> [-o <output>]

   For example,

     nasm -f elf myfile.asm

   will assemble `myfile.asm' into an ELF object file `myfile.o'.  And

     nasm -f bin myfile.asm -o myfile.com

   will assemble `myfile.asm' into a raw binary file `myfile.com'.

   To produce a listing file, with the hex codes output from NASM
displayed on the left of the original sources, use the `-l' option to
give a listing file name, for example:

     nasm -f coff myfile.asm -l myfile.lst

   To get further usage instructions from NASM, try typing

     nasm -h

   This will also list the available output file formats, and what they
are.

   If you use Linux but aren't sure whether your system is `a.out' or
ELF, type

     file nasm

   (in the directory in which you put the NASM binary when you
installed it).  If it says something like

     nasm: ELF 32-bit LSB executable i386 (386 and up) Version 1

   then your system is ELF, and you should use the option `-f elf' when
you want NASM to produce Linux object files. If it says

     nasm: Linux/i386 demand-paged executable (QMAGIC)

   or something similar, your system is `a.out', and you should use `-f
aout' instead (Linux `a.out' systems are considered obsolete, and are
rare these days.)

   Like Unix compilers and assemblers, NASM is silent unless it goes
wrong: you won't see any output at all, unless it gives error messages.

* Menu:

* Section 2.1.1:: The `-o' Option: Specifying the Output File Name
* Section 2.1.2:: The `-f' Option: Specifying the Output File Format
* Section 2.1.3:: The `-l' Option: Generating a Listing File
* Section 2.1.4:: The `-E' Option: Send Errors to a File
* Section 2.1.5:: The `-s' Option: Send Errors to `stdout'
* Section 2.1.6:: The `-i' Option: Include File Search Directories
* Section 2.1.7:: The `-p' Option: Pre-Include a File
* Section 2.1.8:: The `-d' Option:  Pre-Define a Macro
* Section 2.1.9:: The `-u' Option:  Undefine a Macro
* Section 2.1.10:: The `-e' Option: Preprocess Only
* Section 2.1.11:: The `-a' Option: Don't Preprocess At All
* Section 2.1.12:: The `-w' Option: Enable or Disable Assembly Warnings
* Section 2.1.13:: The `NASM' Environment Variable


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2.1.1. The `-o' Option: Specifying the Output File Name
*******************************************************

   NASM will normally choose the name of your output file for you;
precisely how it does this is dependent on the object file format. For
Microsoft object file formats (`obj' and `win32'), it will remove the
`.asm' extension (or whatever extension you like to use - NASM doesn't
care) from your source file name and substitute `.obj'. For Unix object
file formats (`aout', `coff', `elf' and `as86') it will substitute
`.o'. For `rdf', it will use `.rdf', and for the `bin' format it will
simply remove the extension, so that `myfile.asm' produces the output
file `myfile'.

   If the output file already exists, NASM will overwrite it, unless it
has the same name as the input file, in which case it will give a
warning and use `nasm.out' as the output file name instead.

   For situations in which this behaviour is unacceptable, NASM
provides the `-o' command-line option, which allows you to specify your
desired output file name. You invoke `-o' by following it with the name
you wish for the output file, either with or without an intervening
space. For example:

     nasm -f bin program.asm -o program.com
     nasm -f bin driver.asm -odriver.sys


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2.1.2. The `-f' Option: Specifying the Output File Format
*********************************************************

   If you do not supply the `-f' option to NASM, it will choose an
output file format for you itself. In the distribution versions of
NASM, the default is always `bin'; if you've compiled your own copy of
NASM, you can redefine `OF_DEFAULT' at compile time and choose what you
want the default to be.

   Like `-o', the intervening space between `-f' and the output file
format is optional; so `-f elf' and `-felf' are both valid.

   A complete list of the available output file formats can be given by
issuing the command `nasm -h'.


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2.1.3. The `-l' Option: Generating a Listing File
*************************************************

   If you supply the `-l' option to NASM, followed (with the usual
optional space) by a file name, NASM will generate a source-listing file
for you, in which addresses and generated code are listed on the left,
and the actual source code, with expansions of multi-line macros
(except those which specifically request no expansion in source
listings: see *Note Section 4.2.9::) on the right. For example:

     nasm -f elf myfile.asm -l myfile.lst


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2.1.4. The `-E' Option: Send Errors to a File
*********************************************

   Under MS-DOS it can be difficult (though there are ways) to redirect
the standard-error output of a program to a file. Since NASM usually
produces its warning and error messages on `stderr', this can make it
hard to capture the errors if (for example) you want to load them into
an editor.

   NASM therefore provides the `-E' option, taking a filename argument
which causes errors to be sent to the specified files rather than
standard error. Therefore you can redirect the errors into a file by
typing

     nasm -E myfile.err -f obj myfile.asm


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2.1.5. The `-s' Option: Send Errors to `stdout'
***********************************************

   The `-s' option redirects error messages to `stdout' rather than
`stderr', so it can be redirected under MS-DOS. To assemble the file
`myfile.asm' and pipe its output to the `more' program, you can type:

     nasm -s -f obj myfile.asm | more

   See also the `-E' option, *Note Section 2.1.4::.


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2.1.6. The `-i' Option: Include File Search Directories
*******************************************************

   When NASM sees the `%include' directive in a source file (see *Note
Section 4.5::), it will search for the given file not only in the
current directory, but also in any directories specified on the command
line by the use of the `-i' option. Therefore you can include files
from a macro library, for example, by typing

     nasm -ic:\macrolib\ -f obj myfile.asm

   (As usual, a space between `-i' and the path name is allowed, and
optional).

   NASM, in the interests of complete source-code portability, does not
understand the file naming conventions of the OS it is running on; the
string you provide as an argument to the `-i' option will be prepended
exactly as written to the name of the include file. Therefore the
trailing backslash in the above example is necessary. Under Unix, a
trailing forward slash is similarly necessary.

   (You can use this to your advantage, if you're really perverse, by
noting that the option `-ifoo' will cause `%include "bar.i"' to search
for the file `foobar.i'...)

   If you want to define a _standard_ include search path, similar to
`/usr/include' on Unix systems, you should place one or more `-i'
directives in the `NASM' environment variable (see *Note Section
2.1.13::).

   For Makefile compatibility with many C compilers, this option can
also be specified as `-I'.


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2.1.7. The `-p' Option: Pre-Include a File
******************************************

   NASM allows you to specify files to be _pre-included_ into your
source file, by the use of the `-p' option. So running

     nasm myfile.asm -p myinc.inc

   is equivalent to running `nasm myfile.asm' and placing the directive
`%include "myinc.inc"' at the start of the file.

   For consistency with the `-I', `-D' and `-U' options, this option
can also be specified as `-P'.


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2.1.8. The `-d' Option:  Pre-Define a Macro
*******************************************

   Just as the `-p' option gives an alternative to placing `%include'
directives at the start of a source file, the `-d' option gives an
alternative to placing a `%define' directive. You could code

     nasm myfile.asm -dFOO=100

   as an alternative to placing the directive

     %define FOO 100

   at the start of the file. You can miss off the macro value, as well:
the option `-dFOO' is equivalent to coding `%define FOO'. This form of
the directive may be useful for selecting assembly-time options which
are then tested using `%ifdef', for example `-dDEBUG'.

   For Makefile compatibility with many C compilers, this option can
also be specified as `-D'.


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2.1.9. The `-u' Option:  Undefine a Macro
*****************************************

   The `-u' option undefines a macro that would otherwise have been pre-
defined, either automatically or by a `-p' or `-d' option specified
earlier on the command lines.

   For example, the following command line:

     nasm myfile.asm -dFOO=100 -uFOO

   would result in `FOO' _not_ being a predefined macro in the program.
This is useful to override options specified at a different point in a
Makefile.

   For Makefile compatibility with many C compilers, this option can
also be specified as `-U'.


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2.1.10. The `-e' Option: Preprocess Only
****************************************

   NASM allows the preprocessor to be run on its own, up to a point.
Using the `-e' option (which requires no arguments) will cause NASM to
preprocess its input file, expand all the macro references, remove all
the comments and preprocessor directives, and print the resulting file
on standard output (or save it to a file, if the `-o' option is also
used).

   This option cannot be applied to programs which require the
preprocessor to evaluate expressions which depend on the values of
symbols: so code such as

     %assign tablesize ($-tablestart)

   will cause an error in preprocess-only mode.


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2.1.11. The `-a' Option: Don't Preprocess At All
************************************************

   If NASM is being used as the back end to a compiler, it might be
desirable to suppress preprocessing completely and assume the compiler
has already done it, to save time and increase compilation speeds. The
`-a' option, requiring no argument, instructs NASM to replace its
powerful preprocessor with a stub preprocessor which does nothing.


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2.1.12. The `-w' Option: Enable or Disable Assembly Warnings
************************************************************

   NASM can observe many conditions during the course of assembly which
are worth mentioning to the user, but not a sufficiently severe error to
justify NASM refusing to generate an output file. These conditions are
reported like errors, but come up with the word `warning' before the
message. Warnings do not prevent NASM from generating an output file and
returning a success status to the operating system.

   Some conditions are even less severe than that: they are only
sometimes worth mentioning to the user. Therefore NASM supports the `-w'
command-line option, which enables or disables certain classes of
assembly warning. Such warning classes are described by a name, for
example `orphan-labels'; you can enable warnings of this class by the
command- line option `-w+orphan-labels' and disable it by
`-w-orphan-labels'.

   The suppressible warning classes are:

   * `macro-params' covers warnings about multi-line macros being
     invoked with the wrong number of parameters. This warning class is
     enabled by default; see *Note Section 4.2.1:: for an example of
     why you might want to disable it.

   * `orphan-labels' covers warnings about source lines which contain no
     instruction but define a label without a trailing colon. NASM does
     not warn about this somewhat obscure condition by default; see
     *Note Section 3.1:: for an example of why you might want it to.

   * `number-overflow' covers warnings about numeric constants which
     don't fit in 32 bits (for example, it's easy to type one too many
     Fs and produce `0x7ffffffff' by mistake). This warning class is
     enabled by default.


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2.1.13. The `NASM' Environment Variable
***************************************

   If you define an environment variable called `NASM', the program will
interpret it as a list of extra command-line options, which are
processed before the real command line. You can use this to define
standard search directories for include files, by putting `-i' options
in the `NASM' variable.

   The value of the variable is split up at white space, so that the
value `-s -ic:\nasmlib' will be treated as two separate options.
However, that means that the value `-dNAME="my name"' won't do what you
might want, because it will be split at the space and the NASM
command-line processing will get confused by the two nonsensical words
`-dNAME="my' and `name"'.

   To get round this, NASM provides a feature whereby, if you begin the
`NASM' environment variable with some character that isn't a minus
sign, then NASM will treat this character as the separator character for
options. So setting the `NASM' variable to the value `!-s!-ic:\nasmlib'
is equivalent to setting it to `-s -ic:\nasmlib', but `!-dNAME="my
name"' will work.


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2.2. Quick Start for MASM Users
*******************************

   If you're used to writing programs with MASM, or with TASM in MASM-
compatible (non-Ideal) mode, or with `a86', this section attempts to
outline the major differences between MASM's syntax and NASM's. If
you're not already used to MASM, it's probably worth skipping this
section.

* Menu:

* Section 2.2.1:: NASM Is Case-Sensitive
* Section 2.2.2:: NASM Requires Square Brackets For Memory References
* Section 2.2.3:: NASM Doesn't Store Variable Types
* Section 2.2.4:: NASM Doesn't `ASSUME'
* Section 2.2.5:: NASM Doesn't Support Memory Models
* Section 2.2.6:: Floating-Point Differences
* Section 2.2.7:: Other Differences


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2.2.1. NASM Is Case-Sensitive
*****************************

   One simple difference is that NASM is case-sensitive. It makes a
difference whether you call your label `foo', `Foo' or `FOO'. If you're
assembling to DOS or OS/2 `.OBJ' files, you can invoke the `UPPERCASE'
directive (documented in *Note Section 6.2::) to ensure that all
symbols exported to other code modules are forced to be upper case; but
even then, _within_ a single module, NASM will distinguish between
labels differing only in case.


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2.2.2. NASM Requires Square Brackets For Memory References
**********************************************************

   NASM was designed with simplicity of syntax in mind. One of the
design goals of NASM is that it should be possible, as far as is
practical, for the user to look at a single line of NASM code and tell
what opcode is generated by it. You can't do this in MASM: if you
declare, for example,

     foo       equ 1
     bar       dw 2

   then the two lines of code

               mov ax,foo
               mov ax,bar

   generate completely different opcodes, despite having
identical-looking syntaxes.

   NASM avoids this undesirable situation by having a much simpler
syntax for memory references. The rule is simply that any access to the
_contents_ of a memory location requires square brackets around the
address, and any access to the _address_ of a variable doesn't. So an
instruction of the form `mov ax,foo' will _always_ refer to a
compile-time constant, whether it's an `EQU' or the address of a
variable; and to access the _contents_ of the variable `bar', you must
code `mov ax,[bar]'.

   This also means that NASM has no need for MASM's `OFFSET' keyword,
since the MASM code `mov ax,offset bar' means exactly the same thing as
NASM's `mov ax,bar'. If you're trying to get large amounts of MASM code
to assemble sensibly under NASM, you can always code `%idefine offset'
to make the preprocessor treat the `OFFSET' keyword as a no-op.

   This issue is even more confusing in `a86', where declaring a label
with a trailing colon defines it to be a `label' as opposed to a
`variable' and causes `a86' to adopt NASM-style semantics; so in `a86',
`mov ax,var' has different behaviour depending on whether `var' was
declared as `var: dw 0' (a label) or `var dw 0' (a word-size variable).
NASM is very simple by comparison: _everything_ is a label.

   NASM, in the interests of simplicity, also does not support the
hybrid syntaxes supported by MASM and its clones, such as `mov
ax,table[bx]', where a memory reference is denoted by one portion
outside square brackets and another portion inside. The correct syntax
for the above is `mov ax,[table+bx]'. Likewise, `mov ax,es:[di]' is
wrong and `mov ax,[es:di]' is right.


File: nasm.info,  Node: Section 2.2.3,  Next: Section 2.2.4,  Prev: Section 2.2.2,  Up: Section 2.2

2.2.3. NASM Doesn't Store Variable Types
****************************************

   NASM, by design, chooses not to remember the types of variables you
declare. Whereas MASM will remember, on seeing `var dw 0', that you
declared `var' as a word-size variable, and will then be able to fill
in the ambiguity in the size of the instruction `mov var,2', NASM will
deliberately remember nothing about the symbol `var' except where it
begins, and so you must explicitly code `mov word [var],2'.

   For this reason, NASM doesn't support the `LODS', `MOVS', `STOS',
`SCAS', `CMPS', `INS', or `OUTS' instructions, but only supports the
forms such as `LODSB', `MOVSW', and `SCASD', which explicitly specify
the size of the components of the strings being manipulated.


File: nasm.info,  Node: Section 2.2.4,  Next: Section 2.2.5,  Prev: Section 2.2.3,  Up: Section 2.2

2.2.4. NASM Doesn't `ASSUME'
****************************

   As part of NASM's drive for simplicity, it also does not support the
`ASSUME' directive. NASM will not keep track of what values you choose
to put in your segment registers, and will never _automatically_
generate a segment override prefix.


File: nasm.info,  Node: Section 2.2.5,  Next: Section 2.2.6,  Prev: Section 2.2.4,  Up: Section 2.2

2.2.5. NASM Doesn't Support Memory Models
*****************************************

   NASM also does not have any directives to support different 16-bit
memory models. The programmer has to keep track of which functions are
supposed to be called with a far call and which with a near call, and
is responsible for putting the correct form of `RET' instruction
(`RETN' or `RETF'; NASM accepts `RET' itself as an alternate form for
`RETN'); in addition, the programmer is responsible for coding CALL FAR
instructions where necessary when calling _external_ functions, and
must also keep track of which external variable definitions are far and
which are near.


File: nasm.info,  Node: Section 2.2.6,  Next: Section 2.2.7,  Prev: Section 2.2.5,  Up: Section 2.2

2.2.6. Floating-Point Differences
*********************************

   NASM uses different names to refer to floating-point registers from
MASM: where MASM would call them `ST(0)', `ST(1)' and so on, and `a86'
would call them simply `0', `1' and so on, NASM chooses to call them
`st0', `st1' etc.

   As of version 0.96, NASM now treats the instructions with `nowait'
forms in the same way as MASM-compatible assemblers. The idiosyncratic
treatment employed by 0.95 and earlier was based on a misunderstanding
by the authors.


File: nasm.info,  Node: Section 2.2.7,  Next: Chapter 3,  Prev: Section 2.2.6,  Up: Section 2.2

2.2.7. Other Differences
************************

   For historical reasons, NASM uses the keyword `TWORD' where MASM and
compatible assemblers use `TBYTE'.

   NASM does not declare uninitialised storage in the same way as MASM:
where a MASM programmer might use `stack db 64 dup (?)', NASM requires
`stack resb 64', intended to be read as `reserve 64 bytes'. For a
limited amount of compatibility, since NASM treats `?' as a valid
character in symbol names, you can code `? equ 0' and then writing `dw
?' will at least do something vaguely useful. `DUP' is still not a
supported syntax, however.

   In addition to all of this, macros and directives work completely
differently to MASM. See *Note Chapter 4:: and *Note Chapter 5:: for
further details.


File: nasm.info,  Node: Chapter 3,  Next: Section 3.1,  Prev: Section 2.2.7,  Up: Top

Chapter 3: The NASM Language
****************************

* Menu:

* Section 3.1:: Layout of a NASM Source Line
* Section 3.2:: Pseudo-Instructions
* Section 3.3:: Effective Addresses
* Section 3.4:: Constants
* Section 3.5:: Expressions
* Section 3.6:: `SEG' and `WRT'
* Section 3.7:: Critical Expressions
* Section 3.8:: Local Labels


File: nasm.info,  Node: Section 3.1,  Next: Section 3.2,  Prev: Chapter 3,  Up: Chapter 3

3.1. Layout of a NASM Source Line
*********************************

   Like most assemblers, each NASM source line contains (unless it is a
macro, a preprocessor directive or an assembler directive: see *Note
Chapter 4:: and *Note Chapter 5::) some combination of the four fields

     label:    instruction operands        ; comment

   As usual, most of these fields are optional; the presence or absence
of any combination of a label, an instruction and a comment is allowed.
Of course, the operand field is either required or forbidden by the
presence and nature of the instruction field.

   NASM places no restrictions on white space within a line: labels may
have white space before them, or instructions may have no space before
them, or anything. The colon after a label is also optional. (Note that
this means that if you intend to code `lodsb' alone on a line, and type
`lodab' by accident, then that's still a valid source line which does
nothing but define a label. Running NASM with the command-line option
`-w+orphan-labels' will cause it to warn you if you define a label
alone on a line without a trailing colon.)

   Valid characters in labels are letters, numbers, `_', `$', `#', `@',
`~', `.', and `?'. The only characters which may be used as the _first_
character of an identifier are letters, `.' (with special meaning: see
*Note Section 3.8::), `_' and `?'. An identifier may also be prefixed
with a `$' to indicate that it is intended to be read as an identifier
and not a reserved word; thus, if some other module you are linking
with defines a symbol called `eax', you can refer to `$eax' in NASM
code to distinguish the symbol from the register.

   The instruction field may contain any machine instruction: Pentium
and P6 instructions, FPU instructions, MMX instructions and even
undocumented instructions are all supported. The instruction may be
prefixed by `LOCK', `REP', `REPE'/`REPZ' or `REPNE'/`REPNZ', in the
usual way. Explicit address-size and operand-size prefixes `A16',
`A32', `O16' and `O32' are provided - one example of their use is given
in *Note Chapter 9::. You can also use the name of a segment register
as an instruction prefix: coding `es mov [bx],ax' is equivalent to
coding `mov [es:bx],ax'. We recommend the latter syntax, since it is
consistent with other syntactic features of the language, but for
instructions such as `LODSB', which has no operands and yet can require
a segment override, there is no clean syntactic way to proceed apart
from `es lodsb'.

   An instruction is not required to use a prefix: prefixes such as
`CS', `A32', `LOCK' or `REPE' can appear on a line by themselves, and
NASM will just generate the prefix bytes.

   In addition to actual machine instructions, NASM also supports a
number of pseudo-instructions, described in *Note Section 3.2::.

   Instruction operands may take a number of forms: they can be
registers, described simply by the register name (e.g. `ax', `bp',
`ebx', `cr0': NASM does not use the `gas'-style syntax in which
register names must be prefixed by a `%' sign), or they can be
effective addresses (see *Note Section 3.3::), constants (*Note Section
3.4::) or expressions (*Note Section 3.5::).

   For floating-point instructions, NASM accepts a wide range of
syntaxes: you can use two-operand forms like MASM supports, or you can
use NASM's native single-operand forms in most cases. Details of all
forms of each supported instruction are given in *Note Appendix A::.
For example, you can code:

               fadd st1               ; this sets st0 := st0 + st1
               fadd st0,st1           ; so does this
     
               fadd st1,st0           ; this sets st1 := st1 + st0
               fadd to st1            ; so does this

   Almost any floating-point instruction that references memory must
use one of the prefixes `DWORD', `QWORD' or `TWORD' to indicate what
size of memory operand it refers to.


File: nasm.info,  Node: Section 3.2,  Next: Section 3.2.1,  Prev: Section 3.1,  Up: Chapter 3

3.2. Pseudo-Instructions
************************

   Pseudo-instructions are things which, though not real x86 machine
instructions, are used in the instruction field anyway because that's
the most convenient place to put them. The current pseudo-instructions
are `DB', `DW', `DD', `DQ' and `DT', their uninitialised counterparts
`RESB', `RESW', `RESD', `RESQ' and `REST', the `INCBIN' command, the
`EQU' command, and the `TIMES' prefix.

* Menu:

* Section 3.2.1:: `DB' and friends: Declaring Initialised Data
* Section 3.2.2:: `RESB' and friends: Declaring Uninitialised Data
* Section 3.2.3:: `INCBIN': Including External Binary Files
* Section 3.2.4:: `EQU': Defining Constants
* Section 3.2.5:: `TIMES': Repeating Instructions or Data


File: nasm.info,  Node: Section 3.2.1,  Next: Section 3.2.2,  Prev: Section 3.2,  Up: Section 3.2

3.2.1. `DB' and friends: Declaring Initialised Data
***************************************************

   `DB', `DW', `DD', `DQ' and `DT' are used, much as in MASM, to
declare initialised data in the output file. They can be invoked in a
wide range of ways:

               db 0x55                ; just the byte 0x55
               db 0x55,0x56,0x57      ; three bytes in succession
               db 'a',0x55            ; character constants are OK
               db 'hello',13,10,'$'   ; so are string constants
               dw 0x1234              ; 0x34 0x12
               dw 'a'                 ; 0x41 0x00 (it's just a number)
               dw 'ab'                ; 0x41 0x42 (character constant)
               dw 'abc'               ; 0x41 0x42 0x43 0x00 (string)
               dd 0x12345678          ; 0x78 0x56 0x34 0x12
               dd 1.234567e20         ; floating-point constant
               dq 1.234567e20         ; double-precision float
               dt 1.234567e20         ; extended-precision float

   `DQ' and `DT' do not accept numeric constants or string constants as
operands.


File: nasm.info,  Node: Section 3.2.2,  Next: Section 3.2.3,  Prev: Section 3.2.1,  Up: Section 3.2

3.2.2. `RESB' and friends: Declaring Uninitialised Data
*******************************************************

   `RESB', `RESW', `RESD', `RESQ' and `REST' are designed to be used in
the BSS section of a module: they declare _uninitialised_ storage
space. Each takes a single operand, which is the number of bytes,
words, doublewords or whatever to reserve. As stated in *Note Section
2.2.7::, NASM does not support the MASM/TASM syntax of reserving
uninitialised space by writing `DW ?' or similar things: this is what
it does instead. The operand to a `RESB'-type pseudo- instruction is a
_critical expression_: see *Note Section 3.7::.

   For example:

     buffer:   resb 64                ; reserve 64 bytes
     wordvar:  resw 1                 ; reserve a word
     realarray resq 10                ; array of ten reals


File: nasm.info,  Node: Section 3.2.3,  Next: Section 3.2.4,  Prev: Section 3.2.2,  Up: Section 3.2

3.2.3. `INCBIN': Including External Binary Files
************************************************

   `INCBIN' is borrowed from the old Amiga assembler DevPac: it includes
a binary file verbatim into the output file. This can be handy for (for
example) including graphics and sound data directly into a game
executable file. It can be called in one of these three ways:

               incbin "file.dat"      ; include the whole file
               incbin "file.dat",1024 ; skip the first 1024 bytes
               incbin "file.dat",1024,512 ; skip the first 1024, and
                                      ; actually include at most 512


File: nasm.info,  Node: Section 3.2.4,  Next: Section 3.2.5,  Prev: Section 3.2.3,  Up: Section 3.2

3.2.4. `EQU': Defining Constants
********************************

   `EQU' defines a symbol to a given constant value: when `EQU' is
used, the source line must contain a label. The action of `EQU' is to
define the given label name to the value of its (only) operand. This
definition is absolute, and cannot change later. So, for example,

     message   db 'hello, world'
     msglen    equ $-message

   defines `msglen' to be the constant 12. `msglen' may not then be
redefined later. This is not a preprocessor definition either: the
value of `msglen' is evaluated _once_, using the value of `$' (see
*Note Section 3.5:: for an explanation of `$') at the point of
definition, rather than being evaluated wherever it is referenced and
using the value of `$' at the point of reference. Note that the operand
to an `EQU' is also a critical expression (*Note Section 3.7::).


File: nasm.info,  Node: Section 3.2.5,  Next: Section 3.3,  Prev: Section 3.2.4,  Up: Section 3.2

3.2.5. `TIMES': Repeating Instructions or Data
**********************************************

   The `TIMES' prefix causes the instruction to be assembled multiple
times. This is partly present as NASM's equivalent of the `DUP' syntax
supported by MASM-compatible assemblers, in that you can code

     zerobuf:  times 64 db 0

   or similar things; but `TIMES' is more versatile than that. The
argument to `TIMES' is not just a numeric constant, but a numeric
_expression_, so you can do things like

     buffer:   db 'hello, world'
               times 64-$+buffer db ' '

   which will store exactly enough spaces to make the total length of
`buffer' up to 64. Finally, `TIMES' can be applied to ordinary
instructions, so you can code trivial unrolled loops in it:

               times 100 movsb

   Note that there is no effective difference between `times 100 resb 1'
and `resb 100', except that the latter will be assembled about 100
times faster due to the internal structure of the assembler.

   The operand to `TIMES', like that of `EQU' and those of `RESB' and
friends, is a critical expression (*Note Section 3.7::).

   Note also that `TIMES' can't be applied to macros: the reason for
this is that `TIMES' is processed after the macro phase, which allows
the argument to `TIMES' to contain expressions such as `64-$+buffer' as
above. To repeat more than one line of code, or a complex macro, use the
preprocessor `%rep' directive.


File: nasm.info,  Node: Section 3.3,  Next: Section 3.4,  Prev: Section 3.2.5,  Up: Chapter 3

3.3. Effective Addresses
************************

   An effective address is any operand to an instruction which
references memory. Effective addresses, in NASM, have a very simple
syntax: they consist of an expression evaluating to the desired
address, enclosed in square brackets. For example:

     wordvar   dw 123
               mov ax,[wordvar]
               mov ax,[wordvar+1]
               mov ax,[es:wordvar+bx]

   Anything not conforming to this simple system is not a valid memory
reference in NASM, for example `es:wordvar[bx]'.

   More complicated effective addresses, such as those involving more
than one register, work in exactly the same way:

               mov eax,[ebx*2+ecx+offset]
               mov ax,[bp+di+8]

   NASM is capable of doing algebra on these effective addresses, so
that things which don't necessarily _look_ legal are perfectly all
right:

               mov eax,[ebx*5]        ; assembles as [ebx*4+ebx]
               mov eax,[label1*2-label2] ; ie [label1+(label1-label2)]

   Some forms of effective address have more than one assembled form;
in most such cases NASM will generate the smallest form it can. For
example, there are distinct assembled forms for the 32-bit effective
addresses `[eax*2+0]' and `[eax+eax]', and NASM will generally generate
the latter on the grounds that the former requires four bytes to store
a zero offset.

   NASM has a hinting mechanism which will cause `[eax+ebx]' and
`[ebx+eax]' to generate different opcodes; this is occasionally useful
because `[esi+ebp]' and `[ebp+esi]' have different default segment
registers.

   However, you can force NASM to generate an effective address in a
particular form by the use of the keywords `BYTE', `WORD', `DWORD' and
`NOSPLIT'. If you need `[eax+3]' to be assembled using a double-word
offset field instead of the one byte NASM will normally generate, you
can code `[dword eax+3]'. Similarly, you can force NASM to use a byte
offset for a small value which it hasn't seen on the first pass (see
*Note Section 3.7:: for an example of such a code fragment) by using
`[byte eax+offset]'. As special cases, `[byte eax]' will code `[eax+0]'
with a byte offset of zero, and `[dword eax]' will code it with a
double-word offset of zero. The normal form, `[eax]', will be coded
with no offset field.

   Similarly, NASM will split `[eax*2]' into `[eax+eax]' because that
allows the offset field to be absent and space to be saved; in fact, it
will also split `[eax*2+offset]' into `[eax+eax+offset]'. You can
combat this behaviour by the use of the `NOSPLIT' keyword: `[nosplit
eax*2]' will force `[eax*2+0]' to be generated literally.


File: nasm.info,  Node: Section 3.4,  Next: Section 3.4.1,  Prev: Section 3.3,  Up: Chapter 3

3.4. Constants
**************

   NASM understands four different types of constant: numeric,
character, string and floating-point.

* Menu:

* Section 3.4.1:: Numeric Constants
* Section 3.4.2:: Character Constants
* Section 3.4.3:: String Constants
* Section 3.4.4:: Floating-Point Constants


File: nasm.info,  Node: Section 3.4.1,  Next: Section 3.4.2,  Prev: Section 3.4,  Up: Section 3.4

3.4.1. Numeric Constants
************************

   A numeric constant is simply a number. NASM allows you to specify
numbers in a variety of number bases, in a variety of ways: you can
suffix `H', `Q' and `B' for hex, octal and binary, or you can prefix
`0x' for hex in the style of C, or you can prefix `$' for hex in the
style of Borland Pascal. Note, though, that the `$' prefix does double
duty as a prefix on identifiers (see *Note Section 3.1::), so a hex
number prefixed with a `$' sign must have a digit after the `$' rather
than a letter.

   Some examples:

               mov ax,100             ; decimal
               mov ax,0a2h            ; hex
               mov ax,$0a2            ; hex again: the 0 is required
               mov ax,0xa2            ; hex yet again
               mov ax,777q            ; octal
               mov ax,10010011b       ; binary


File: nasm.info,  Node: Section 3.4.2,  Next: Section 3.4.3,  Prev: Section 3.4.1,  Up: Section 3.4

3.4.2. Character Constants
**************************

   A character constant consists of up to four characters enclosed in
either single or double quotes. The type of quote makes no difference
to NASM, except of course that surrounding the constant with single
quotes allows double quotes to appear within it and vice versa.

   A character constant with more than one character will be arranged
with little-endian order in mind: if you code

               mov eax,'abcd'

   then the constant generated is not `0x61626364', but `0x64636261',
so that if you were then to store the value into memory, it would read
`abcd' rather than `dcba'. This is also the sense of character
constants understood by the Pentium's `CPUID' instruction (see *Note
Section A.22::).


File: nasm.info,  Node: Section 3.4.3,  Next: Section 3.4.4,  Prev: Section 3.4.2,  Up: Section 3.4

3.4.3. String Constants
***********************

   String constants are only acceptable to some pseudo-instructions,
namely the `DB' family and `INCBIN'.

   A string constant looks like a character constant, only longer. It is
treated as a concatenation of maximum-size character constants for the
conditions. So the following are equivalent:

               db 'hello'             ; string constant
               db 'h','e','l','l','o' ; equivalent character constants

   And the following are also equivalent:

               dd 'ninechars'         ; doubleword string constant
               dd 'nine','char','s'   ; becomes three doublewords
               db 'ninechars',0,0,0   ; and really looks like this

   Note that when used as an operand to `db', a constant like `'ab'' is
treated as a string constant despite being short enough to be a
character constant, because otherwise `db 'ab'' would have the same
effect as `db 'a'', which would be silly. Similarly, three-character or
four-character constants are treated as strings when they are operands
to `dw'.


File: nasm.info,  Node: Section 3.4.4,  Next: Section 3.5,  Prev: Section 3.4.3,  Up: Section 3.4

3.4.4. Floating-Point Constants
*******************************

   Floating-point constants are acceptable only as arguments to `DD',
`DQ' and `DT'. They are expressed in the traditional form: digits, then
a period, then optionally more digits, then optionally an `E' followed
by an exponent. The period is mandatory, so that NASM can distinguish
between `dd 1', which declares an integer constant, and `dd 1.0' which
declares a floating-point constant.

   Some examples:

               dd 1.2                 ; an easy one
               dq 1.e10               ; 10,000,000,000
               dq 1.e+10              ; synonymous with 1.e10
               dq 1.e-10              ; 0.000 000 000 1
               dt 3.141592653589793238462 ; pi

   NASM cannot do compile-time arithmetic on floating-point constants.
This is because NASM is designed to be portable - although it always
generates code to run on x86 processors, the assembler itself can run
on any system with an ANSI C compiler. Therefore, the assembler cannot
guarantee the presence of a floating-point unit capable of handling the
Intel number formats, and so for NASM to be able to do floating
arithmetic it would have to include its own complete set of
floating-point routines, which would significantly increase the size of
the assembler for very little benefit.


File: nasm.info,  Node: Section 3.5,  Next: Section 3.5.1,  Prev: Section 3.4.4,  Up: Chapter 3

3.5. Expressions
****************

   Expressions in NASM are similar in syntax to those in C.

   NASM does not guarantee the size of the integers used to evaluate
expressions at compile time: since NASM can compile and run on 64-bit
systems quite happily, don't assume that expressions are evaluated in
32- bit registers and so try to make deliberate use of integer
overflow. It might not always work. The only thing NASM will guarantee
is what's guaranteed by ANSI C: you always have _at least_ 32 bits to
work in.

   NASM supports two special tokens in expressions, allowing
calculations to involve the current assembly position: the `$' and `$$'
tokens.  `$' evaluates to the assembly position at the beginning of the
line containing the expression; so you can code an infinite loop using
`JMP $'. `$$' evaluates to the beginning of the current section; so you
can tell how far into the section you are by using `($-$$)'.

   The arithmetic operators provided by NASM are listed here, in
increasing order of precedence.

* Menu:

* Section 3.5.1:: `|': Bitwise OR Operator
* Section 3.5.2:: `^': Bitwise XOR Operator
* Section 3.5.3:: `&': Bitwise AND Operator
* Section 3.5.4:: `<<' and `>>': Bit Shift Operators
* Section 3.5.5:: `+' and `-': Addition and Subtraction Operators
* Section 3.5.6:: `*', `/', `//', `%' and `%%': Multiplication and Division
* Section 3.5.7:: Unary Operators: `+', `-', `~' and `SEG'


File: nasm.info,  Node: Section 3.5.1,  Next: Section 3.5.2,  Prev: Section 3.5,  Up: Section 3.5

3.5.1. `|': Bitwise OR Operator
*******************************

   The `|' operator gives a bitwise OR, exactly as performed by the
`OR' machine instruction. Bitwise OR is the lowest-priority arithmetic
operator supported by NASM.


File: nasm.info,  Node: Section 3.5.2,  Next: Section 3.5.3,  Prev: Section 3.5.1,  Up: Section 3.5

3.5.2. `^': Bitwise XOR Operator
********************************

   `^' provides the bitwise XOR operation.


File: nasm.info,  Node: Section 3.5.3,  Next: Section 3.5.4,  Prev: Section 3.5.2,  Up: Section 3.5

3.5.3. `&': Bitwise AND Operator
********************************

   `&' provides the bitwise AND operation.


File: nasm.info,  Node: Section 3.5.4,  Next: Section 3.5.5,  Prev: Section 3.5.3,  Up: Section 3.5

3.5.4. `<<' and `>>': Bit Shift Operators
*****************************************

   `<<' gives a bit-shift to the left, just as it does in C. So `5<<3'
evaluates to 5 times 8, or 40. `>>' gives a bit-shift to the right; in
NASM, such a shift is _always_ unsigned, so that the bits shifted in
from the left-hand end are filled with zero rather than a
sign-extension of the previous highest bit.


File: nasm.info,  Node: Section 3.5.5,  Next: Section 3.5.6,  Prev: Section 3.5.4,  Up: Section 3.5

3.5.5. `+' and `-': Addition and Subtraction Operators
******************************************************

   The `+' and `-' operators do perfectly ordinary addition and
subtraction.


File: nasm.info,  Node: Section 3.5.6,  Next: Section 3.5.7,  Prev: Section 3.5.5,  Up: Section 3.5

3.5.6. `*', `/', `//', `%' and `%%': Multiplication and Division
****************************************************************

   `*' is the multiplication operator. `/' and `//' are both division
operators: `/' is unsigned division and `//' is signed division.
Similarly, `%' and `%%' provide unsigned and signed modulo operators
respectively.

   NASM, like ANSI C, provides no guarantees about the sensible
operation of the signed modulo operator.

   Since the `%' character is used extensively by the macro
preprocessor, you should ensure that both the signed and unsigned
modulo operators are followed by white space wherever they appear.


File: nasm.info,  Node: Section 3.5.7,  Next: Section 3.6,  Prev: Section 3.5.6,  Up: Section 3.5

3.5.7. Unary Operators: `+', `-', `~' and `SEG'
***********************************************

   The highest-priority operators in NASM's expression grammar are
those which only apply to one argument. `-' negates its operand, `+'
does nothing (it's provided for symmetry with `-'), `~' computes the
one's complement of its operand, and `SEG' provides the segment address
of its operand (explained in more detail in *Note Section 3.6::).


File: nasm.info,  Node: Section 3.6,  Next: Section 3.7,  Prev: Section 3.5.7,  Up: Chapter 3

3.6. `SEG' and `WRT'
********************

   When writing large 16-bit programs, which must be split into multiple
segments, it is often necessary to be able to refer to the segment part
of the address of a symbol. NASM supports the `SEG' operator to perform
this function.

   The `SEG' operator returns the _preferred_ segment base of a symbol,
defined as the segment base relative to which the offset of the symbol
makes sense. So the code

               mov ax,seg symbol
               mov es,ax
               mov bx,symbol

   will load `ES:BX' with a valid pointer to the symbol `symbol'.

   Things can be more complex than this: since 16-bit segments and
groups may overlap, you might occasionally want to refer to some symbol
using a different segment base from the preferred one. NASM lets you do
this, by the use of the `WRT' (With Reference To) keyword. So you can
do things like

               mov ax,weird_seg       ; weird_seg is a segment base
               mov es,ax
               mov bx,symbol wrt weird_seg

   to load `ES:BX' with a different, but functionally equivalent,
pointer to the symbol `symbol'.

   NASM supports far (inter-segment) calls and jumps by means of the
syntax `call segment:offset', where `segment' and `offset' both
represent immediate values. So to call a far procedure, you could code
either of

               call (seg procedure):procedure
               call weird_seg:(procedure wrt weird_seg)

   (The parentheses are included for clarity, to show the intended
parsing of the above instructions. They are not necessary in practice.)

   NASM supports the syntax `call far procedure' as a synonym for the
first of the above usages. `JMP' works identically to `CALL' in these
examples.

   To declare a far pointer to a data item in a data segment, you must
code

               dw symbol, seg symbol

   NASM supports no convenient synonym for this, though you can always
invent one using the macro processor.


File: nasm.info,  Node: Section 3.7,  Next: Section 3.8,  Prev: Section 3.6,  Up: Chapter 3

3.7. Critical Expressions
*************************

   A limitation of NASM is that it is a two-pass assembler; unlike TASM
and others, it will always do exactly two assembly passes. Therefore it
is unable to cope with source files that are complex enough to require
three or more passes.

   The first pass is used to determine the size of all the assembled
code and data, so that the second pass, when generating all the code,
knows all the symbol addresses the code refers to. So one thing NASM
can't handle is code whose size depends on the value of a symbol
declared after the code in question. For example,

               times (label-$) db 0
     label:    db 'Where am I?'

   The argument to `TIMES' in this case could equally legally evaluate
to anything at all; NASM will reject this example because it cannot
tell the size of the `TIMES' line when it first sees it. It will just
as firmly reject the slightly paradoxical code

               times (label-$+1) db 0
     label:    db 'NOW where am I?'

   in which _any_ value for the `TIMES' argument is by definition wrong!

   NASM rejects these examples by means of a concept called a _critical
expression_, which is defined to be an expression whose value is
required to be computable in the first pass, and which must therefore
depend only on symbols defined before it. The argument to the `TIMES'
prefix is a critical expression; for the same reason, the arguments to
the `RESB' family of pseudo-instructions are also critical expressions.

   Critical expressions can crop up in other contexts as well: consider
the following code.

               mov ax,symbol1
     symbol1   equ symbol2
     symbol2:

   On the first pass, NASM cannot determine the value of `symbol1',
because `symbol1' is defined to be equal to `symbol2' which NASM hasn't
seen yet. On the second pass, therefore, when it encounters the line
`mov ax,symbol1', it is unable to generate the code for it because it
still doesn't know the value of `symbol1'. On the next line, it would
see the `EQU' again and be able to determine the value of `symbol1',
but by then it would be too late.

   NASM avoids this problem by defining the right-hand side of an `EQU'
statement to be a critical expression, so the definition of `symbol1'
would be rejected in the first pass.

   There is a related issue involving forward references: consider this
code fragment.

               mov eax,[ebx+offset]
     offset    equ 10

   NASM, on pass one, must calculate the size of the instruction `mov
eax,[ebx+offset]' without knowing the value of `offset'. It has no way
of knowing that `offset' is small enough to fit into a one- byte offset
field and that it could therefore get away with generating a shorter
form of the effective-address encoding; for all it knows, in pass one,
`offset' could be a symbol in the code segment, and it might need the
full four-byte form. So it is forced to compute the size of the
instruction to accommodate a four-byte address part. In pass two, having
made this decision, it is now forced to honour it and keep the
instruction large, so the code generated in this case is not as small
as it could have been. This problem can be solved by defining `offset'
before using it, or by forcing byte size in the effective address by
coding `[byte ebx+offset]'.


File: nasm.info,  Node: Section 3.8,  Next: Chapter 4,  Prev: Section 3.7,  Up: Chapter 3

3.8. Local Labels
*****************

   NASM gives special treatment to symbols beginning with a period. A
label beginning with a single period is treated as a _local_ label,
which means that it is associated with the previous non-local label.
So, for example:

     label1    ; some code
     .loop     ; some more code
               jne .loop
               ret
     label2    ; some code
     .loop     ; some more code
               jne .loop
               ret

   In the above code fragment, each `JNE' instruction jumps to the line
immediately before it, because the two definitions of `.loop' are kept
separate by virtue of each being associated with the previous non-local
label.

   This form of local label handling is borrowed from the old Amiga
assembler DevPac; however, NASM goes one step further, in allowing
access to local labels from other parts of the code. This is achieved
by means of _defining_ a local label in terms of the previous non-local
label: the first definition of `.loop' above is really defining a
symbol called `label1.loop', and the second defines a symbol called
`label2.loop'. So, if you really needed to, you could write

     label3    ; some more code
               ; and some more
               jmp label1.loop

   Sometimes it is useful - in a macro, for instance - to be able to
define a label which can be referenced from anywhere but which doesn't
interfere with the normal local-label mechanism. Such a label can't be
non-local because it would interfere with subsequent definitions of,
and references to, local labels; and it can't be local because the
macro that defined it wouldn't know the label's full name. NASM
therefore introduces a third type of label, which is probably only
useful in macro definitions: if a label begins with the special prefix
`..@', then it does nothing to the local label mechanism. So you could
code

     label1:   ; a non-local label
     .local:   ; this is really label1.local
     ..@foo:   ; this is a special symbol
     label2:   ; another non-local label
     .local:   ; this is really label2.local
               jmp ..@foo             ; this will jump three lines up

   NASM has the capacity to define other special symbols beginning with
a double period: for example, `..start' is used to specify the entry
point in the `obj' output format (see *Note Section 6.2.6::).


File: nasm.info,  Node: Chapter 4,  Next: Section 4.1,  Prev: Section 3.8,  Up: Top

Chapter 4: The NASM Preprocessor
********************************

   NASM contains a powerful macro processor, which supports conditional
assembly, multi-level file inclusion, two forms of macro (single-line
and multi-line), and a `context stack' mechanism for extra macro power.
Preprocessor directives all begin with a `%' sign.

* Menu:

* Section 4.1:: Single-Line Macros
* Section 4.2:: Multi-Line Macros: `%macro'
* Section 4.3:: Conditional Assembly
* Section 4.4:: Preprocessor Loops: `%rep'
* Section 4.5:: Including Other Files
* Section 4.6:: The Context Stack
* Section 4.7:: Standard Macros


File: nasm.info,  Node: Section 4.1,  Next: Section 4.1.1,  Prev: Chapter 4,  Up: Chapter 4

4.1. Single-Line Macros
***********************

* Menu:

* Section 4.1.1:: The Normal Way: `%define'
* Section 4.1.2:: Undefining macros: `%undef'
* Section 4.1.3:: Preprocessor Variables: `%assign'


File: nasm.info,  Node: Section 4.1.1,  Next: Section 4.1.2,  Prev: Section 4.1,  Up: Section 4.1

4.1.1. The Normal Way: `%define'
********************************

   Single-line macros are defined using the `%define' preprocessor
directive. The definitions work in a similar way to C; so you can do
things like

     %define ctrl 0x1F &
     %define param(a,b) ((a)+(a)*(b))
               mov byte [param(2,ebx)], ctrl 'D'

   which will expand to

               mov byte [(2)+(2)*(ebx)], 0x1F & 'D'

   When the expansion of a single-line macro contains tokens which
invoke another macro, the expansion is performed at invocation time,
not at definition time. Thus the code

     %define a(x) 1+b(x)
     %define b(x) 2*x
               mov ax,a(8)

   will evaluate in the expected way to `mov ax,1+2*8', even though the
macro `b' wasn't defined at the time of definition of `a'.

   Macros defined with `%define' are case sensitive: after `%define foo
bar', only `foo' will expand to `bar': `Foo' or `FOO' will not. By
using `%idefine' instead of `%define' (the `i' stands for
`insensitive') you can define all the case variants of a macro at once,
so that `%idefine foo bar' would cause `foo', `Foo', `FOO', `fOO' and
so on all to expand to `bar'.

   There is a mechanism which detects when a macro call has occurred as
a result of a previous expansion of the same macro, to guard against
circular references and infinite loops. If this happens, the
preprocessor will only expand the first occurrence of the macro. Hence,
if you code

     %define a(x) 1+a(x)
               mov ax,a(3)

   the macro `a(3)' will expand once, becoming `1+a(3)', and will then
expand no further. This behaviour can be useful: see *Note Section 8.1::
for an example of its use.

   You can overload single-line macros: if you write

     %define foo(x) 1+x
     %define foo(x,y) 1+x*y

   the preprocessor will be able to handle both types of macro call, by
counting the parameters you pass; so `foo(3)' will become `1+3' whereas
`foo(ebx,2)' will become `1+ebx*2'. However, if you define

     %define foo bar

   then no other definition of `foo' will be accepted: a macro with no
parameters prohibits the definition of the same name as a macro _with_
parameters, and vice versa.

   This doesn't prevent single-line macros being _redefined_: you can
perfectly well define a macro with

     %define foo bar

   and then re-define it later in the same source file with

     %define foo baz

   Then everywhere the macro `foo' is invoked, it will be expanded
according to the most recent definition. This is particularly useful
when defining single-line macros with `%assign' (see *Note Section
4.1.3::).

   You can pre-define single-line macros using the `-d' option on the
NASM command line: see *Note Section 2.1.8::.


File: nasm.info,  Node: Section 4.1.2,  Next: Section 4.1.3,  Prev: Section 4.1.1,  Up: Section 4.1

4.1.2. Undefining macros: `%undef'
**********************************

   Single-line macros can be removed with the `%undef' command. For
example, the following sequence:

     %define foo bar
     %undef foo
     		mov eax, foo

   will expand to the instruction `mov eax, foo', since after `%undef'
the macro `foo' is no longer defined.

   Macros that would otherwise be pre-defined can be undefined on the
command- line using the `-u' option on the NASM command line: see *Note
Section 2.1.9::.


File: nasm.info,  Node: Section 4.1.3,  Next: Section 4.2,  Prev: Section 4.1.2,  Up: Section 4.1

4.1.3. Preprocessor Variables: `%assign'
****************************************

   An alternative way to define single-line macros is by means of the
`%assign' command (and its case sensitivecase-insensitive counterpart
`%iassign', which differs from `%assign' in exactly the same way that
`%idefine' differs from `%define').

   `%assign' is used to define single-line macros which take no
parameters and have a numeric value. This value can be specified in the
form of an expression, and it will be evaluated once, when the
`%assign' directive is processed.

   Like `%define', macros defined using `%assign' can be re-defined
later, so you can do things like

     %assign i i+1

   to increment the numeric value of a macro.

   `%assign' is useful for controlling the termination of `%rep'
preprocessor loops: see *Note Section 4.4:: for an example of this.
Another use for `%assign' is given in *Note Section 7.4:: and *Note
Section 8.1::.

   The expression passed to `%assign' is a critical expression (see
*Note Section 3.7::), and must also evaluate to a pure number (rather
than a relocatable reference such as a code or data address, or
anything involving a register).


File: nasm.info,  Node: Section 4.2,  Next: Section 4.2.1,  Prev: Section 4.1.3,  Up: Chapter 4

4.2. Multi-Line Macros: `%macro'
********************************

   Multi-line macros are much more like the type of macro seen in MASM
and TASM: a multi-line macro definition in NASM looks something like
this.

     %macro prologue 1
               push ebp
               mov ebp,esp
               sub esp,%1
     %endmacro

   This defines a C-like function prologue as a macro: so you would
invoke the macro with a call such as

     myfunc:   prologue 12

   which would expand to the three lines of code

     myfunc:   push ebp
               mov ebp,esp
               sub esp,12

   The number `1' after the macro name in the `%macro' line defines the
number of parameters the macro `prologue' expects to receive. The use
of `%1' inside the macro definition refers to the first parameter to
the macro call. With a macro taking more than one parameter, subsequent
parameters would be referred to as `%2', `%3' and so on.

   Multi-line macros, like single-line macros, are case-sensitive,
unless you define them using the alternative directive `%imacro'.

   If you need to pass a comma as _part_ of a parameter to a multi-line
macro, you can do that by enclosing the entire parameter in braces. So
you could code things like

     %macro silly 2
     %2:       db %1
     %endmacro
               silly 'a', letter_a    ; letter_a:  db 'a'
               silly 'ab', string_ab  ; string_ab: db 'ab'
               silly {13,10}, crlf    ; crlf:      db 13,10

* Menu:

* Section 4.2.1:: Overloading Multi-Line Macros
* Section 4.2.2:: Macro-Local Labels
* Section 4.2.3:: Greedy Macro Parameters
* Section 4.2.4:: Default Macro Parameters
* Section 4.2.5:: `%0': Macro Parameter Counter
* Section 4.2.6:: `%rotate': Rotating Macro Parameters
* Section 4.2.7:: Concatenating Macro Parameters
* Section 4.2.8:: Condition Codes as Macro Parameters
* Section 4.2.9:: Disabling Listing Expansion


File: nasm.info,  Node: Section 4.2.1,  Next: Section 4.2.2,  Prev: Section 4.2,  Up: Section 4.2

4.2.1. Overloading Multi-Line Macros
************************************

   As with single-line macros, multi-line macros can be overloaded by
defining the same macro name several times with different numbers of
parameters.  This time, no exception is made for macros with no
parameters at all. So you could define

     %macro prologue 0
               push ebp
               mov ebp,esp
     %endmacro

   to define an alternative form of the function prologue which
allocates no local stack space.

   Sometimes, however, you might want to `overload' a machine
instruction; for example, you might want to define

     %macro push 2
               push %1
               push %2
     %endmacro

   so that you could code

               push ebx               ; this line is not a macro call
               push eax,ecx           ; but this one is

   Ordinarily, NASM will give a warning for the first of the above two
lines, since `push' is now defined to be a macro, and is being invoked
with a number of parameters for which no definition has been given. The
correct code will still be generated, but the assembler will give a
warning. This warning can be disabled by the use of the
`-w-macro-params' command- line option (see *Note Section 2.1.12::).


File: nasm.info,  Node: Section 4.2.2,  Next: Section 4.2.3,  Prev: Section 4.2.1,  Up: Section 4.2

4.2.2. Macro-Local Labels
*************************

   NASM allows you to define labels within a multi-line macro
definition in such a way as to make them local to the macro call: so
calling the same macro multiple times will use a different label each
time. You do this by prefixing `%%' to the label name. So you can
invent an instruction which executes a `RET' if the `Z' flag is set by
doing this:

     %macro retz 0
               jnz %%skip
               ret
     %%skip:
     %endmacro

   You can call this macro as many times as you want, and every time
you call it NASM will make up a different `real' name to substitute for
the label `%%skip'. The names NASM invents are of the form
`..@2345.skip', where the number 2345 changes with every macro call.
The `..@' prefix prevents macro-local labels from interfering with the
local label mechanism, as described in *Note Section 3.8::. You should
avoid defining your own labels in this form (the `..@' prefix, then a
number, then another period) in case they interfere with macro-local
labels.


File: nasm.info,  Node: Section 4.2.3,  Next: Section 4.2.4,  Prev: Section 4.2.2,  Up: Section 4.2

4.2.3. Greedy Macro Parameters
******************************

   Occasionally it is useful to define a macro which lumps its entire
command line into one parameter definition, possibly after extracting
one or two smaller parameters from the front. An example might be a
macro to write a text string to a file in MS-DOS, where you might want
to be able to write

               writefile [filehandle],"hello, world",13,10

   NASM allows you to define the last parameter of a macro to be
_greedy_, meaning that if you invoke the macro with more parameters
than it expects, all the spare parameters get lumped into the last
defined one along with the separating commas. So if you code:

     %macro writefile 2+
               jmp %%endstr
     %%str:    db %2
     %%endstr: mov dx,%%str
               mov cx,%%endstr-%%str
               mov bx,%1
               mov ah,0x40
               int 0x21
     %endmacro

   then the example call to `writefile' above will work as expected: the
text before the first comma, `[filehandle]', is used as the first macro
parameter and expanded when `%1' is referred to, and all the subsequent
text is lumped into `%2' and placed after the `db'.

   The greedy nature of the macro is indicated to NASM by the use of the
`+' sign after the parameter count on the `%macro' line.

   If you define a greedy macro, you are effectively telling NASM how it
should expand the macro given _any_ number of parameters from the
actual number specified up to infinity; in this case, for example, NASM
now knows what to do when it sees a call to `writefile' with 2, 3, 4 or
more parameters. NASM will take this into account when overloading
macros, and will not allow you to define another form of `writefile'
taking 4 parameters (for example).

   Of course, the above macro could have been implemented as a
non-greedy macro, in which case the call to it would have had to look
like

               writefile [filehandle], {"hello, world",13,10}

   NASM provides both mechanisms for putting commas in macro
parameters, and you choose which one you prefer for each macro
definition.

   See *Note Section 5.2.1:: for a better way to write the above macro.


File: nasm.info,  Node: Section 4.2.4,  Next: Section 4.2.5,  Prev: Section 4.2.3,  Up: Section 4.2

4.2.4. Default Macro Parameters
*******************************

   NASM also allows you to define a multi-line macro with a _range_ of
allowable parameter counts. If you do this, you can specify defaults for
omitted parameters. So, for example:

     %macro die 0-1 "Painful program death has occurred."
               writefile 2,%1
               mov ax,0x4c01
               int 0x21
     %endmacro

   This macro (which makes use of the `writefile' macro defined in
*Note Section 4.2.3::) can be called with an explicit error message,
which it will display on the error output stream before exiting, or it
can be called with no parameters, in which case it will use the default
error message supplied in the macro definition.

   In general, you supply a minimum and maximum number of parameters
for a macro of this type; the minimum number of parameters are then
required in the macro call, and then you provide defaults for the
optional ones. So if a macro definition began with the line

     %macro foobar 1-3 eax,[ebx+2]

   then it could be called with between one and three parameters, and
`%1' would always be taken from the macro call. `%2', if not specified
by the macro call, would default to `eax', and `%3' if not specified
would default to `[ebx+2]'.

   You may omit parameter defaults from the macro definition, in which
case the parameter default is taken to be blank. This can be useful for
macros which can take a variable number of parameters, since the `%0'
token (see *Note Section 4.2.5::) allows you to determine how many
parameters were really passed to the macro call.

   This defaulting mechanism can be combined with the greedy-parameter
mechanism; so the `die' macro above could be made more powerful, and
more useful, by changing the first line of the definition to

     %macro die 0-1+ "Painful program death has occurred.",13,10

   The maximum parameter count can be infinite, denoted by `*'. In this
case, of course, it is impossible to provide a _full_ set of default
parameters. Examples of this usage are shown in *Note Section 4.2.6::.


File: nasm.info,  Node: Section 4.2.5,  Next: Section 4.2.6,  Prev: Section 4.2.4,  Up: Section 4.2

4.2.5. `%0': Macro Parameter Counter
************************************

   For a macro which can take a variable number of parameters, the
parameter reference `%0' will return a numeric constant giving the
number of parameters passed to the macro. This can be used as an
argument to `%rep' (see *Note Section 4.4::) in order to iterate
through all the parameters of a macro. Examples are given in *Note
Section 4.2.6::.


File: nasm.info,  Node: Section 4.2.6,  Next: Section 4.2.7,  Prev: Section 4.2.5,  Up: Section 4.2

4.2.6. `%rotate': Rotating Macro Parameters
*******************************************

   Unix shell programmers will be familiar with the `shift' shell
command, which allows the arguments passed to a shell script
(referenced as `$1', `$2' and so on) to be moved left by one place, so
that the argument previously referenced as `$2' becomes available as
`$1', and the argument previously referenced as `$1' is no longer
available at all.

   NASM provides a similar mechanism, in the form of `%rotate'. As its
name suggests, it differs from the Unix `shift' in that no parameters
are lost: parameters rotated off the left end of the argument list
reappear on the right, and vice versa.

   `%rotate' is invoked with a single numeric argument (which may be an
expression). The macro parameters are rotated to the left by that many
places. If the argument to `%rotate' is negative, the macro parameters
are rotated to the right.

   So a pair of macros to save and restore a set of registers might
work as follows:

     %macro multipush 1-*
     %rep %0
               push %1
     %rotate 1
     %endrep
     %endmacro

   This macro invokes the `PUSH' instruction on each of its arguments in
turn, from left to right. It begins by pushing its first argument,
`%1', then invokes `%rotate' to move all the arguments one place to the
left, so that the original second argument is now available as `%1'.
Repeating this procedure as many times as there were arguments
(achieved by supplying `%0' as the argument to `%rep') causes each
argument in turn to be pushed.

   Note also the use of `*' as the maximum parameter count, indicating
that there is no upper limit on the number of parameters you may supply
to the `multipush' macro.

   It would be convenient, when using this macro, to have a `POP'
equivalent, which _didn't_ require the arguments to be given in reverse
order. Ideally, you would write the `multipush' macro call, then
cut-and-paste the line to where the pop needed to be done, and change
the name of the called macro to `multipop', and the macro would take
care of popping the registers in the opposite order from the one in
which they were pushed.

   This can be done by the following definition:

     %macro multipop 1-*
     %rep %0
     %rotate -1
               pop %1
     %endrep
     %endmacro

   This macro begins by rotating its arguments one place to the _right_,
so that the original _last_ argument appears as `%1'. This is then
popped, and the arguments are rotated right again, so the second-to-
last argument becomes `%1'. Thus the arguments are iterated through in
reverse order.


File: nasm.info,  Node: Section 4.2.7,  Next: Section 4.2.8,  Prev: Section 4.2.6,  Up: Section 4.2

4.2.7. Concatenating Macro Parameters
*************************************

   NASM can concatenate macro parameters on to other text surrounding
them.  This allows you to declare a family of symbols, for example, in
a macro definition. If, for example, you wanted to generate a table of
key codes along with offsets into the table, you could code something
like

     %macro keytab_entry 2
     keypos%1 equ $-keytab
               db %2
     %endmacro
     keytab:
               keytab_entry F1,128+1
               keytab_entry F2,128+2
               keytab_entry Return,13

   which would expand to

     keytab:
     keyposF1 equ $-keytab
               db 128+1
     keyposF2 equ $-keytab
               db 128+2
     keyposReturn equ $-keytab
               db 13

   You can just as easily concatenate text on to the other end of a
macro parameter, by writing `%1foo'.

   If you need to append a _digit_ to a macro parameter, for example
defining labels `foo1' and `foo2' when passed the parameter `foo', you
can't code `%11' because that would be taken as the eleventh macro
parameter. Instead, you must code `%{1}1', which will separate the
first `1' (giving the number of the macro parameter) from the second
(literal text to be concatenated to the parameter).

   This concatenation can also be applied to other preprocessor in-line
objects, such as macro-local labels (*Note Section 4.2.2::) and
context-local labels (*Note Section 4.6.2::). In all cases, ambiguities
in syntax can be resolved by enclosing everything after the `%' sign
and before the literal text in braces: so `%{%foo}bar' concatenates the
text `bar' to the end of the real name of the macro-local label
`%%foo'. (This is unnecessary, since the form NASM uses for the real
names of macro-local labels means that the two usages `%{%foo}bar' and
`%%foobar' would both expand to the same thing anyway; nevertheless,
the capability is there.)


File: nasm.info,  Node: Section 4.2.8,  Next: Section 4.2.9,  Prev: Section 4.2.7,  Up: Section 4.2

4.2.8. Condition Codes as Macro Parameters
******************************************

   NASM can give special treatment to a macro parameter which contains a
condition code. For a start, you can refer to the macro parameter `%1'
by means of the alternative syntax `%+1', which informs NASM that this
macro parameter is supposed to contain a condition code, and will cause
the preprocessor to report an error message if the macro is called with
a parameter which is _not_ a valid condition code.

   Far more usefully, though, you can refer to the macro parameter by
means of `%-1', which NASM will expand as the _inverse_ condition code.
So the `retz' macro defined in *Note Section 4.2.2:: can be replaced by
a general conditional-return macro like this:

     %macro retc 1
               j%-1 %%skip
               ret
     %%skip:
     %endmacro

   This macro can now be invoked using calls like `retc ne', which will
cause the conditional-jump instruction in the macro expansion to come
out as `JE', or `retc po' which will make the jump a `JPE'.

   The `%+1' macro-parameter reference is quite happy to interpret the
arguments `CXZ' and `ECXZ' as valid condition codes; however, `%-1'
will report an error if passed either of these, because no inverse
condition code exists.


File: nasm.info,  Node: Section 4.2.9,  Next: Section 4.3,  Prev: Section 4.2.8,  Up: Section 4.2

4.2.9. Disabling Listing Expansion
**********************************

   When NASM is generating a listing file from your program, it will
generally expand multi-line macros by means of writing the macro call
and then listing each line of the expansion. This allows you to see
which instructions in the macro expansion are generating what code;
however, for some macros this clutters the listing up unnecessarily.

   NASM therefore provides the `.nolist' qualifier, which you can
include in a macro definition to inhibit the expansion of the macro in
the listing file. The `.nolist' qualifier comes directly after the
number of parameters, like this:

     %macro foo 1.nolist

   Or like this:

     %macro bar 1-5+.nolist a,b,c,d,e,f,g,h


File: nasm.info,  Node: Section 4.3,  Next: Section 4.3.1,  Prev: Section 4.2.9,  Up: Chapter 4

4.3. Conditional Assembly
*************************

   Similarly to the C preprocessor, NASM allows sections of a source
file to be assembled only if certain conditions are met. The general
syntax of this feature looks like this:

     %if<condition>
     ; some code which only appears if <condition> is met
     %elif<condition2>
     ; only appears if <condition> is not met but <condition2> is
     %else
     ; this appears if neither <condition> nor <condition2> was met
     %endif

   The `%else' clause is optional, as is the `%elif' clause. You can
have more than one `%elif' clause as well.

* Menu:

* Section 4.3.1:: `%ifdef': Testing Single-Line Macro Existence
* Section 4.3.2:: `%ifctx': Testing the Context Stack
* Section 4.3.3:: `%if': Testing Arbitrary Numeric Expressions
* Section 4.3.4:: `%ifidn' and `%ifidni': Testing Exact Text Identity
* Section 4.3.5:: `%ifid', `%ifnum', `%ifstr': Testing Token Types
* Section 4.3.6:: `%error': Reporting User-Defined Errors


File: nasm.info,  Node: Section 4.3.1,  Next: Section 4.3.2,  Prev: Section 4.3,  Up: Section 4.3

4.3.1. `%ifdef': Testing Single-Line Macro Existence
****************************************************

   Beginning a conditional-assembly block with the line `%ifdef MACRO'
will assemble the subsequent code if, and only if, a single-line macro
called `MACRO' is defined. If not, then the `%elif' and `%else' blocks
(if any) will be processed instead.

   For example, when debugging a program, you might want to write code
such as

               ; perform some function
     %ifdef DEBUG
               writefile 2,"Function performed successfully",13,10
     %endif
               ; go and do something else

   Then you could use the command-line option `-dDEBUG' to create a
version of the program which produced debugging messages, and remove the
option to generate the final release version of the program.

   You can test for a macro _not_ being defined by using `%ifndef'
instead of `%ifdef'. You can also test for macro definitions in `%elif'
blocks by using `%elifdef' and `%elifndef'.


File: nasm.info,  Node: Section 4.3.2,  Next: Section 4.3.3,  Prev: Section 4.3.1,  Up: Section 4.3

4.3.2. `%ifctx': Testing the Context Stack
******************************************

   The conditional-assembly construct `%ifctx ctxname' will cause the
subsequent code to be assembled if and only if the top context on the
preprocessor's context stack has the name `ctxname'. As with `%ifdef',
the inverse and `%elif' forms `%ifnctx', `%elifctx' and `%elifnctx' are
also supported.

   For more details of the context stack, see *Note Section 4.6::. For
a sample use of `%ifctx', see *Note Section 4.6.5::.


File: nasm.info,  Node: Section 4.3.3,  Next: Section 4.3.4,  Prev: Section 4.3.2,  Up: Section 4.3

4.3.3. `%if': Testing Arbitrary Numeric Expressions
***************************************************

   The conditional-assembly construct `%if expr' will cause the
subsequent code to be assembled if and only if the value of the numeric
expression `expr' is non-zero. An example of the use of this feature is
in deciding when to break out of a `%rep' preprocessor loop: see *Note
Section 4.4:: for a detailed example.

   The expression given to `%if', and its counterpart `%elif', is a
critical expression (see *Note Section 3.7::).

   `%if' extends the normal NASM expression syntax, by providing a set
of relational operators which are not normally available in
expressions. The operators `=', `<', `>', `<=', `>=' and `<>' test
equality, less-than, greater-than, less-or-equal, greater-or-equal and
not-equal respectively. The C-like forms `==' and `!=' are supported as
alternative forms of `=' and `<>'. In addition, low- priority logical
operators `&&', `^^' and `||' are provided, supplying logical AND,
logical XOR and logical OR. These work like the C logical operators
(although C has no logical XOR), in that they always return either 0 or
1, and treat any non-zero input as 1 (so that `^^', for example,
returns 1 if exactly one of its inputs is zero, and 0 otherwise). The
relational operators also return 1 for true and 0 for false.


File: nasm.info,  Node: Section 4.3.4,  Next: Section 4.3.5,  Prev: Section 4.3.3,  Up: Section 4.3

4.3.4. `%ifidn' and `%ifidni': Testing Exact Text Identity
**********************************************************

   The construct `%ifidn text1,text2' will cause the subsequent code to
be assembled if and only if `text1' and `text2', after expanding
single-line macros, are identical pieces of text. Differences in white
space are not counted.

   `%ifidni' is similar to `%ifidn', but is case-insensitive.

   For example, the following macro pushes a register or number on the
stack, and allows you to treat `IP' as a real register:

     %macro pushparam 1
     %ifidni %1,ip
               call %%label
     %%label:
     %else
               push %1
     %endif
     %endmacro

   Like most other `%if' constructs, `%ifidn' has a counterpart
`%elifidn', and negative forms `%ifnidn' and `%elifnidn'.  Similarly,
`%ifidni' has counterparts `%elifidni', `%ifnidni' and `%elifnidni'.


File: nasm.info,  Node: Section 4.3.5,  Next: Section 4.3.6,  Prev: Section 4.3.4,  Up: Section 4.3

4.3.5. `%ifid', `%ifnum', `%ifstr': Testing Token Types
*******************************************************

   Some macros will want to perform different tasks depending on
whether they are passed a number, a string, or an identifier. For
example, a string output macro might want to be able to cope with being
passed either a string constant or a pointer to an existing string.

   The conditional assembly construct `%ifid', taking one parameter
(which may be blank), assembles the subsequent code if and only if the
first token in the parameter exists and is an identifier. `%ifnum'
works similarly, but tests for the token being a numeric constant;
`%ifstr' tests for it being a string.

   For example, the `writefile' macro defined in *Note Section 4.2.3::
can be extended to take advantage of `%ifstr' in the following fashion:

     %macro writefile 2-3+
     %ifstr %2
               jmp %%endstr
     %if %0 = 3
     %%str:	  db %2,%3
     %else
     %%str:	  db %2
     %endif
     %%endstr: mov dx,%%str
               mov cx,%%endstr-%%str
     %else
     	  mov dx,%2
     	  mov cx,%3
     %endif
               mov bx,%1
               mov ah,0x40
               int 0x21
     %endmacro

   Then the `writefile' macro can cope with being called in either of
the following two ways:

               writefile [file], strpointer, length
               writefile [file], "hello", 13, 10

   In the first, `strpointer' is used as the address of an already-
declared string, and `length' is used as its length; in the second, a
string is given to the macro, which therefore declares it itself and
works out the address and length for itself.

   Note the use of `%if' inside the `%ifstr': this is to detect whether
the macro was passed two arguments (so the string would be a single
string constant, and `db %2' would be adequate) or more (in which case,
all but the first two would be lumped together into `%3', and `db
%2,%3' would be required).

   The usual `%elifXXX', `%ifnXXX' and `%elifnXXX' versions exist for
each of `%ifid', `%ifnum' and `%ifstr'.


File: nasm.info,  Node: Section 4.3.6,  Next: Section 4.4,  Prev: Section 4.3.5,  Up: Section 4.3

4.3.6. `%error': Reporting User-Defined Errors
**********************************************

   The preprocessor directive `%error' will cause NASM to report an
error if it occurs in assembled code. So if other users are going to
try to assemble your source files, you can ensure that they define the
right macros by means of code like this:

     %ifdef SOME_MACRO
     ; do some setup
     %elifdef SOME_OTHER_MACRO
     ; do some different setup
     %else
     %error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined.
     %endif

   Then any user who fails to understand the way your code is supposed
to be assembled will be quickly warned of their mistake, rather than
having to wait until the program crashes on being run and then not
knowing what went wrong.


File: nasm.info,  Node: Section 4.4,  Next: Section 4.5,  Prev: Section 4.3.6,  Up: Chapter 4

4.4. Preprocessor Loops: `%rep'
*******************************

   NASM's `TIMES' prefix, though useful, cannot be used to invoke a
multi-line macro multiple times, because it is processed by NASM after
macros have already been expanded. Therefore NASM provides another form
of loop, this time at the preprocessor level: `%rep'.

   The directives `%rep' and `%endrep' (`%rep' takes a numeric
argument, which can be an expression; `%endrep' takes no arguments) can
be used to enclose a chunk of code, which is then replicated as many
times as specified by the preprocessor:

     %assign i 0
     %rep 64
               inc word [table+2*i]
     %assign i i+1
     %endrep

   This will generate a sequence of 64 `INC' instructions, incrementing
every word of memory from `[table]' to `[table+126]'.

   For more complex termination conditions, or to break out of a repeat
loop part way along, you can use the `%exitrep' directive to terminate
the loop, like this:

     fibonacci:
     %assign i 0
     %assign j 1
     %rep 100
     %if j > 65535
     %exitrep
     %endif
               dw j
     %assign k j+i
     %assign i j
     %assign j k
     %endrep
     fib_number equ ($-fibonacci)/2

   This produces a list of all the Fibonacci numbers that will fit in
16 bits.  Note that a maximum repeat count must still be given to
`%rep'. This is to prevent the possibility of NASM getting into an
infinite loop in the preprocessor, which (on multitasking or multi-user
systems) would typically cause all the system memory to be gradually
used up and other applications to start crashing.


File: nasm.info,  Node: Section 4.5,  Next: Section 4.6,  Prev: Section 4.4,  Up: Chapter 4

4.5. Including Other Files
**************************

   Using, once again, a very similar syntax to the C preprocessor,
NASM's preprocessor lets you include other source files into your code.
This is done by the use of the `%include' directive:

     %include "macros.mac"

   will include the contents of the file `macros.mac' into the source
file containing the `%include' directive.

   Include files are searched for in the current directory (the
directory you're in when you run NASM, as opposed to the location of
the NASM executable or the location of the source file), plus any
directories specified on the NASM command line using the `-i' option.

   The standard C idiom for preventing a file being included more than
once is just as applicable in NASM: if the file `macros.mac' has the
form

     %ifndef MACROS_MAC
     %define MACROS_MAC
     ; now define some macros
     %endif

   then including the file more than once will not cause errors,
because the second time the file is included nothing will happen
because the macro `MACROS_MAC' will already be defined.

   You can force a file to be included even if there is no `%include'
directive that explicitly includes it, by using the `-p' option on the
NASM command line (see *Note Section 2.1.7::).


File: nasm.info,  Node: Section 4.6,  Next: Section 4.6.1,  Prev: Section 4.5,  Up: Chapter 4

4.6. The Context Stack
**********************

   Having labels that are local to a macro definition is sometimes not
quite powerful enough: sometimes you want to be able to share labels
between several macro calls. An example might be a `REPEAT' ... `UNTIL'
loop, in which the expansion of the `REPEAT' macro would need to be
able to refer to a label which the `UNTIL' macro had defined. However,
for such a macro you would also want to be able to nest these loops.

   NASM provides this level of power by means of a _context stack_. The
preprocessor maintains a stack of _contexts_, each of which is
characterised by a name. You add a new context to the stack using the
`%push' directive, and remove one using `%pop'. You can define labels
that are local to a particular context on the stack.

* Menu:

* Section 4.6.1:: `%push' and `%pop': Creating and Removing Contexts
* Section 4.6.2:: Context-Local Labels
* Section 4.6.3:: Context-Local Single-Line Macros
* Section 4.6.4:: `%repl': Renaming a Context
* Section 4.6.5:: Example Use of the Context Stack: Block IFs


File: nasm.info,  Node: Section 4.6.1,  Next: Section 4.6.2,  Prev: Section 4.6,  Up: Section 4.6

4.6.1. `%push' and `%pop': Creating and Removing Contexts
*********************************************************

   The `%push' directive is used to create a new context and place it on
the top of the context stack. `%push' requires one argument, which is
the name of the context. For example:

     %push foobar

   This pushes a new context called `foobar' on the stack. You can have
several contexts on the stack with the same name: they can still be
distinguished.

   The directive `%pop', requiring no arguments, removes the top context
from the context stack and destroys it, along with any labels associated
with it.


File: nasm.info,  Node: Section 4.6.2,  Next: Section 4.6.3,  Prev: Section 4.6.1,  Up: Section 4.6

4.6.2. Context-Local Labels
***************************

   Just as the usage `%%foo' defines a label which is local to the
particular macro call in which it is used, the usage `%$foo' is used to
define a label which is local to the context on the top of the context
stack. So the `REPEAT' and `UNTIL' example given above could be
implemented by means of:

     %macro repeat 0
     %push repeat
     %$begin:
     %endmacro

     %macro until 1
               j%-1 %$begin
     %pop
     %endmacro

   and invoked by means of, for example,

               mov cx,string
               repeat
               add cx,3
               scasb
               until e

   which would scan every fourth byte of a string in search of the byte
in `AL'.

   If you need to define, or access, labels local to the context _below_
the top one on the stack, you can use `%$$foo', or `%$$$foo' for the
context below that, and so on.


File: nasm.info,  Node: Section 4.6.3,  Next: Section 4.6.4,  Prev: Section 4.6.2,  Up: Section 4.6

4.6.3. Context-Local Single-Line Macros
***************************************

   NASM also allows you to define single-line macros which are local to
a particular context, in just the same way:

     %define %$localmac 3

   will define the single-line macro `%$localmac' to be local to the top
context on the stack. Of course, after a subsequent `%push', it can
then still be accessed by the name `%$$localmac'.


File: nasm.info,  Node: Section 4.6.4,  Next: Section 4.6.5,  Prev: Section 4.6.3,  Up: Section 4.6

4.6.4. `%repl': Renaming a Context
**********************************

   If you need to change the name of the top context on the stack (in
order, for example, to have it respond differently to `%ifctx'), you can
execute a `%pop' followed by a `%push'; but this will have the side
effect of destroying all context-local labels and macros associated
with the context that was just popped.

   NASM provides the directive `%repl', which _replaces_ a context with
a different name, without touching the associated macros and labels.
So you could replace the destructive code

     %pop
     %push newname

   with the non-destructive version `%repl newname'.


File: nasm.info,  Node: Section 4.6.5,  Next: Section 4.7,  Prev: Section 4.6.4,  Up: Section 4.6

4.6.5. Example Use of the Context Stack: Block IFs
**************************************************

   This example makes use of almost all the context-stack features,
including the conditional-assembly construct `%ifctx', to implement a
block IF statement as a set of macros.

     %macro if 1
         %push if
         j%-1 %$ifnot
     %endmacro

     %macro else 0
         %ifctx if
             %repl else
             jmp %$ifend
             %$ifnot:
         %else
             %error "expected `if' before `else'"
         %endif
     %endmacro

     %macro endif 0
         %ifctx if
             %$ifnot:
             %pop
         %elifctx else
             %$ifend:
             %pop
         %else
             %error "expected `if' or `else' before `endif'"
         %endif
     %endmacro

   This code is more robust than the `REPEAT' and `UNTIL' macros given
in *Note Section 4.6.2::, because it uses conditional assembly to check
that the macros are issued in the right order (for example, not calling
`endif' before `if') and issues a `%error' if they're not.

   In addition, the `endif' macro has to be able to cope with the two
distinct cases of either directly following an `if', or following an
`else'. It achieves this, again, by using conditional assembly to do
different things depending on whether the context on top of the stack is
`if' or `else'.

   The `else' macro has to preserve the context on the stack, in order
to have the `%$ifnot' referred to by the `if' macro be the same as the
one defined by the `endif' macro, but has to change the context's name
so that `endif' will know there was an intervening `else'.  It does
this by the use of `%repl'.

   A sample usage of these macros might look like:

               cmp ax,bx
               if ae
                 cmp bx,cx
                 if ae
                   mov ax,cx
                 else
                   mov ax,bx
                 endif
               else
                 cmp ax,cx
                 if ae
                   mov ax,cx
                 endif
               endif

   The block-`IF' macros handle nesting quite happily, by means of
pushing another context, describing the inner `if', on top of the one
describing the outer `if'; thus `else' and `endif' always refer to the
last unmatched `if' or `else'.


File: nasm.info,  Node: Section 4.7,  Next: Section 4.7.1,  Prev: Section 4.6.5,  Up: Chapter 4

4.7. Standard Macros
********************

   NASM defines a set of standard macros, which are already defined
when it starts to process any source file. If you really need a program
to be assembled with no pre-defined macros, you can use the `%clear'
directive to empty the preprocessor of everything.

   Most user-level assembler directives (see *Note Chapter 5::) are
implemented as macros which invoke primitive directives; these are
described in *Note Chapter 5::. The rest of the standard macro set is
described here.

* Menu:

* Section 4.7.1:: `__NASM_MAJOR__' and `__NASM_MINOR__': NASM Version
* Section 4.7.2:: `__FILE__' and `__LINE__': File Name and Line Number
* Section 4.7.3:: `STRUC' and `ENDSTRUC': Declaring Structure Data Types
* Section 4.7.4:: `ISTRUC', `AT' and `IEND': Declaring Instances of Structures
* Section 4.7.5:: `ALIGN' and `ALIGNB': Data Alignment


File: nasm.info,  Node: Section 4.7.1,  Next: Section 4.7.2,  Prev: Section 4.7,  Up: Section 4.7

4.7.1. `__NASM_MAJOR__' and `__NASM_MINOR__': NASM Version
**********************************************************

   The single-line macros `__NASM_MAJOR__' and `__NASM_MINOR__' expand
to the major and minor parts of the version number of NASM being used.
So, under NASM 0.96 for example, `__NASM_MAJOR__' would be defined to
be 0 and `__NASM_MINOR__' would be defined as 96.


File: nasm.info,  Node: Section 4.7.2,  Next: Section 4.7.3,  Prev: Section 4.7.1,  Up: Section 4.7

4.7.2. `__FILE__' and `__LINE__': File Name and Line Number
***********************************************************

   Like the C preprocessor, NASM allows the user to find out the file
name and line number containing the current instruction. The macro
`__FILE__' expands to a string constant giving the name of the current
input file (which may change through the course of assembly if
`%include' directives are used), and `__LINE__' expands to a numeric
constant giving the current line number in the input file.

   These macros could be used, for example, to communicate debugging
information to a macro, since invoking `__LINE__' inside a macro
definition (either single-line or multi-line) will return the line
number of the macro _call_, rather than _definition_. So to determine
where in a piece of code a crash is occurring, for example, one could
write a routine `stillhere', which is passed a line number in `EAX' and
outputs something like `line 155: still here'. You could then write a
macro

     %macro notdeadyet 0
               push eax
               mov eax,__LINE__
               call stillhere
               pop eax
     %endmacro

   and then pepper your code with calls to `notdeadyet' until you find
the crash point.


File: nasm.info,  Node: Section 4.7.3,  Next: Section 4.7.4,  Prev: Section 4.7.2,  Up: Section 4.7

4.7.3. `STRUC' and `ENDSTRUC': Declaring Structure Data Types
*************************************************************

   The core of NASM contains no intrinsic means of defining data
structures; instead, the preprocessor is sufficiently powerful that
data structures can be implemented as a set of macros. The macros
`STRUC' and `ENDSTRUC' are used to define a structure data type.

   `STRUC' takes one parameter, which is the name of the data type. This
name is defined as a symbol with the value zero, and also has the suffix
`_size' appended to it and is then defined as an `EQU' giving the size
of the structure. Once `STRUC' has been issued, you are defining the
structure, and should define fields using the `RESB' family of
pseudo-instructions, and then invoke `ENDSTRUC' to finish the
definition.

   For example, to define a structure called `mytype' containing a
longword, a word, a byte and a string of bytes, you might code

               struc mytype
     mt_long:  resd 1
     mt_word:  resw 1
     mt_byte:  resb 1
     mt_str:   resb 32
               endstruc

   The above code defines six symbols: `mt_long' as 0 (the offset from
the beginning of a `mytype' structure to the longword field), `mt_word'
as 4, `mt_byte' as 6, `mt_str' as 7, `mytype_size' as 39, and `mytype'
itself as zero.

   The reason why the structure type name is defined at zero is a side
effect of allowing structures to work with the local label mechanism:
if your structure members tend to have the same names in more than one
structure, you can define the above structure like this:

               struc mytype
     .long:    resd 1
     .word:    resw 1
     .byte:    resb 1
     .str:     resb 32
               endstruc

   This defines the offsets to the structure fields as `mytype.long',
`mytype.word', `mytype.byte' and `mytype.str'.

   NASM, since it has no _intrinsic_ structure support, does not support
any form of period notation to refer to the elements of a structure once
you have one (except the above local-label notation), so code such as
`mov ax,[mystruc.mt_word]' is not valid. `mt_word' is a constant just
like any other constant, so the correct syntax is `mov
ax,[mystruc+mt_word]' or `mov ax,[mystruc+mytype.word]'.


File: nasm.info,  Node: Section 4.7.4,  Next: Section 4.7.5,  Prev: Section 4.7.3,  Up: Section 4.7

4.7.4. `ISTRUC', `AT' and `IEND': Declaring Instances of Structures
*******************************************************************

   Having defined a structure type, the next thing you typically want
to do is to declare instances of that structure in your data segment.
NASM provides an easy way to do this in the `ISTRUC' mechanism. To
declare a structure of type `mytype' in a program, you code something
like this:

     mystruc:  istruc mytype
               at mt_long, dd 123456
               at mt_word, dw 1024
               at mt_byte, db 'x'
               at mt_str, db 'hello, world', 13, 10, 0
               iend

   The function of the `AT' macro is to make use of the `TIMES' prefix
to advance the assembly position to the correct point for the specified
structure field, and then to declare the specified data.  Therefore the
structure fields must be declared in the same order as they were
specified in the structure definition.

   If the data to go in a structure field requires more than one source
line to specify, the remaining source lines can easily come after the
`AT' line. For example:

               at mt_str, db 123,134,145,156,167,178,189
               db 190,100,0

   Depending on personal taste, you can also omit the code part of the
`AT' line completely, and start the structure field on the next line:

               at mt_str
               db 'hello, world'
               db 13,10,0


File: nasm.info,  Node: Section 4.7.5,  Next: Chapter 5,  Prev: Section 4.7.4,  Up: Section 4.7

4.7.5. `ALIGN' and `ALIGNB': Data Alignment
*******************************************

   The `ALIGN' and `ALIGNB' macros provides a convenient way to align
code or data on a word, longword, paragraph or other boundary. (Some
assemblers call this directive `EVEN'.) The syntax of the `ALIGN' and
`ALIGNB' macros is

               align 4                ; align on 4-byte boundary
               align 16               ; align on 16-byte boundary
               align 8,db 0           ; pad with 0s rather than NOPs
               align 4,resb 1         ; align to 4 in the BSS
               alignb 4               ; equivalent to previous line

   Both macros require their first argument to be a power of two; they
both compute the number of additional bytes required to bring the
length of the current section up to a multiple of that power of two,
and then apply the `TIMES' prefix to their second argument to perform
the alignment.

   If the second argument is not specified, the default for `ALIGN' is
`NOP', and the default for `ALIGNB' is `RESB 1'. So if the second
argument is specified, the two macros are equivalent. Normally, you can
just use `ALIGN' in code and data sections and `ALIGNB' in BSS
sections, and never need the second argument except for special
purposes.

   `ALIGN' and `ALIGNB', being simple macros, perform no error
checking: they cannot warn you if their first argument fails to be a
power of two, or if their second argument generates more than one byte
of code.  In each of these cases they will silently do the wrong thing.

   `ALIGNB' (or `ALIGN' with a second argument of `RESB 1') can be used
within structure definitions:

               struc mytype2
     mt_byte:  resb 1
               alignb 2
     mt_word:  resw 1
               alignb 4
     mt_long:  resd 1
     mt_str:   resb 32
               endstruc

   This will ensure that the structure members are sensibly aligned
relative to the base of the structure.

   A final caveat: `ALIGN' and `ALIGNB' work relative to the beginning
of the _section_, not the beginning of the address space in the final
executable. Aligning to a 16-byte boundary when the section you're in
is only guaranteed to be aligned to a 4-byte boundary, for example, is
a waste of effort. Again, NASM does not check that the section's
alignment characteristics are sensible for the use of `ALIGN' or
`ALIGNB'.


File: nasm.info,  Node: Chapter 5,  Next: Section 5.1,  Prev: Section 4.7.5,  Up: Top

Chapter 5: Assembler Directives
*******************************

   NASM, though it attempts to avoid the bureaucracy of assemblers like
MASM and TASM, is nevertheless forced to support a _few_ directives.
These are described in this chapter.

   NASM's directives come in two types: user-level
directives_user-level_ directives and primitive directives_primitive_
directives. Typically, each directive has a user-level form and a
primitive form. In almost all cases, we recommend that users use the
user-level forms of the directives, which are implemented as macros
which call the primitive forms.

   Primitive directives are enclosed in square brackets; user-level
directives are not.

   In addition to the universal directives described in this chapter,
each object file format can optionally supply extra directives in order
to control particular features of that file format. These
format-specific directives_format-specific_ directives are documented
along with the formats that implement them, in *Note Chapter 6::.

* Menu:

* Section 5.1:: `BITS': Specifying Target Processor Mode
* Section 5.2:: `SECTION' or `SEGMENT': Changing and Defining Sections
* Section 5.3:: `ABSOLUTE': Defining Absolute Labels
* Section 5.4:: `EXTERN': Importing Symbols from Other Modules
* Section 5.5:: `GLOBAL': Exporting Symbols to Other Modules
* Section 5.6:: `COMMON': Defining Common Data Areas


File: nasm.info,  Node: Section 5.1,  Next: Section 5.2,  Prev: Chapter 5,  Up: Chapter 5

5.1. `BITS': Specifying Target Processor Mode
*********************************************

   The `BITS' directive specifies whether NASM should generate code
designed to run on a processor operating in 16-bit mode, or code
designed to run on a processor operating in 32-bit mode. The syntax is
`BITS 16' or `BITS 32'.

   In most cases, you should not need to use `BITS' explicitly. The
`aout', `coff', `elf' and `win32' object formats, which are designed
for use in 32-bit operating systems, all cause NASM to select 32-bit
mode by default. The `obj' object format allows you to specify each
segment you define as either `USE16' or `USE32', and NASM will set its
operating mode accordingly, so the use of the `BITS' directive is once
again unnecessary.

   The most likely reason for using the `BITS' directive is to write 32-
bit code in a flat binary file; this is because the `bin' output format
defaults to 16-bit mode in anticipation of it being used most
frequently to write DOS `.COM' programs, DOS `.SYS' device drivers and
boot loader software.

   You do _not_ need to specify `BITS 32' merely in order to use 32-
bit instructions in a 16-bit DOS program; if you do, the assembler will
generate incorrect code because it will be writing code targeted at a
32- bit platform, to be run on a 16-bit one.

   When NASM is in `BITS 16' state, instructions which use 32-bit data
are prefixed with an 0x66 byte, and those referring to 32-bit addresses
have an 0x67 prefix. In `BITS 32' state, the reverse is true: 32-bit
instructions require no prefixes, whereas instructions using 16-bit data
need an 0x66 and those working in 16-bit addresses need an 0x67.

   The `BITS' directive has an exactly equivalent primitive form,
`[BITS 16]' and `[BITS 32]'. The user-level form is a macro which has
no function other than to call the primitive form.


File: nasm.info,  Node: Section 5.2,  Next: Section 5.2.1,  Prev: Section 5.1,  Up: Chapter 5

5.2. `SECTION' or `SEGMENT': Changing and Defining Sections
***********************************************************

   The `SECTION' directive (`SEGMENT' is an exactly equivalent synonym)
changes which section of the output file the code you write will be
assembled into. In some object file formats, the number and names of
sections are fixed; in others, the user may make up as many as they
wish.  Hence `SECTION' may sometimes give an error message, or may
define a new section, if you try to switch to a section that does not
(yet) exist.

   The Unix object formats, and the `bin' object format, all support the
standardised section names `.text', `.data' and `.bss' for the code,
data and uninitialised-data sections. The `obj' format, by contrast,
does not recognise these section names as being special, and indeed
will strip off the leading period of any section name that has one.

* Menu:

* Section 5.2.1:: The `__SECT__' Macro


File: nasm.info,  Node: Section 5.2.1,  Next: Section 5.3,  Prev: Section 5.2,  Up: Section 5.2

5.2.1. The `__SECT__' Macro
***************************

   The `SECTION' directive is unusual in that its user-level form
functions differently from its primitive form. The primitive form,
`[SECTION xyz]', simply switches the current target section to the one
given. The user-level form, `SECTION xyz', however, first defines the
single-line macro `__SECT__' to be the primitive `[SECTION]' directive
which it is about to issue, and then issues it. So the user-level
directive

               SECTION .text

   expands to the two lines

     %define __SECT__ [SECTION .text]
               [SECTION .text]

   Users may find it useful to make use of this in their own macros. For
example, the `writefile' macro defined in *Note Section 4.2.3:: can be
usefully rewritten in the following more sophisticated form:

     %macro writefile 2+
               [section .data]
     %%str:    db %2
     %%endstr:
               __SECT__
               mov dx,%%str
               mov cx,%%endstr-%%str
               mov bx,%1
               mov ah,0x40
               int 0x21
     %endmacro

   This form of the macro, once passed a string to output, first
switches temporarily to the data section of the file, using the
primitive form of the `SECTION' directive so as not to modify
`__SECT__'. It then declares its string in the data section, and then
invokes `__SECT__' to switch back to _whichever_ section the user was
previously working in. It thus avoids the need, in the previous version
of the macro, to include a `JMP' instruction to jump over the data, and
also does not fail if, in a complicated `OBJ' format module, the user
could potentially be assembling the code in any of several separate code
sections.


File: nasm.info,  Node: Section 5.3,  Next: Section 5.4,  Prev: Section 5.2.1,  Up: Chapter 5

5.3. `ABSOLUTE': Defining Absolute Labels
*****************************************

   The `ABSOLUTE' directive can be thought of as an alternative form of
`SECTION': it causes the subsequent code to be directed at no physical
section, but at the hypothetical section starting at the given absolute
address. The only instructions you can use in this mode are the `RESB'
family.

   `ABSOLUTE' is used as follows:

               absolute 0x1A
     kbuf_chr  resw 1
     kbuf_free resw 1
     kbuf      resw 16

   This example describes a section of the PC BIOS data area, at segment
address 0x40: the above code defines `kbuf_chr' to be 0x1A, `kbuf_free'
to be 0x1C, and `kbuf' to be 0x1E.

   The user-level form of `ABSOLUTE', like that of `SECTION', redefines
the `__SECT__' macro when it is invoked.

   `STRUC' and `ENDSTRUC' are defined as macros which use `ABSOLUTE'
(and also `__SECT__').

   `ABSOLUTE' doesn't have to take an absolute constant as an argument:
it can take an expression (actually, a critical expression: see *Note
Section 3.7::) and it can be a value in a segment. For example, a TSR
can re-use its setup code as run-time BSS like this:

               org 100h               ; it's a .COM program
               jmp setup              ; setup code comes last
               ; the resident part of the TSR goes here
     setup:    ; now write the code that installs the TSR here
               absolute setup
     runtimevar1 resw 1
     runtimevar2 resd 20
     tsr_end:

   This defines some variables `on top of' the setup code, so that
after the setup has finished running, the space it took up can be
re-used as data storage for the running TSR. The symbol `tsr_end' can
be used to calculate the total size of the part of the TSR that needs
to be made resident.


File: nasm.info,  Node: Section 5.4,  Next: Section 5.5,  Prev: Section 5.3,  Up: Chapter 5

5.4. `EXTERN': Importing Symbols from Other Modules
***************************************************

   `EXTERN' is similar to the MASM directive `EXTRN' and the C keyword
`extern': it is used to declare a symbol which is not defined anywhere
in the module being assembled, but is assumed to be defined in some
other module and needs to be referred to by this one. Not every
object-file format can support external variables: the `bin' format
cannot.

   The `EXTERN' directive takes as many arguments as you like. Each
argument is the name of a symbol:

               extern _printf
               extern _sscanf,_fscanf

   Some object-file formats provide extra features to the `EXTERN'
directive. In all cases, the extra features are used by suffixing a
colon to the symbol name followed by object-format specific text. For
example, the `obj' format allows you to declare that the default
segment base of an external should be the group `dgroup' by means of
the directive

               extern _variable:wrt dgroup

   The primitive form of `EXTERN' differs from the user-level form only
in that it can take only one argument at a time: the support for
multiple arguments is implemented at the preprocessor level.

   You can declare the same variable as `EXTERN' more than once: NASM
will quietly ignore the second and later redeclarations. You can't
declare a variable as `EXTERN' as well as something else, though.


File: nasm.info,  Node: Section 5.5,  Next: Section 5.6,  Prev: Section 5.4,  Up: Chapter 5

5.5. `GLOBAL': Exporting Symbols to Other Modules
*************************************************

   `GLOBAL' is the other end of `EXTERN': if one module declares a
symbol as `EXTERN' and refers to it, then in order to prevent linker
errors, some other module must actually _define_ the symbol and declare
it as `GLOBAL'. Some assemblers use the name `PUBLIC' for this purpose.

   The `GLOBAL' directive applying to a symbol must appear _before_ the
definition of the symbol.

   `GLOBAL' uses the same syntax as `EXTERN', except that it must refer
to symbols which _are_ defined in the same module as the `GLOBAL'
directive. For example:

               global _main
     _main:    ; some code

   `GLOBAL', like `EXTERN', allows object formats to define private
extensions by means of a colon. The `elf' object format, for example,
lets you specify whether global data items are functions or data:

               global hashlookup:function, hashtable:data

   Like `EXTERN', the primitive form of `GLOBAL' differs from the
user-level form only in that it can take only one argument at a time.


File: nasm.info,  Node: Section 5.6,  Next: Chapter 6,  Prev: Section 5.5,  Up: Chapter 5

5.6. `COMMON': Defining Common Data Areas
*****************************************

   The `COMMON' directive is used to declare _common variables_. A
common variable is much like a global variable declared in the
uninitialised data section, so that

               common intvar 4

   is similar in function to

               global intvar
               section .bss
     intvar    resd 1

   The difference is that if more than one module defines the same
common variable, then at link time those variables will be _merged_, and
references to `intvar' in all modules will point at the same piece of
memory.

   Like `GLOBAL' and `EXTERN', `COMMON' supports object-format specific
extensions. For example, the `obj' format allows common variables to be
NEAR or FAR, and the `elf' format allows you to specify the alignment
requirements of a common variable:

               common commvar 4:near  ; works in OBJ
               common intarray 100:4  ; works in ELF: 4 byte aligned

   Once again, like `EXTERN' and `GLOBAL', the primitive form of
`COMMON' differs from the user-level form only in that it can take only
one argument at a time.


File: nasm.info,  Node: Chapter 6,  Next: Section 6.1,  Prev: Section 5.6,  Up: Top

Chapter 6: Output Formats
*************************

   NASM is a portable assembler, designed to be able to compile on any
ANSI C- supporting platform and produce output to run on a variety of
Intel x86 operating systems. For this reason, it has a large number of
available output formats, selected using the `-f' option on the NASM
command line. Each of these formats, along with its extensions to the
base NASM syntax, is detailed in this chapter.

   As stated in *Note Section 2.1.1::, NASM chooses a default name for
your output file based on the input file name and the chosen output
format. This will be generated by removing the extension (`.asm', `.s',
or whatever you like to use) from the input file name, and substituting
an extension defined by the output format. The extensions are given
with each format below.

* Menu:

* Section 6.1:: `bin': Flat-Form Binary Output
* Section 6.2:: `obj': Microsoft OMF Object Files
* Section 6.3:: `win32': Microsoft Win32 Object Files
* Section 6.4:: `coff': Common Object File Format
* Section 6.5:: `elf': Linux ELFObject Files
* Section 6.6:: `aout': Linux `a.out' Object Files
* Section 6.7:: `aoutb': NetBSD/FreeBSD/OpenBSD `a.out' Object Files
* Section 6.8:: `as86': Linux `as86' Object Files
* Section 6.9:: `rdf': Relocatable Dynamic Object File Format
* Section 6.10:: `dbg': Debugging Format


File: nasm.info,  Node: Section 6.1,  Next: Section 6.1.1,  Prev: Chapter 6,  Up: Chapter 6

6.1. `bin': Flat-Form Binary Output
***********************************

   The `bin' format does not produce object files: it generates nothing
in the output file except the code you wrote. Such `pure binary' files
are used by MS-DOS: `.COM' executables and `.SYS' device drivers are
pure binary files. Pure binary output is also useful for
operating-system and boot loader development.

   `bin' supports the three standardised section names `.text', `.data'
and `.bss' only. The file NASM outputs will contain the contents of the
`.text' section first, followed by the contents of the `.data' section,
aligned on a four-byte boundary. The `.bss' section is not stored in
the output file at all, but is assumed to appear directly after the end
of the `.data' section, again aligned on a four-byte boundary.

   If you specify no explicit `SECTION' directive, the code you write
will be directed by default into the `.text' section.

   Using the `bin' format puts NASM by default into 16-bit mode (see
*Note Section 5.1::). In order to use `bin' to write 32-bit code such as
an OS kernel, you need to explicitly issue the `BITS 32' directive.

   `bin' has no default output file name extension: instead, it leaves
your file name as it is once the original extension has been removed.
Thus, the default is for NASM to assemble `binprog.asm' into a binary
file called `binprog'.

* Menu:

* Section 6.1.1:: `ORG': Binary File Program Origin
* Section 6.1.2:: `bin' Extensions to the `SECTION' Directive


File: nasm.info,  Node: Section 6.1.1,  Next: Section 6.1.2,  Prev: Section 6.1,  Up: Section 6.1

6.1.1. `ORG': Binary File Program Origin
****************************************

   The `bin' format provides an additional directive to the list given
in *Note Chapter 5::: `ORG'. The function of the `ORG' directive is to
specify the origin address which NASM will assume the program begins at
when it is loaded into memory.

   For example, the following code will generate the longword
`0x00000104':

               org 0x100
               dd label
     label:

   Unlike the `ORG' directive provided by MASM-compatible assemblers,
which allows you to jump around in the object file and overwrite code
you have already generated, NASM's `ORG' does exactly what the directive
says: _origin_. Its sole function is to specify one offset which is
added to all internal address references within the file; it does not
permit any of the trickery that MASM's version does. See *Note Section
10.1.3:: for further comments.


File: nasm.info,  Node: Section 6.1.2,  Next: Section 6.2,  Prev: Section 6.1.1,  Up: Section 6.1

6.1.2. `bin' Extensions to the `SECTION' Directive
**************************************************

   The `bin' output format extends the `SECTION' (or `SEGMENT')
directive to allow you to specify the alignment requirements of
segments.  This is done by appending the `ALIGN' qualifier to the end
of the section-definition line. For example,

               section .data align=16

   switches to the section `.data' and also specifies that it must be
aligned on a 16-byte boundary.

   The parameter to `ALIGN' specifies how many low bits of the section
start address must be forced to zero. The alignment value given may be
any power of two.


File: nasm.info,  Node: Section 6.2,  Next: Section 6.2.1,  Prev: Section 6.1.2,  Up: Chapter 6

6.2. `obj': Microsoft OMF Object Files
**************************************

   The `obj' file format (NASM calls it `obj' rather than `omf' for
historical reasons) is the one produced by MASM and TASM, which is
typically fed to 16-bit DOS linkers to produce `.EXE' files. It is also
the format used by OS/2.

   `obj' provides a default output file-name extension of `.obj'.

   `obj' is not exclusively a 16-bit format, though: NASM has full
support for the 32-bit extensions to the format. In particular, 32-bit
`obj' format files are used by Borland's Win32 compilers, instead of
using Microsoft's newer `win32' object file format.

   The `obj' format does not define any special segment names: you can
call your segments anything you like. Typical names for segments in
`obj' format files are `CODE', `DATA' and `BSS'.

   If your source file contains code before specifying an explicit
`SEGMENT' directive, then NASM will invent its own segment called
`__NASMDEFSEG' for you.

   When you define a segment in an `obj' file, NASM defines the segment
name as a symbol as well, so that you can access the segment address of
the segment. So, for example:

               segment data
     dvar:     dw 1234
               segment code
     function: mov ax,data            ; get segment address of data
               mov ds,ax              ; and move it into DS
               inc word [dvar]        ; now this reference will work
               ret

   The `obj' format also enables the use of the `SEG' and `WRT'
operators, so that you can write code which does things like

               extern foo
               mov ax,seg foo         ; get preferred segment of foo
               mov ds,ax
               mov ax,data            ; a different segment
               mov es,ax
               mov ax,[ds:foo]        ; this accesses `foo'
               mov [es:foo wrt data],bx  ; so does this

* Menu:

* Section 6.2.1:: `obj' Extensions to the `SEGMENT' Directive
* Section 6.2.2:: `GROUP': Defining Groups of Segments
* Section 6.2.3:: `UPPERCASE': Disabling Case Sensitivity in Output
* Section 6.2.4:: `IMPORT': Importing DLL Symbols
* Section 6.2.5:: `EXPORT': Exporting DLL Symbols
* Section 6.2.6:: `..start': Defining the Program Entry Point
* Section 6.2.7:: `obj' Extensions to the `EXTERN' Directive
* Section 6.2.8:: `obj' Extensions to the `COMMON' Directive


File: nasm.info,  Node: Section 6.2.1,  Next: Section 6.2.2,  Prev: Section 6.2,  Up: Section 6.2

6.2.1. `obj' Extensions to the `SEGMENT' Directive
**************************************************

   The `obj' output format extends the `SEGMENT' (or `SECTION')
directive to allow you to specify various properties of the segment you
are defining. This is done by appending extra qualifiers to the end of
the segment-definition line. For example,

               segment code private align=16

   defines the segment `code', but also declares it to be a private
segment, and requires that the portion of it described in this code
module must be aligned on a 16-byte boundary.

   The available qualifiers are:

   * `PRIVATE', `PUBLIC', `COMMON' and `STACK' specify the combination
     characteristics of the segment. `PRIVATE' segments do not get
     combined with any others by the linker; `PUBLIC' and `STACK'
     segments get concatenated together at link time; and `COMMON'
     segments all get overlaid on top of each other rather than stuck
     end-to-end.

   * `ALIGN' is used, as shown above, to specify how many low bits of
     the segment start address must be forced to zero. The alignment
     value given may be any power of two from 1 to 4096; in reality,
     the only values supported are 1, 2, 4, 16, 256 and 4096, so if 8
     is specified it will be rounded up to 16, and 32, 64 and 128 will
     all be rounded up to 256, and so on. Note that alignment to
     4096-byte boundaries is a PharLap extension to the format and may
     not be supported by all linkers.

   * `CLASS' can be used to specify the segment class; this feature
     indicates to the linker that segments of the same class should be
     placed near each other in the output file. The class name can be
     any word, e.g.  `CLASS=CODE'.

   * `OVERLAY', like `CLASS', is specified with an arbitrary word as an
     argument, and provides overlay information to an overlay-capable
     linker.

   * Segments can be declared as `USE16' or `USE32', which has the
     effect of recording the choice in the object file and also
     ensuring that NASM's default assembly mode when assembling in that
     segment is 16-bit or 32-bit respectively.

   * When writing OS/2 object files, you should declare 32-bit segments
     as `FLAT', which causes the default segment base for anything in
     the segment to be the special group `FLAT', and also defines the
     group if it is not already defined.

   * The `obj' file format also allows segments to be declared as
     having a pre-defined absolute segment address, although no linkers
     are currently known to make sensible use of this feature;
     nevertheless, NASM allows you to declare a segment such as
     `SEGMENT SCREEN ABSOLUTE=0xB800' if you need to. The `ABSOLUTE'
     and `ALIGN' keywords are mutually exclusive.

   NASM's default segment attributes are `PUBLIC', `ALIGN=1', no class,
no overlay, and `USE16'.


File: nasm.info,  Node: Section 6.2.2,  Next: Section 6.2.3,  Prev: Section 6.2.1,  Up: Section 6.2

6.2.2. `GROUP': Defining Groups of Segments
*******************************************

   The `obj' format also allows segments to be grouped, so that a single
segment register can be used to refer to all the segments in a group.
NASM therefore supplies the `GROUP' directive, whereby you can code

               segment data
               ; some data
               segment bss
               ; some uninitialised data
               group dgroup data bss

   which will define a group called `dgroup' to contain the segments
`data' and `bss'. Like `SEGMENT', `GROUP' causes the group name to be
defined as a symbol, so that you can refer to a variable `var' in the
`data' segment as `var wrt data' or as `var wrt dgroup', depending on
which segment value is currently in your segment register.

   If you just refer to `var', however, and `var' is declared in a
segment which is part of a group, then NASM will default to giving you
the offset of `var' from the beginning of the _group_, not the
_segment_. Therefore `SEG var', also, will return the group base rather
than the segment base.

   NASM will allow a segment to be part of more than one group, but will
generate a warning if you do this. Variables declared in a segment
which is part of more than one group will default to being relative to
the first group that was defined to contain the segment.

   A group does not have to contain any segments; you can still make
`WRT' references to a group which does not contain the variable you are
referring to. OS/2, for example, defines the special group `FLAT' with
no segments in it.


File: nasm.info,  Node: Section 6.2.3,  Next: Section 6.2.4,  Prev: Section 6.2.2,  Up: Section 6.2

6.2.3. `UPPERCASE': Disabling Case Sensitivity in Output
********************************************************

   Although NASM itself is case sensitive, some OMF linkers are not;
therefore it can be useful for NASM to output single-case object files.
The `UPPERCASE' format-specific directive causes all segment, group and
symbol names that are written to the object file to be forced to upper
case just before being written. Within a source file, NASM is still
case- sensitive; but the object file can be written entirely in upper
case if desired.

   `UPPERCASE' is used alone on a line; it requires no parameters.


File: nasm.info,  Node: Section 6.2.4,  Next: Section 6.2.5,  Prev: Section 6.2.3,  Up: Section 6.2

6.2.4. `IMPORT': Importing DLL Symbols
**************************************

   The `IMPORT' format-specific directive defines a symbol to be
imported from a DLL, for use if you are writing a DLL's import library
in NASM. You still need to declare the symbol as `EXTERN' as well as
using the `IMPORT' directive.

   The `IMPORT' directive takes two required parameters, separated by
white space, which are (respectively) the name of the symbol you wish to
import and the name of the library you wish to import it from. For
example:

               import WSAStartup wsock32.dll

   A third optional parameter gives the name by which the symbol is
known in the library you are importing it from, in case this is not the
same as the name you wish the symbol to be known by to your code once
you have imported it. For example:

               import asyncsel wsock32.dll WSAAsyncSelect


File: nasm.info,  Node: Section 6.2.5,  Next: Section 6.2.6,  Prev: Section 6.2.4,  Up: Section 6.2

6.2.5. `EXPORT': Exporting DLL Symbols
**************************************

   The `EXPORT' format-specific directive defines a global symbol to be
exported as a DLL symbol, for use if you are writing a DLL in NASM. You
still need to declare the symbol as `GLOBAL' as well as using the
`EXPORT' directive.

   `EXPORT' takes one required parameter, which is the name of the
symbol you wish to export, as it was defined in your source file. An
optional second parameter (separated by white space from the first)
gives the _external_ name of the symbol: the name by which you wish the
symbol to be known to programs using the DLL. If this name is the same
as the internal name, you may leave the second parameter off.

   Further parameters can be given to define attributes of the exported
symbol. These parameters, like the second, are separated by white
space. If further parameters are given, the external name must also be
specified, even if it is the same as the internal name. The available
attributes are:

   * `resident' indicates that the exported name is to be kept resident
     by the system loader. This is an optimisation for frequently used
     symbols imported by name.

   * `nodata' indicates that the exported symbol is a function which
     does not make use of any initialised data.

   * `parm=NNN', where `NNN' is an integer, sets the number of
     parameter words for the case in which the symbol is a call gate
     between 32- bit and 16-bit segments.

   * An attribute which is just a number indicates that the symbol
     should be exported with an identifying number (ordinal), and gives
     the desired number.

   For example:

               export myfunc
               export myfunc TheRealMoreFormalLookingFunctionName
               export myfunc myfunc 1234  ; export by ordinal
               export myfunc myfunc resident parm=23 nodata


File: nasm.info,  Node: Section 6.2.6,  Next: Section 6.2.7,  Prev: Section 6.2.5,  Up: Section 6.2

6.2.6. `..start': Defining the Program Entry Point
**************************************************

   OMF linkers require exactly one of the object files being linked to
define the program entry point, where execution will begin when the
program is run. If the object file that defines the entry point is
assembled using NASM, you specify the entry point by declaring the
special symbol `..start' at the point where you wish execution to begin.


File: nasm.info,  Node: Section 6.2.7,  Next: Section 6.2.8,  Prev: Section 6.2.6,  Up: Section 6.2

6.2.7. `obj' Extensions to the `EXTERN' Directive
*************************************************

   If you declare an external symbol with the directive

               extern foo

   then references such as `mov ax,foo' will give you the offset of
`foo' from its preferred segment base (as specified in whichever module
`foo' is actually defined in). So to access the contents of `foo' you
will usually need to do something like

               mov ax,seg foo         ; get preferred segment base
               mov es,ax              ; move it into ES
               mov ax,[es:foo]        ; and use offset `foo' from it

   This is a little unwieldy, particularly if you know that an external
is going to be accessible from a given segment or group, say `dgroup'.
So if `DS' already contained `dgroup', you could simply code

               mov ax,[foo wrt dgroup]

   However, having to type this every time you want to access `foo' can
be a pain; so NASM allows you to declare `foo' in the alternative form

               extern foo:wrt dgroup

   This form causes NASM to pretend that the preferred segment base of
`foo' is in fact `dgroup'; so the expression `seg foo' will now return
`dgroup', and the expression `foo' is equivalent to `foo wrt dgroup'.

   This default-`WRT' mechanism can be used to make externals appear to
be relative to any group or segment in your program. It can also be
applied to common variables: see *Note Section 6.2.8::.


File: nasm.info,  Node: Section 6.2.8,  Next: Section 6.3,  Prev: Section 6.2.7,  Up: Section 6.2

6.2.8. `obj' Extensions to the `COMMON' Directive
*************************************************

   The `obj' format allows common variables to be either near or far;
NASM allows you to specify which your variables should be by the use of
the syntax

               common nearvar 2:near  ; `nearvar' is a near common
               common farvar 10:far   ; and `farvar' is far

   Far common variables may be greater in size than 64Kb, and so the OMF
specification says that they are declared as a number of _elements_ of
a given size. So a 10-byte far common variable could be declared as ten
one-byte elements, five two-byte elements, two five-byte elements or one
ten-byte element.

   Some OMF linkers require the element size, as well as the variable
size, to match when resolving common variables declared in more than
one module.  Therefore NASM must allow you to specify the element size
on your far common variables. This is done by the following syntax:

               common c_5by2 10:far 5 ; two five-byte elements
               common c_2by5 10:far 2 ; five two-byte elements

   If no element size is specified, the default is 1. Also, the `FAR'
keyword is not required when an element size is specified, since only
far commons may have element sizes at all. So the above declarations
could equivalently be

               common c_5by2 10:5     ; two five-byte elements
               common c_2by5 10:2     ; five two-byte elements

   In addition to these extensions, the `COMMON' directive in `obj'
also supports default-`WRT' specification like `EXTERN' does (explained
in *Note Section 6.2.7::). So you can also declare things like

               common foo 10:wrt dgroup
               common bar 16:far 2:wrt data
               common baz 24:wrt data:6


File: nasm.info,  Node: Section 6.3,  Next: Section 6.3.1,  Prev: Section 6.2.8,  Up: Chapter 6

6.3. `win32': Microsoft Win32 Object Files
******************************************

   The `win32' output format generates Microsoft Win32 object files,
suitable for passing to Microsoft linkers such as Visual C++. Note that
Borland Win32 compilers do not use this format, but use `obj' instead
(see *Note Section 6.2::).

   `win32' provides a default output file-name extension of `.obj'.

   Note that although Microsoft say that Win32 object files follow the
COFF (Common Object File Format) standard, the object files produced by
Microsoft Win32 compilers are not compatible with COFF linkers such as
DJGPP's, and vice versa. This is due to a difference of opinion over the
precise semantics of PC-relative relocations. To produce COFF files
suitable for DJGPP, use NASM's `coff' output format; conversely, the
`coff' format does not produce object files that Win32 linkers can
generate correct output from.

* Menu:

* Section 6.3.1:: `win32' Extensions to the `SECTION' Directive


File: nasm.info,  Node: Section 6.3.1,  Next: Section 6.4,  Prev: Section 6.3,  Up: Section 6.3

6.3.1. `win32' Extensions to the `SECTION' Directive
****************************************************

   Like the `obj' format, `win32' allows you to specify additional
information on the `SECTION' directive line, to control the type and
properties of sections you declare. Section types and properties are
generated automatically by NASM for the standard section names `.text',
`.data' and `.bss', but may still be overridden by these qualifiers.

   The available qualifiers are:

   * `code', or equivalently `text', defines the section to be a code
     section. This marks the section as readable and executable, but not
     writable, and also indicates to the linker that the type of the
     section is code.

   * `data' and `bss' define the section to be a data section,
     analogously to `code'. Data sections are marked as readable and
     writable, but not executable. `data' declares an initialised data
     section, whereas `bss' declares an uninitialised data section.

   * `info' defines the section to be an informational section, which is
     not included in the executable file by the linker, but may (for
     example) pass information _to_ the linker. For example, declaring
     an `info'-type section called `.drectve' causes the linker to
     interpret the contents of the section as command-line options.

   * `align=', used with a trailing number as in `obj', gives the
     alignment requirements of the section. The maximum you may specify
     is 64: the Win32 object file format contains no means to request a
     greater section alignment than this. If alignment is not
     explicitly specified, the defaults are 16-byte alignment for code
     sections, and 4-byte alignment for data (and BSS) sections.
     Informational sections get a default alignment of 1 byte (no
     alignment), though the value does not matter.

   The defaults assumed by NASM if you do not specify the above
qualifiers are:

               section .text code align=16
               section .data data align=4
               section .bss bss align=4

   Any other section name is treated by default like `.text'.


File: nasm.info,  Node: Section 6.4,  Next: Section 6.5,  Prev: Section 6.3.1,  Up: Chapter 6

6.4. `coff': Common Object File Format
**************************************

   The `coff' output type produces COFF object files suitable for
linking with the DJGPP linker.

   `coff' provides a default output file-name extension of `.o'.

   The `coff' format supports the same extensions to the `SECTION'
directive as `win32' does, except that the `align' qualifier and the
`info' section type are not supported.


File: nasm.info,  Node: Section 6.5,  Next: Section 6.5.1,  Prev: Section 6.4,  Up: Chapter 6

6.5. `elf': Linux ELFObject Files
*********************************

   The `elf' output format generates ELF32 (Executable and Linkable
Format) object files, as used by Linux. `elf' provides a default output
file-name extension of `.o'.

* Menu:

* Section 6.5.1:: `elf' Extensions to the `SECTION' Directive
* Section 6.5.2:: Position-Independent Code: `elf' Special Symbols and `WRT'
* Section 6.5.3:: `elf' Extensions to the `GLOBAL' Directive
* Section 6.5.4:: `elf' Extensions to the `COMMON' Directive


File: nasm.info,  Node: Section 6.5.1,  Next: Section 6.5.2,  Prev: Section 6.5,  Up: Section 6.5

6.5.1. `elf' Extensions to the `SECTION' Directive
**************************************************

   Like the `obj' format, `elf' allows you to specify additional
information on the `SECTION' directive line, to control the type and
properties of sections you declare. Section types and properties are
generated automatically by NASM for the standard section names `.text',
`.data' and `.bss', but may still be overridden by these qualifiers.

   The available qualifiers are:

   * `alloc' defines the section to be one which is loaded into memory
     when the program is run. `noalloc' defines it to be one which is
     not, such as an informational or comment section.

   * `exec' defines the section to be one which should have execute
     permission when the program is run. `noexec' defines it as one
     which should not.

   * `write' defines the section to be one which should be writable when
     the program is run. `nowrite' defines it as one which should not.

   * `progbits' defines the section to be one with explicit contents
     stored in the object file: an ordinary code or data section, for
     example, `nobits' defines the section to be one with no explicit
     contents given, such as a BSS section.

   * `align=', used with a trailing number as in `obj', gives the
     alignment requirements of the section.

   The defaults assumed by NASM if you do not specify the above
qualifiers are:

               section .text progbits alloc   exec nowrite align=16
               section .data progbits alloc noexec   write align=4
               section .bss    nobits alloc noexec   write align=4
               section other progbits alloc noexec nowrite align=1

   (Any section name other than `.text', `.data' and `.bss' is treated
by default like `other' in the above code.)


File: nasm.info,  Node: Section 6.5.2,  Next: Section 6.5.3,  Prev: Section 6.5.1,  Up: Section 6.5

6.5.2. Position-Independent Code: `elf' Special Symbols and `WRT'
*****************************************************************

   The ELF specification contains enough features to allow position-
independent code (PIC) to be written, which makes ELF shared libraries
very flexible. However, it also means NASM has to be able to generate a
variety of strange relocation types in ELF object files, if it is to be
an assembler which can write PIC.

   Since ELF does not support segment-base references, the `WRT'
operator is not used for its normal purpose; therefore NASM's `elf'
output format makes use of `WRT' for a different purpose, namely the
PIC- specific relocation types.

   `elf' defines five special symbols which you can use as the
right-hand side of the `WRT' operator to obtain PIC relocation types.
They are `..gotpc', `..gotoff', `..got', `..plt' and `..sym'. Their
functions are summarised here:

   * Referring to the symbol marking the global offset table base using
     `wrt ..gotpc' will end up giving the distance from the beginning of
     the current section to the global offset table.
     (`_GLOBAL_OFFSET_TABLE_' is the standard symbol name used to refer
     to the GOT.) So you would then need to add `$$' to the result to
     get the real address of the GOT.

   * Referring to a location in one of your own sections using `wrt
     ..gotoff' will give the distance from the beginning of the GOT to
     the specified location, so that adding on the address of the GOT
     would give the real address of the location you wanted.

   * Referring to an external or global symbol using `wrt ..got' causes
     the linker to build an entry _in_ the GOT containing the address
     of the symbol, and the reference gives the distance from the
     beginning of the GOT to the entry; so you can add on the address
     of the GOT, load from the resulting address, and end up with the
     address of the symbol.

   * Referring to a procedure name using `wrt ..plt' causes the linker
     to build a procedure linkage table entry for the symbol, and the
     reference gives the address of the PLT entry. You can only use
     this in contexts which would generate a PC-relative relocation
     normally (i.e. as the destination for `CALL' or `JMP'), since ELF
     contains no relocation type to refer to PLT entries absolutely.

   * Referring to a symbol name using `wrt ..sym' causes NASM to write
     an ordinary relocation, but instead of making the relocation
     relative to the start of the section and then adding on the offset
     to the symbol, it will write a relocation record aimed directly at
     the symbol in question. The distinction is a necessary one due to
     a peculiarity of the dynamic linker.

   A fuller explanation of how to use these relocation types to write
shared libraries entirely in NASM is given in *Note Section 8.2::.


File: nasm.info,  Node: Section 6.5.3,  Next: Section 6.5.4,  Prev: Section 6.5.2,  Up: Section 6.5

6.5.3. `elf' Extensions to the `GLOBAL' Directive
*************************************************

   ELF object files can contain more information about a global symbol
than just its address: they can contain the size of the symbol and its
type as well. These are not merely debugger conveniences, but are
actually necessary when the program being written is a shared library.
NASM therefore supports some extensions to the `GLOBAL' directive,
allowing you to specify these features.

   You can specify whether a global variable is a function or a data
object by suffixing the name with a colon and the word `function' or
`data'. (`object' is a synonym for `data'.) For example:

               global hashlookup:function, hashtable:data

   exports the global symbol `hashlookup' as a function and `hashtable'
as a data object.

   You can also specify the size of the data associated with the
symbol, as a numeric expression (which may involve labels, and even
forward references) after the type specifier. Like this:

               global hashtable:data (hashtable.end - hashtable)
     hashtable:
               db this,that,theother  ; some data here
     .end:

   This makes NASM automatically calculate the length of the table and
place that information into the ELF symbol table.

   Declaring the type and size of global symbols is necessary when
writing shared library code. For more information, see *Note Section
8.2.4::.


File: nasm.info,  Node: Section 6.5.4,  Next: Section 6.6,  Prev: Section 6.5.3,  Up: Section 6.5

6.5.4. `elf' Extensions to the `COMMON' Directive
*************************************************

   ELF also allows you to specify alignment requirements on common
variables.  This is done by putting a number (which must be a power of
two) after the name and size of the common variable, separated (as
usual) by a colon. For example, an array of doublewords would benefit
from 4-byte alignment:

               common dwordarray 128:4

   This declares the total size of the array to be 128 bytes, and
requires that it be aligned on a 4-byte boundary.


File: nasm.info,  Node: Section 6.6,  Next: Section 6.7,  Prev: Section 6.5.4,  Up: Chapter 6

6.6. `aout': Linux `a.out' Object Files
***************************************

   The `aout' format generates `a.out' object files, in the form used
by early Linux systems. (These differ from other `a.out' object files
in that the magic number in the first four bytes of the file is
different. Also, some implementations of `a.out', for example NetBSD's,
support position-independent code, which Linux's implementation
doesn't.)

   `a.out' provides a default output file-name extension of `.o'.

   `a.out' is a very simple object format. It supports no special
directives, no special symbols, no use of `SEG' or `WRT', and no
extensions to any standard directives. It supports only the three
standard section names `.text', `.data' and `.bss'.


File: nasm.info,  Node: Section 6.7,  Next: Section 6.8,  Prev: Section 6.6,  Up: Chapter 6

6.7. `aoutb': NetBSD/FreeBSD/OpenBSD `a.out' Object Files
*********************************************************

   The `aoutb' format generates `a.out' object files, in the form used
by the various free BSD Unix clones, NetBSD, FreeBSD and OpenBSD. For
simple object files, this object format is exactly the same as `aout'
except for the magic number in the first four bytes of the file.
However, the `aoutb' format supports position-independent code in the
same way as the `elf' format, so you can use it to write BSD shared
libraries.

   `aoutb' provides a default output file-name extension of `.o'.

   `aoutb' supports no special directives, no special symbols, and only
the three standard section names `.text', `.data' and `.bss'. However,
it also supports the same use of `WRT' as `elf' does, to provide
position-independent code relocation types. See *Note Section 6.5.2::
for full documentation of this feature.

   `aoutb' also supports the same extensions to the `GLOBAL' directive
as `elf' does: see *Note Section 6.5.3:: for documentation of this.


File: nasm.info,  Node: Section 6.8,  Next: Section 6.9,  Prev: Section 6.7,  Up: Chapter 6

6.8. `as86': Linux `as86' Object Files
**************************************

   The Linux 16-bit assembler `as86' has its own non-standard object
file format. Although its companion linker `ld86' produces something
close to ordinary `a.out' binaries as output, the object file format
used to communicate between `as86' and `ld86' is not itself `a.out'.

   NASM supports this format, just in case it is useful, as `as86'.
`as86' provides a default output file-name extension of `.o'.

   `as86' is a very simple object format (from the NASM user's point of
view). It supports no special directives, no special symbols, no use of
`SEG' or `WRT', and no extensions to any standard directives. It
supports only the three standard section names `.text', `.data' and
`.bss'.


File: nasm.info,  Node: Section 6.9,  Next: Section 6.9.1,  Prev: Section 6.8,  Up: Chapter 6

6.9. `rdf': Relocatable Dynamic Object File Format
**************************************************

   The `rdf' output format produces RDOFF object files. RDOFF
(Relocatable Dynamic Object File Format) is a home-grown object-file
format, designed alongside NASM itself and reflecting in its file format
the internal structure of the assembler.

   RDOFF is not used by any well-known operating systems. Those writing
their own systems, however, may well wish to use RDOFF as their object
format, on the grounds that it is designed primarily for simplicity and
contains very little file-header bureaucracy.

   The Unix NASM archive, and the DOS archive which includes sources,
both contain an `rdoff' subdirectory holding a set of RDOFF utilities:
an RDF linker, an RDF static-library manager, an RDF file dump utility,
and a program which will load and execute an RDF executable under Linux.

   `rdf' supports only the standard section names `.text', `.data' and
`.bss'.

* Menu:

* Section 6.9.1:: Requiring a Library: The `LIBRARY' Directive


File: nasm.info,  Node: Section 6.9.1,  Next: Section 6.10,  Prev: Section 6.9,  Up: Section 6.9

6.9.1. Requiring a Library: The `LIBRARY' Directive
***************************************************

   RDOFF contains a mechanism for an object file to demand a given
library to be linked to the module, either at load time or run time.
This is done by the `LIBRARY' directive, which takes one argument which
is the name of the module:

               library mylib.rdl


File: nasm.info,  Node: Section 6.10,  Next: Chapter 7,  Prev: Section 6.9.1,  Up: Chapter 6

6.10. `dbg': Debugging Format
*****************************

   The `dbg' output format is not built into NASM in the default
configuration. If you are building your own NASM executable from the
sources, you can define `OF_DBG' in `outform.h' or on the compiler
command line, and obtain the `dbg' output format.

   The `dbg' format does not output an object file as such; instead, it
outputs a text file which contains a complete list of all the
transactions between the main body of NASM and the output-format back
end module. It is primarily intended to aid people who want to write
their own output drivers, so that they can get a clearer idea of the
various requests the main program makes of the output driver, and in
what order they happen.

   For simple files, one can easily use the `dbg' format like this:

     nasm -f dbg filename.asm

   which will generate a diagnostic file called `filename.dbg'. However,
this will not work well on files which were designed for a different
object format, because each object format defines its own macros
(usually user- level forms of directives), and those macros will not be
defined in the `dbg' format. Therefore it can be useful to run NASM
twice, in order to do the preprocessing with the native object format
selected:

     nasm -e -f rdf -o rdfprog.i rdfprog.asm
     nasm -a -f dbg rdfprog.i

   This preprocesses `rdfprog.asm' into `rdfprog.i', keeping the `rdf'
object format selected in order to make sure RDF special directives are
converted into primitive form correctly. Then the preprocessed source
is fed through the `dbg' format to generate the final diagnostic output.

   This workaround will still typically not work for programs intended
for `obj' format, because the `obj' `SEGMENT' and `GROUP' directives
have side effects of defining the segment and group names as symbols;
`dbg' will not do this, so the program will not assemble. You will have
to work around that by defining the symbols yourself (using `EXTERN',
for example) if you really need to get a `dbg' trace of an
`obj'-specific source file.

   `dbg' accepts any section name and any directives at all, and logs
them all to its output file.


File: nasm.info,  Node: Chapter 7,  Next: Section 7.1,  Prev: Section 6.10,  Up: Top

Chapter 7: Writing 16-bit Code (DOS, Windows 3/3.1)
***************************************************

   This chapter attempts to cover some of the common issues encountered
when writing 16-bit code to run under MS-DOS or Windows 3.x. It covers
how to link programs to produce `.EXE' or `.COM' files, how to write
`.SYS' device drivers, and how to interface assembly language code with
16-bit C compilers and with Borland Pascal.

* Menu:

* Section 7.1:: Producing `.EXE' Files
* Section 7.2:: Producing `.COM' Files
* Section 7.3:: Producing `.SYS' Files
* Section 7.4:: Interfacing to 16-bit C Programs
* Section 7.5:: Interfacing to Borland Pascal Programs


File: nasm.info,  Node: Section 7.1,  Next: Section 7.1.1,  Prev: Chapter 7,  Up: Chapter 7

7.1. Producing `.EXE' Files
***************************

   Any large program written under DOS needs to be built as a `.EXE'
file: only `.EXE' files have the necessary internal structure required
to span more than one 64K segment. Windows programs, also, have to be
built as `.EXE' files, since Windows does not support the `.COM' format.

   In general, you generate `.EXE' files by using the `obj' output
format to produce one or more `.OBJ' files, and then linking them
together using a linker. However, NASM also supports the direct
generation of simple DOS `.EXE' files using the `bin' output format (by
using `DB' and `DW' to construct the `.EXE' file header), and a macro
package is supplied to do this. Thanks to Yann Guidon for contributing
the code for this.

   NASM may also support `.EXE' natively as another output format in
future releases.

* Menu:

* Section 7.1.1:: Using the `obj' Format To Generate `.EXE' Files
* Section 7.1.2:: Using the `bin' Format To Generate `.EXE' Files


File: nasm.info,  Node: Section 7.1.1,  Next: Section 7.1.2,  Prev: Section 7.1,  Up: Section 7.1

7.1.1. Using the `obj' Format To Generate `.EXE' Files
******************************************************

   This section describes the usual method of generating `.EXE' files by
linking `.OBJ' files together.

   Most 16-bit programming language packages come with a suitable
linker; if you have none of these, there is a free linker called VAL,
available in `LZH' archive format from `x2ftp.oulu.fi'. An LZH archiver
can be found at `ftp.simtel.net'. There is another `free' linker
(though this one doesn't come with sources) called FREELINK, available
from `www.pcorner.com'. A third, `djlink', written by DJ Delorie, is
available at `www.delorie.com'.

   When linking several `.OBJ' files into a `.EXE' file, you should
ensure that exactly one of them has a start point defined (using the
`..start' special symbol defined by the `obj' format: see *Note Section
6.2.6::). If no module defines a start point, the linker will not know
what value to give the entry-point field in the output file header; if
more than one defines a start point, the linker will not know _which_
value to use.

   An example of a NASM source file which can be assembled to a `.OBJ'
file and linked on its own to a `.EXE' is given here. It demonstrates
the basic principles of defining a stack, initialising the segment
registers, and declaring a start point. This file is also provided in
the `test' subdirectory of the NASM archives, under the name
`objexe.asm'.

               segment code
     
     ..start:  mov ax,data
               mov ds,ax
               mov ax,stack
               mov ss,ax
               mov sp,stacktop

   This initial piece of code sets up `DS' to point to the data segment,
and initialises `SS' and `SP' to point to the top of the provided
stack. Notice that interrupts are implicitly disabled for one
instruction after a move into `SS', precisely for this situation, so
that there's no chance of an interrupt occurring between the loads of
`SS' and `SP' and not having a stack to execute on.

   Note also that the special symbol `..start' is defined at the
beginning of this code, which means that will be the entry point into
the resulting executable file.

               mov dx,hello
               mov ah,9
               int 0x21

   The above is the main program: load `DS:DX' with a pointer to the
greeting message (`hello' is implicitly relative to the segment `data',
which was loaded into `DS' in the setup code, so the full pointer is
valid), and call the DOS print-string function.

               mov ax,0x4c00
               int 0x21

   This terminates the program using another DOS system call.

               segment data
     hello:    db 'hello, world', 13, 10, '$'

   The data segment contains the string we want to display.

               segment stack stack
               resb 64
     stacktop:

   The above code declares a stack segment containing 64 bytes of
uninitialised stack space, and points `stacktop' at the top of it. The
directive `segment stack stack' defines a segment _called_ `stack', and
also of _type_ `STACK'. The latter is not necessary to the correct
running of the program, but linkers are likely to issue warnings or
errors if your program has no segment of type `STACK'.

   The above file, when assembled into a `.OBJ' file, will link on its
own to a valid `.EXE' file, which when run will print `hello, world'
and then exit.


File: nasm.info,  Node: Section 7.1.2,  Next: Section 7.2,  Prev: Section 7.1.1,  Up: Section 7.1

7.1.2. Using the `bin' Format To Generate `.EXE' Files
******************************************************

   The `.EXE' file format is simple enough that it's possible to build a
`.EXE' file by writing a pure-binary program and sticking a 32-byte
header on the front. This header is simple enough that it can be
generated using `DB' and `DW' commands by NASM itself, so that you can
use the `bin' output format to directly generate `.EXE' files.

   Included in the NASM archives, in the `misc' subdirectory, is a file
`exebin.mac' of macros. It defines three macros: `EXE_begin',
`EXE_stack' and `EXE_end'.

   To produce a `.EXE' file using this method, you should start by using
`%include' to load the `exebin.mac' macro package into your source
file. You should then issue the `EXE_begin' macro call (which takes no
arguments) to generate the file header data. Then write code as normal
for the `bin' format - you can use all three standard sections `.text',
`.data' and `.bss'. At the end of the file you should call the
`EXE_end' macro (again, no arguments), which defines some symbols to
mark section sizes, and these symbols are referred to in the header
code generated by `EXE_begin'.

   In this model, the code you end up writing starts at `0x100', just
like a `.COM' file - in fact, if you strip off the 32-byte header from
the resulting `.EXE' file, you will have a valid `.COM' program. All
the segment bases are the same, so you are limited to a 64K program,
again just like a `.COM' file. Note that an `ORG' directive is issued
by the `EXE_begin' macro, so you should not explicitly issue one of
your own.

   You can't directly refer to your segment base value, unfortunately,
since this would require a relocation in the header, and things would
get a lot more complicated. So you should get your segment base by
copying it out of `CS' instead.

   On entry to your `.EXE' file, `SS:SP' are already set up to point to
the top of a 2Kb stack. You can adjust the default stack size of 2Kb by
calling the `EXE_stack' macro. For example, to change the stack size of
your program to 64 bytes, you would call `EXE_stack 64'.

   A sample program which generates a `.EXE' file in this way is given
in the `test' subdirectory of the NASM archive, as `binexe.asm'.


File: nasm.info,  Node: Section 7.2,  Next: Section 7.2.1,  Prev: Section 7.1.2,  Up: Chapter 7

7.2. Producing `.COM' Files
***************************

   While large DOS programs must be written as `.EXE' files, small ones
are often better written as `.COM' files. `.COM' files are pure binary,
and therefore most easily produced using the `bin' output format.

* Menu:

* Section 7.2.1:: Using the `bin' Format To Generate `.COM' Files
* Section 7.2.2:: Using the `obj' Format To Generate `.COM' Files


File: nasm.info,  Node: Section 7.2.1,  Next: Section 7.2.2,  Prev: Section 7.2,  Up: Section 7.2

7.2.1. Using the `bin' Format To Generate `.COM' Files
******************************************************

   `.COM' files expect to be loaded at offset `100h' into their segment
(though the segment may change). Execution then begins at `100h', i.e.
right at the start of the program. So to write a `.COM' program, you
would create a source file looking like

               org 100h
               section .text
     start:    ; put your code here
               section .data
               ; put data items here
               section .bss
               ; put uninitialised data here

   The `bin' format puts the `.text' section first in the file, so you
can declare data or BSS items before beginning to write code if you
want to and the code will still end up at the front of the file where it
belongs.

   The BSS (uninitialised data) section does not take up space in the
`.COM' file itself: instead, addresses of BSS items are resolved to
point at space beyond the end of the file, on the grounds that this
will be free memory when the program is run. Therefore you should not
rely on your BSS being initialised to all zeros when you run.

   To assemble the above program, you should use a command line like

     nasm myprog.asm -fbin -o myprog.com

   The `bin' format would produce a file called `myprog' if no explicit
output file name were specified, so you have to override it and give
the desired file name.


File: nasm.info,  Node: Section 7.2.2,  Next: Section 7.3,  Prev: Section 7.2.1,  Up: Section 7.2

7.2.2. Using the `obj' Format To Generate `.COM' Files
******************************************************

   If you are writing a `.COM' program as more than one module, you may
wish to assemble several `.OBJ' files and link them together into a
`.COM' program. You can do this, provided you have a linker capable of
outputting `.COM' files directly (TLINK does this), or alternatively a
converter program such as `EXE2BIN' to transform the `.EXE' file output
from the linker into a `.COM' file.

   If you do this, you need to take care of several things:

   * The first object file containing code should start its code
     segment with a line like `RESB 100h'. This is to ensure that the
     code begins at offset `100h' relative to the beginning of the code
     segment, so that the linker or converter program does not have to
     adjust address references within the file when generating the
     `.COM' file. Other assemblers use an `ORG' directive for this
     purpose, but `ORG' in NASM is a format-specific directive to the
     `bin' output format, and does not mean the same thing as it does
     in MASM-compatible assemblers.

   * You don't need to define a stack segment.

   * All your segments should be in the same group, so that every time
     your code or data references a symbol offset, all offsets are
     relative to the same segment base. This is because, when a `.COM'
     file is loaded, all the segment registers contain the same value.


File: nasm.info,  Node: Section 7.3,  Next: Section 7.4,  Prev: Section 7.2.2,  Up: Chapter 7

7.3. Producing `.SYS' Files
***************************

   MS-DOS device drivers - `.SYS' files - are pure binary files,
similar to `.COM' files, except that they start at origin zero rather
than `100h'. Therefore, if you are writing a device driver using the
`bin' format, you do not need the `ORG' directive, since the default
origin for `bin' is zero. Similarly, if you are using `obj', you do not
need the `RESB 100h' at the start of your code segment.

   `.SYS' files start with a header structure, containing pointers to
the various routines inside the driver which do the work. This
structure should be defined at the start of the code segment, even
though it is not actually code.

   For more information on the format of `.SYS' files, and the data
which has to go in the header structure, a list of books is given in the
Frequently Asked Questions list for the newsgroup
`comp.os.msdos.programmer'.


File: nasm.info,  Node: Section 7.4,  Next: Section 7.4.1,  Prev: Section 7.3,  Up: Chapter 7

7.4. Interfacing to 16-bit C Programs
*************************************

   This section covers the basics of writing assembly routines that
call, or are called from, C programs. To do this, you would typically
write an assembly module as a `.OBJ' file, and link it with your C
modules to produce a mixed-language program.

* Menu:

* Section 7.4.1:: External Symbol Names
* Section 7.4.2:: Memory Models
* Section 7.4.3:: Function Definitions and Function Calls
* Section 7.4.4:: Accessing Data Items
* Section 7.4.5:: `c16.mac': Helper Macros for the 16-bit C Interface


File: nasm.info,  Node: Section 7.4.1,  Next: Section 7.4.2,  Prev: Section 7.4,  Up: Section 7.4

7.4.1. External Symbol Names
****************************

   C compilers have the convention that the names of all global symbols
(functions or data) they define are formed by prefixing an underscore to
the name as it appears in the C program. So, for example, the function
a C programmer thinks of as `printf' appears to an assembly language
programmer as `_printf'. This means that in your assembly programs, you
can define symbols without a leading underscore, and not have to worry
about name clashes with C symbols.

   If you find the underscores inconvenient, you can define macros to
replace the `GLOBAL' and `EXTERN' directives as follows:

     %macro cglobal 1
               global _%1
     %define %1 _%1
     %endmacro

     %macro cextern 1
               extern _%1
     %define %1 _%1
     %endmacro

   (These forms of the macros only take one argument at a time; a `%rep'
construct could solve this.)

   If you then declare an external like this:

               cextern printf

   then the macro will expand it as

               extern _printf
     %define printf _printf

   Thereafter, you can reference `printf' as if it was a symbol, and the
preprocessor will put the leading underscore on where necessary.

   The `cglobal' macro works similarly. You must use `cglobal' before
defining the symbol in question, but you would have had to do that
anyway if you used `GLOBAL'.


File: nasm.info,  Node: Section 7.4.2,  Next: Section 7.4.3,  Prev: Section 7.4.1,  Up: Section 7.4

7.4.2. Memory Models
********************

   NASM contains no mechanism to support the various C memory models
directly; you have to keep track yourself of which one you are writing
for. This means you have to keep track of the following things:

   * In models using a single code segment (tiny, small and compact),
     functions are near. This means that function pointers, when stored
     in data segments or pushed on the stack as function arguments, are
     16 bits long and contain only an offset field (the `CS' register
     never changes its value, and always gives the segment part of the
     full function address), and that functions are called using
     ordinary near `CALL' instructions and return using `RETN' (which,
     in NASM, is synonymous with `RET' anyway). This means both that
     you should write your own routines to return with `RETN', and that
     you should call external C routines with near `CALL' instructions.

   * In models using more than one code segment (medium, large and
     huge), functions are far. This means that function pointers are 32
     bits long (consisting of a 16-bit offset followed by a 16-bit
     segment), and that functions are called using `CALL FAR' (or `CALL
     seg:offset') and return using `RETF'. Again, you should therefore
     write your own routines to return with `RETF' and use `CALL FAR'
     to call external routines.

   * In models using a single data segment (tiny, small and medium),
     data pointers are 16 bits long, containing only an offset field
     (the `DS' register doesn't change its value, and always gives the
     segment part of the full data item address).

   * In models using more than one data segment (compact, large and
     huge), data pointers are 32 bits long, consisting of a 16-bit
     offset followed by a 16- bit segment. You should still be careful
     not to modify `DS' in your routines without restoring it
     afterwards, but `ES' is free for you to use to access the contents
     of 32-bit data pointers you are passed.

   * The huge memory model allows single data items to exceed 64K in
     size. In all other memory models, you can access the whole of a
     data item just by doing arithmetic on the offset field of the
     pointer you are given, whether a segment field is present or not;
     in huge model, you have to be more careful of your pointer
     arithmetic.

   * In most memory models, there is a _default_ data segment, whose
     segment address is kept in `DS' throughout the program. This data
     segment is typically the same segment as the stack, kept in `SS',
     so that functions' local variables (which are stored on the stack)
     and global data items can both be accessed easily without changing
     `DS'.  Particularly large data items are typically stored in other
     segments.  However, some memory models (though not the standard
     ones, usually) allow the assumption that `SS' and `DS' hold the
     same value to be removed. Be careful about functions' local
     variables in this latter case.

   In models with a single code segment, the segment is called `_TEXT',
so your code segment must also go by this name in order to be linked
into the same place as the main code segment. In models with a single
data segment, or with a default data segment, it is called `_DATA'.


File: nasm.info,  Node: Section 7.4.3,  Next: Section 7.4.4,  Prev: Section 7.4.2,  Up: Section 7.4

7.4.3. Function Definitions and Function Calls
**********************************************

   The C calling convention in 16-bit programs is as follows. In the
following description, the words _caller_ and _callee_ are used to
denote the function doing the calling and the function which gets
called.

   * The caller pushes the function's parameters on the stack, one after
     another, in reverse order (right to left, so that the first
     argument specified to the function is pushed last).

   * The caller then executes a `CALL' instruction to pass control to
     the callee. This `CALL' is either near or far depending on the
     memory model.

   * The callee receives control, and typically (although this is not
     actually necessary, in functions which do not need to access their
     parameters) starts by saving the value of `SP' in `BP' so as to be
     able to use `BP' as a base pointer to find its parameters on the
     stack.  However, the caller was probably doing this too, so part
     of the calling convention states that `BP' must be preserved by
     any C function. Hence the callee, if it is going to set up `BP' as
     a _frame pointer_, must push the previous value first.

   * The callee may then access its parameters relative to `BP'. The
     word at `[BP]' holds the previous value of `BP' as it was pushed;
     the next word, at `[BP+2]', holds the offset part of the return
     address, pushed implicitly by `CALL'. In a small-model (near)
     function, the parameters start after that, at `[BP+4]'; in a
     large-model (far) function, the segment part of the return address
     lives at `[BP+4]', and the parameters begin at `[BP+6]'. The
     leftmost parameter of the function, since it was pushed last, is
     accessible at this offset from `BP'; the others follow, at
     successively greater offsets. Thus, in a function such as `printf'
     which takes a variable number of parameters, the pushing of the
     parameters in reverse order means that the function knows where to
     find its first parameter, which tells it the number and type of
     the remaining ones.

   * The callee may also wish to decrease `SP' further, so as to
     allocate space on the stack for local variables, which will then
     be accessible at negative offsets from `BP'.

   * The callee, if it wishes to return a value to the caller, should
     leave the value in `AL', `AX' or `DX:AX' depending on the size of
     the value. Floating-point results are sometimes (depending on the
     compiler) returned in `ST0'.

   * Once the callee has finished processing, it restores `SP' from
     `BP' if it had allocated local stack space, then pops the previous
     value of `BP', and returns via `RETN' or `RETF' depending on
     memory model.

   * When the caller regains control from the callee, the function
     parameters are still on the stack, so it typically adds an
     immediate constant to `SP' to remove them (instead of executing a
     number of slow `POP' instructions). Thus, if a function is
     accidentally called with the wrong number of parameters due to a
     prototype mismatch, the stack will still be returned to a sensible
     state since the caller, which _knows_ how many parameters it
     pushed, does the removing.

   It is instructive to compare this calling convention with that for
Pascal programs (described in *Note Section 7.5.1::). Pascal has a
simpler convention, since no functions have variable numbers of
parameters.  Therefore the callee knows how many parameters it should
have been passed, and is able to deallocate them from the stack itself
by passing an immediate argument to the `RET' or `RETF' instruction, so
the caller does not have to do it. Also, the parameters are pushed in
left-to- right order, not right-to-left, which means that a compiler
can give better guarantees about sequence points without performance
suffering.

   Thus, you would define a function in C style in the following way.
The following example is for small model:

               global _myfunc
     _myfunc:  push bp
               mov bp,sp
               sub sp,0x40            ; 64 bytes of local stack space
               mov bx,[bp+4]          ; first parameter to function
               ; some more code
               mov sp,bp              ; undo "sub sp,0x40" above
               pop bp
               ret

   For a large-model function, you would replace `RET' by `RETF', and
look for the first parameter at `[BP+6]' instead of `[BP+4]'.  Of
course, if one of the parameters is a pointer, then the offsets of
_subsequent_ parameters will change depending on the memory model as
well: far pointers take up four bytes on the stack when passed as a
parameter, whereas near pointers take up two.

   At the other end of the process, to call a C function from your
assembly code, you would do something like this:

               extern _printf
               ; and then, further down...
               push word [myint]      ; one of my integer variables
               push word mystring     ; pointer into my data segment
               call _printf
               add sp,byte 4          ; `byte' saves space
               ; then those data items...
               segment _DATA
     myint     dw 1234
     mystring  db 'This number -> %d <- should be 1234',10,0

   This piece of code is the small-model assembly equivalent of the C
code

         int myint = 1234;
         printf("This number -> %d <- should be 1234\n", myint);

   In large model, the function-call code might look more like this. In
this example, it is assumed that `DS' already holds the segment base of
the segment `_DATA'. If not, you would have to initialise it first.

               push word [myint]
               push word seg mystring ; Now push the segment, and...
               push word mystring     ; ... offset of "mystring"
               call far _printf
               add sp,byte 6

   The integer value still takes up one word on the stack, since large
model does not affect the size of the `int' data type. The first
argument (pushed last) to `printf', however, is a data pointer, and
therefore has to contain a segment and offset part. The segment should
be stored second in memory, and therefore must be pushed first. (Of
course, `PUSH DS' would have been a shorter instruction than `PUSH WORD
SEG mystring', if `DS' was set up as the above example assumed.) Then
the actual call becomes a far call, since functions expect far calls in
large model; and `SP' has to be increased by 6 rather than 4 afterwards
to make up for the extra word of parameters.


File: nasm.info,  Node: Section 7.4.4,  Next: Section 7.4.5,  Prev: Section 7.4.3,  Up: Section 7.4

7.4.4. Accessing Data Items
***************************

   To get at the contents of C variables, or to declare variables which
C can access, you need only declare the names as `GLOBAL' or `EXTERN'.
(Again, the names require leading underscores, as stated in *Note
Section 7.4.1::.) Thus, a C variable declared as `int i' can be
accessed from assembler as

               extern _i
               mov ax,[_i]

   And to declare your own integer variable which C programs can access
as `extern int j', you do this (making sure you are assembling in the
`_DATA' segment, if necessary):

               global _j
     _j        dw 0

   To access a C array, you need to know the size of the components of
the array. For example, `int' variables are two bytes long, so if a C
program declares an array as `int a[10]', you can access `a[3]' by
coding `mov ax,[_a+6]'. (The byte offset 6 is obtained by multiplying
the desired array index, 3, by the size of the array element, 2.) The
sizes of the C base types in 16-bit compilers are: 1 for `char', 2 for
`short' and `int', 4 for `long' and `float', and 8 for `double'.

   To access a C data structure, you need to know the offset from the
base of the structure to the field you are interested in. You can
either do this by converting the C structure definition into a NASM
structure definition (using `STRUC'), or by calculating the one offset
and using just that.

   To do either of these, you should read your C compiler's manual to
find out how it organises data structures. NASM gives no special
alignment to structure members in its own `STRUC' macro, so you have to
specify alignment yourself if the C compiler generates it. Typically,
you might find that a structure like

     struct {
         char c;
         int i;
     } foo;

   might be four bytes long rather than three, since the `int' field
would be aligned to a two-byte boundary. However, this sort of feature
tends to be a configurable option in the C compiler, either using
command- line options or `#pragma' lines, so you have to find out how
your own compiler does it.


File: nasm.info,  Node: Section 7.4.5,  Next: Section 7.5,  Prev: Section 7.4.4,  Up: Section 7.4

7.4.5. `c16.mac': Helper Macros for the 16-bit C Interface
**********************************************************

   Included in the NASM archives, in the `misc' directory, is a file
`c16.mac' of macros. It defines three macros: `proc', `arg' and
`endproc'. These are intended to be used for C-style procedure
definitions, and they automate a lot of the work involved in keeping
track of the calling convention.

   An example of an assembly function using the macro set is given here:

               proc _nearproc
     %$i       arg
     %$j       arg
               mov ax,[bp + %$i]
               mov bx,[bp + %$j]
               add ax,[bx]
               endproc

   This defines `_nearproc' to be a procedure taking two arguments, the
first (`i') an integer and the second (`j') a pointer to an integer. It
returns `i + *j'.

   Note that the `arg' macro has an `EQU' as the first line of its
expansion, and since the label before the macro call gets prepended to
the first line of the expanded macro, the `EQU' works, defining `%$i'
to be an offset from `BP'. A context-local variable is used, local to
the context pushed by the `proc' macro and popped by the `endproc'
macro, so that the same argument name can be used in later procedures.
Of course, you don't _have_ to do that.

   The macro set produces code for near functions (tiny, small and
compact- model code) by default. You can have it generate far functions
(medium, large and huge-model code) by means of coding `%define
FARCODE'. This changes the kind of return instruction generated by
`endproc', and also changes the starting point for the argument
offsets. The macro set contains no intrinsic dependency on whether data
pointers are far or not.

   `arg' can take an optional parameter, giving the size of the
argument.  If no size is given, 2 is assumed, since it is likely that
many function parameters will be of type `int'.

   The large-model equivalent of the above function would look like
this:

     %define FARCODE
               proc _farproc
     %$i       arg
     %$j       arg 4
               mov ax,[bp + %$i]
               mov bx,[bp + %$j]
               mov es,[bp + %$j + 2]
               add ax,[bx]
               endproc

   This makes use of the argument to the `arg' macro to define a
parameter of size 4, because `j' is now a far pointer. When we load
from `j', we must load a segment and an offset.


File: nasm.info,  Node: Section 7.5,  Next: Section 7.5.1,  Prev: Section 7.4.5,  Up: Chapter 7

7.5. Interfacing to Borland Pascal Programs
*******************************************

   Interfacing to Borland Pascal programs is similar in concept to
interfacing to 16-bit C programs. The differences are:

   * The leading underscore required for interfacing to C programs is
     not required for Pascal.

   * The memory model is always large: functions are far, data pointers
     are far, and no data item can be more than 64K long. (Actually,
     some functions are near, but only those functions that are local
     to a Pascal unit and never called from outside it. All assembly
     functions that Pascal calls, and all Pascal functions that
     assembly routines are able to call, are far.)  However, all static
     data declared in a Pascal program goes into the default data
     segment, which is the one whose segment address will be in `DS'
     when control is passed to your assembly code. The only things that
     do not live in the default data segment are local variables (they
     live in the stack segment) and dynamically allocated variables.
     All data _pointers_, however, are far.

   * The function calling convention is different - described below.

   * Some data types, such as strings, are stored differently.

   * There are restrictions on the segment names you are allowed to use
     - Borland Pascal will ignore code or data declared in a segment it
     doesn't like the name of. The restrictions are described below.

* Menu:

* Section 7.5.1:: The Pascal Calling Convention
* Section 7.5.2:: Borland Pascal Segment Name Restrictions
* Section 7.5.3:: Using `c16.mac' With Pascal Programs


File: nasm.info,  Node: Section 7.5.1,  Next: Section 7.5.2,  Prev: Section 7.5,  Up: Section 7.5

7.5.1. The Pascal Calling Convention
************************************

   The 16-bit Pascal calling convention is as follows. In the following
description, the words _caller_ and _callee_ are used to denote the
function doing the calling and the function which gets called.

   * The caller pushes the function's parameters on the stack, one after
     another, in normal order (left to right, so that the first argument
     specified to the function is pushed first).

   * The caller then executes a far `CALL' instruction to pass control
     to the callee.

   * The callee receives control, and typically (although this is not
     actually necessary, in functions which do not need to access their
     parameters) starts by saving the value of `SP' in `BP' so as to be
     able to use `BP' as a base pointer to find its parameters on the
     stack.  However, the caller was probably doing this too, so part
     of the calling convention states that `BP' must be preserved by
     any function. Hence the callee, if it is going to set up `BP' as a
     frame pointer, must push the previous value first.

   * The callee may then access its parameters relative to `BP'. The
     word at `[BP]' holds the previous value of `BP' as it was pushed.
     The next word, at `[BP+2]', holds the offset part of the return
     address, and the next one at `[BP+4]' the segment part. The
     parameters begin at `[BP+6]'. The rightmost parameter of the
     function, since it was pushed last, is accessible at this offset
     from `BP'; the others follow, at successively greater offsets.

   * The callee may also wish to decrease `SP' further, so as to
     allocate space on the stack for local variables, which will then
     be accessible at negative offsets from `BP'.

   * The callee, if it wishes to return a value to the caller, should
     leave the value in `AL', `AX' or `DX:AX' depending on the size of
     the value. Floating-point results are returned in `ST0'. Results
     of type `Real' (Borland's own custom floating-point data type, not
     handled directly by the FPU) are returned in `DX:BX:AX'. To return
     a result of type `String', the caller pushes a pointer to a
     temporary string before pushing the parameters, and the callee
     places the returned string value at that location. The pointer is
     not a parameter, and should not be removed from the stack by the
     `RETF' instruction.

   * Once the callee has finished processing, it restores `SP' from
     `BP' if it had allocated local stack space, then pops the previous
     value of `BP', and returns via `RETF'. It uses the form of `RETF'
     with an immediate parameter, giving the number of bytes taken up
     by the parameters on the stack. This causes the parameters to be
     removed from the stack as a side effect of the return instruction.

   * When the caller regains control from the callee, the function
     parameters have already been removed from the stack, so it needs
     to do nothing further.

   Thus, you would define a function in Pascal style, taking two
`Integer'-type parameters, in the following way:

               global myfunc
     myfunc:   push bp
               mov bp,sp
               sub sp,0x40            ; 64 bytes of local stack space
               mov bx,[bp+8]          ; first parameter to function
               mov bx,[bp+6]          ; second parameter to function
               ; some more code
               mov sp,bp              ; undo "sub sp,0x40" above
               pop bp
               retf 4                 ; total size of params is 4

   At the other end of the process, to call a Pascal function from your
assembly code, you would do something like this:

               extern SomeFunc
               ; and then, further down...
               push word seg mystring ; Now push the segment, and...
               push word mystring     ; ... offset of "mystring"
               push word [myint]      ; one of my variables
               call far SomeFunc

   This is equivalent to the Pascal code

     procedure SomeFunc(String: PChar; Int: Integer);
         SomeFunc(@mystring, myint);


File: nasm.info,  Node: Section 7.5.2,  Next: Section 7.5.3,  Prev: Section 7.5.1,  Up: Section 7.5

7.5.2. Borland Pascal Segment Name Restrictions
***********************************************

   Since Borland Pascal's internal unit file format is completely
different from `OBJ', it only makes a very sketchy job of actually
reading and understanding the various information contained in a real
`OBJ' file when it links that in. Therefore an object file intended to
be linked to a Pascal program must obey a number of restrictions:

   * Procedures and functions must be in a segment whose name is either
     `CODE', `CSEG', or something ending in `_TEXT'.

   * Initialised data must be in a segment whose name is either `CONST'
     or something ending in `_DATA'.

   * Uninitialised data must be in a segment whose name is either
     `DATA', `DSEG', or something ending in `_BSS'.

   * Any other segments in the object file are completely ignored.
     `GROUP' directives and segment attributes are also ignored.


File: nasm.info,  Node: Section 7.5.3,  Next: Chapter 8,  Prev: Section 7.5.2,  Up: Section 7.5

7.5.3. Using `c16.mac' With Pascal Programs
*******************************************

   The `c16.mac' macro package, described in *Note Section 7.4.5::, can
also be used to simplify writing functions to be called from Pascal
programs, if you code `%define PASCAL'. This definition ensures that
functions are far (it implies `FARCODE'), and also causes procedure
return instructions to be generated with an operand.

   Defining `PASCAL' does not change the code which calculates the
argument offsets; you must declare your function's arguments in reverse
order. For example:

     %define PASCAL
               proc _pascalproc
     %$j       arg 4
     %$i       arg
               mov ax,[bp + %$i]
               mov bx,[bp + %$j]
               mov es,[bp + %$j + 2]
               add ax,[bx]
               endproc

   This defines the same routine, conceptually, as the example in *Note
Section 7.4.5::: it defines a function taking two arguments, an integer
and a pointer to an integer, which returns the sum of the integer and
the contents of the pointer. The only difference between this code and
the large-model C version is that `PASCAL' is defined instead of
`FARCODE', and that the arguments are declared in reverse order.


File: nasm.info,  Node: Chapter 8,  Next: Section 8.1,  Prev: Section 7.5.3,  Up: Top

Chapter 8: Writing 32-bit Code (Unix, Win32, DJGPP)
***************************************************

   This chapter attempts to cover some of the common issues involved
when writing 32-bit code, to run under Win32 or Unix, or to be linked
with C code generated by a Unix-style C compiler such as DJGPP. It
covers how to write assembly code to interface with 32-bit C routines,
and how to write position-independent code for shared libraries.

   Almost all 32-bit code, and in particular all code running under
Win32, DJGPP or any of the PC Unix variants, runs in _flat_ memory
model.  This means that the segment registers and paging have already
been set up to give you the same 32-bit 4Gb address space no matter
what segment you work relative to, and that you should ignore all
segment registers completely. When writing flat-model application code,
you never need to use a segment override or modify any segment
register, and the code-section addresses you pass to `CALL' and `JMP'
live in the same address space as the data-section addresses you access
your variables by and the stack-section addresses you access local
variables and procedure parameters by. Every address is 32 bits long
and contains only an offset part.

* Menu:

* Section 8.1:: Interfacing to 32-bit C Programs
* Section 8.2:: Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF Shared Libraries


File: nasm.info,  Node: Section 8.1,  Next: Section 8.1.1,  Prev: Chapter 8,  Up: Chapter 8

8.1. Interfacing to 32-bit C Programs
*************************************

   A lot of the discussion in *Note Section 7.4::, about interfacing to
16-bit C programs, still applies when working in 32 bits. The absence
of memory models or segmentation worries simplifies things a lot.

* Menu:

* Section 8.1.1:: External Symbol Names
* Section 8.1.2:: Function Definitions and Function Calls
* Section 8.1.3:: Accessing Data Items
* Section 8.1.4:: `c32.mac': Helper Macros for the 32-bit C Interface


File: nasm.info,  Node: Section 8.1.1,  Next: Section 8.1.2,  Prev: Section 8.1,  Up: Section 8.1

8.1.1. External Symbol Names
****************************

   Most 32-bit C compilers share the convention used by 16-bit
compilers, that the names of all global symbols (functions or data)
they define are formed by prefixing an underscore to the name as it
appears in the C program.  However, not all of them do: the ELF
specification states that C symbols do _not_ have a leading underscore
on their assembly-language names.

   The older Linux `a.out' C compiler, all Win32 compilers, DJGPP, and
NetBSD and FreeBSD, all use the leading underscore; for these compilers,
the macros `cextern' and `cglobal', as given in *Note Section 7.4.1::,
will still work. For ELF, though, the leading underscore should not be
used.


File: nasm.info,  Node: Section 8.1.2,  Next: Section 8.1.3,  Prev: Section 8.1.1,  Up: Section 8.1

8.1.2. Function Definitions and Function Calls
**********************************************

   The C calling conventionThe C calling convention in 32-bit programs
is as follows. In the following description, the words _caller_ and
_callee_ are used to denote the function doing the calling and the
function which gets called.

   * The caller pushes the function's parameters on the stack, one after
     another, in reverse order (right to left, so that the first
     argument specified to the function is pushed last).

   * The caller then executes a near `CALL' instruction to pass control
     to the callee.

   * The callee receives control, and typically (although this is not
     actually necessary, in functions which do not need to access their
     parameters) starts by saving the value of `ESP' in `EBP' so as to
     be able to use `EBP' as a base pointer to find its parameters on
     the stack.  However, the caller was probably doing this too, so
     part of the calling convention states that `EBP' must be preserved
     by any C function.  Hence the callee, if it is going to set up
     `EBP' as a frame pointer, must push the previous value first.

   * The callee may then access its parameters relative to `EBP'. The
     doubleword at `[EBP]' holds the previous value of `EBP' as it was
     pushed; the next doubleword, at `[EBP+4]', holds the return
     address, pushed implicitly by `CALL'. The parameters start after
     that, at `[EBP+8]'. The leftmost parameter of the function, since
     it was pushed last, is accessible at this offset from `EBP'; the
     others follow, at successively greater offsets. Thus, in a
     function such as `printf' which takes a variable number of
     parameters, the pushing of the parameters in reverse order means
     that the function knows where to find its first parameter, which
     tells it the number and type of the remaining ones.

   * The callee may also wish to decrease `ESP' further, so as to
     allocate space on the stack for local variables, which will then
     be accessible at negative offsets from `EBP'.

   * The callee, if it wishes to return a value to the caller, should
     leave the value in `AL', `AX' or `EAX' depending on the size of the
     value. Floating-point results are typically returned in `ST0'.

   * Once the callee has finished processing, it restores `ESP' from
     `EBP' if it had allocated local stack space, then pops the previous
     value of `EBP', and returns via `RET' (equivalently, `RETN').

   * When the caller regains control from the callee, the function
     parameters are still on the stack, so it typically adds an
     immediate constant to `ESP' to remove them (instead of executing a
     number of slow `POP' instructions). Thus, if a function is
     accidentally called with the wrong number of parameters due to a
     prototype mismatch, the stack will still be returned to a sensible
     state since the caller, which _knows_ how many parameters it
     pushed, does the removing.

   There is an alternative calling convention used by Win32 programs for
Windows API calls, and also for functions called _by_ the Windows API
such as window procedures: they follow what Microsoft calls the
`__stdcall' convention. This is slightly closer to the Pascal
convention, in that the callee clears the stack by passing a parameter
to the `RET' instruction. However, the parameters are still pushed in
right-to-left order.

   Thus, you would define a function in C style in the following way:

               global _myfunc
     _myfunc:  push ebp
               mov ebp,esp
               sub esp,0x40           ; 64 bytes of local stack space
               mov ebx,[ebp+8]        ; first parameter to function
               ; some more code
               leave                  ; mov esp,ebp / pop ebp
               ret

   At the other end of the process, to call a C function from your
assembly code, you would do something like this:

               extern _printf
               ; and then, further down...
               push dword [myint]     ; one of my integer variables
               push dword mystring    ; pointer into my data segment
               call _printf
               add esp,byte 8         ; `byte' saves space
               ; then those data items...
               segment _DATA
     myint     dd 1234
     mystring  db 'This number -> %d <- should be 1234',10,0

   This piece of code is the assembly equivalent of the C code

         int myint = 1234;
         printf("This number -> %d <- should be 1234\n", myint);


File: nasm.info,  Node: Section 8.1.3,  Next: Section 8.1.4,  Prev: Section 8.1.2,  Up: Section 8.1

8.1.3. Accessing Data Items
***************************

   To get at the contents of C variables, or to declare variables which
C can access, you need only declare the names as `GLOBAL' or `EXTERN'.
(Again, the names require leading underscores, as stated in *Note
Section 8.1.1::.) Thus, a C variable declared as `int i' can be
accessed from assembler as

               extern _i
               mov eax,[_i]

   And to declare your own integer variable which C programs can access
as `extern int j', you do this (making sure you are assembling in the
`_DATA' segment, if necessary):

               global _j
     _j        dd 0

   To access a C array, you need to know the size of the components of
the array. For example, `int' variables are four bytes long, so if a C
program declares an array as `int a[10]', you can access `a[3]' by
coding `mov ax,[_a+12]'. (The byte offset 12 is obtained by multiplying
the desired array index, 3, by the size of the array element, 4.) The
sizes of the C base types in 32-bit compilers are: 1 for `char', 2 for
`short', 4 for `int', `long' and `float', and 8 for `double'. Pointers,
being 32-bit addresses, are also 4 bytes long.

   To access a C data structure, you need to know the offset from the
base of the structure to the field you are interested in. You can
either do this by converting the C structure definition into a NASM
structure definition (using `STRUC'), or by calculating the one offset
and using just that.

   To do either of these, you should read your C compiler's manual to
find out how it organises data structures. NASM gives no special
alignment to structure members in its own `STRUC' macro, so you have to
specify alignment yourself if the C compiler generates it. Typically,
you might find that a structure like

     struct {
         char c;
         int i;
     } foo;

   might be eight bytes long rather than five, since the `int' field
would be aligned to a four-byte boundary. However, this sort of feature
is sometimes a configurable option in the C compiler, either using
command- line options or `#pragma' lines, so you have to find out how
your own compiler does it.


File: nasm.info,  Node: Section 8.1.4,  Next: Section 8.2,  Prev: Section 8.1.3,  Up: Section 8.1

8.1.4. `c32.mac': Helper Macros for the 32-bit C Interface
**********************************************************

   Included in the NASM archives, in the `misc' directory, is a file
`c32.mac' of macros. It defines three macros: `proc', `arg' and
`endproc'. These are intended to be used for C-style procedure
definitions, and they automate a lot of the work involved in keeping
track of the calling convention.

   An example of an assembly function using the macro set is given here:

               proc _proc32
     %$i       arg
     %$j       arg
               mov eax,[ebp + %$i]
               mov ebx,[ebp + %$j]
               add eax,[ebx]
               endproc

   This defines `_proc32' to be a procedure taking two arguments, the
first (`i') an integer and the second (`j') a pointer to an integer. It
returns `i + *j'.

   Note that the `arg' macro has an `EQU' as the first line of its
expansion, and since the label before the macro call gets prepended to
the first line of the expanded macro, the `EQU' works, defining `%$i'
to be an offset from `BP'. A context-local variable is used, local to
the context pushed by the `proc' macro and popped by the `endproc'
macro, so that the same argument name can be used in later procedures.
Of course, you don't _have_ to do that.

   `arg' can take an optional parameter, giving the size of the
argument.  If no size is given, 4 is assumed, since it is likely that
many function parameters will be of type `int' or pointers.


File: nasm.info,  Node: Section 8.2,  Next: Section 8.2.1,  Prev: Section 8.1.4,  Up: Chapter 8

8.2. Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF Shared Libraries
******************************************************************

   ELF replaced the older `a.out' object file format under Linux because
it contains support for position-independent code (PIC), which makes
writing shared libraries much easier. NASM supports the ELF position-
independent code features, so you can write Linux ELF shared libraries
in NASM.

   NetBSD, and its close cousins FreeBSD and OpenBSD, take a different
approach by hacking PIC support into the `a.out' format. NASM supports
this as the `aoutb' output format, so you can write BSD shared
libraries in NASM too.

   The operating system loads a PIC shared library by memory-mapping the
library file at an arbitrarily chosen point in the address space of the
running process. The contents of the library's code section must
therefore not depend on where it is loaded in memory.

   Therefore, you cannot get at your variables by writing code like
this:

               mov eax,[myvar]        ; WRONG

   Instead, the linker provides an area of memory called the _global
offset table_, or GOT; the GOT is situated at a constant distance from
your library's code, so if you can find out where your library is
loaded (which is typically done using a `CALL' and `POP' combination),
you can obtain the address of the GOT, and you can then load the
addresses of your variables out of linker-generated entries in the GOT.

   The _data_ section of a PIC shared library does not have these
restrictions: since the data section is writable, it has to be copied
into memory anyway rather than just paged in from the library file, so
as long as it's being copied it can be relocated too. So you can put
ordinary types of relocation in the data section without too much worry
(but see *Note Section 8.2.4:: for a caveat).

* Menu:

* Section 8.2.1:: Obtaining the Address of the GOT
* Section 8.2.2:: Finding Your Local Data Items
* Section 8.2.3:: Finding External and Common Data Items
* Section 8.2.4:: Exporting Symbols to the Library User
* Section 8.2.5:: Calling Procedures Outside the Library
* Section 8.2.6:: Generating the Library File


File: nasm.info,  Node: Section 8.2.1,  Next: Section 8.2.2,  Prev: Section 8.2,  Up: Section 8.2

8.2.1. Obtaining the Address of the GOT
***************************************

   Each code module in your shared library should define the GOT as an
external symbol:

               extern _GLOBAL_OFFSET_TABLE_   ; in ELF
               extern __GLOBAL_OFFSET_TABLE_  ; in BSD a.out

   At the beginning of any function in your shared library which plans
to access your data or BSS sections, you must first calculate the
address of the GOT. This is typically done by writing the function in
this form:

     func:     push ebp
               mov ebp,esp
               push ebx
               call .get_GOT
     .get_GOT: pop ebx
               add ebx,_GLOBAL_OFFSET_TABLE_+$$-.get_GOT wrt ..gotpc
               ; the function body comes here
               mov ebx,[ebp-4]
               mov esp,ebp
               pop ebp
               ret

   (For BSD, again, the symbol `_GLOBAL_OFFSET_TABLE' requires a second
leading underscore.)

   The first two lines of this function are simply the standard C
prologue to set up a stack frame, and the last three lines are standard
C function epilogue. The third line, and the fourth to last line, save
and restore the `EBX' register, because PIC shared libraries use this
register to store the address of the GOT.

   The interesting bit is the `CALL' instruction and the following two
lines. The `CALL' and `POP' combination obtains the address of the
label `.get_GOT', without having to know in advance where the program
was loaded (since the `CALL' instruction is encoded relative to the
current position). The `ADD' instruction makes use of one of the
special PIC relocation types: GOTPC relocation. With the `WRT ..gotpc'
qualifier specified, the symbol referenced (here
`_GLOBAL_OFFSET_TABLE_', the special symbol assigned to the GOT) is
given as an offset from the beginning of the section. (Actually, ELF
encodes it as the offset from the operand field of the `ADD'
instruction, but NASM simplifies this deliberately, so you do things the
same way for both ELF and BSD.) So the instruction then _adds_ the
beginning of the section, to get the real address of the GOT, and
subtracts the value of `.get_GOT' which it knows is in `EBX'.
Therefore, by the time that instruction has finished, `EBX' contains
the address of the GOT.

   If you didn't follow that, don't worry: it's never necessary to
obtain the address of the GOT by any other means, so you can put those
three instructions into a macro and safely ignore them:

     %macro get_GOT 0
               call %%getgot
     %%getgot: pop ebx
               add ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc
     %endmacro


File: nasm.info,  Node: Section 8.2.2,  Next: Section 8.2.3,  Prev: Section 8.2.1,  Up: Section 8.2

8.2.2. Finding Your Local Data Items
************************************

   Having got the GOT, you can then use it to obtain the addresses of
your data items. Most variables will reside in the sections you have
declared; they can be accessed using the `..gotoff' special `WRT' type.
The way this works is like this:

               lea eax,[ebx+myvar wrt ..gotoff]

   The expression `myvar wrt ..gotoff' is calculated, when the shared
library is linked, to be the offset to the local variable `myvar' from
the beginning of the GOT. Therefore, adding it to `EBX' as above will
place the real address of `myvar' in `EAX'.

   If you declare variables as `GLOBAL' without specifying a size for
them, they are shared between code modules in the library, but do not
get exported from the library to the program that loaded it. They will
still be in your ordinary data and BSS sections, so you can access them
in the same way as local variables, using the above `..gotoff'
mechanism.

   Note that due to a peculiarity of the way BSD `a.out' format handles
this relocation type, there must be at least one non-local symbol in the
same section as the address you're trying to access.


File: nasm.info,  Node: Section 8.2.3,  Next: Section 8.2.4,  Prev: Section 8.2.2,  Up: Section 8.2

8.2.3. Finding External and Common Data Items
*********************************************

   If your library needs to get at an external variable (external to the
_library_, not just to one of the modules within it), you must use the
`..got' type to get at it. The `..got' type, instead of giving you the
offset from the GOT base to the variable, gives you the offset from the
GOT base to a GOT _entry_ containing the address of the variable.  The
linker will set up this GOT entry when it builds the library, and the
dynamic linker will place the correct address in it at load time. So to
obtain the address of an external variable `extvar' in `EAX', you would
code

               mov eax,[ebx+extvar wrt ..got]

   This loads the address of `extvar' out of an entry in the GOT. The
linker, when it builds the shared library, collects together every
relocation of type `..got', and builds the GOT so as to ensure it has
every necessary entry present.

   Common variables must also be accessed in this way.


File: nasm.info,  Node: Section 8.2.4,  Next: Section 8.2.5,  Prev: Section 8.2.3,  Up: Section 8.2

8.2.4. Exporting Symbols to the Library User
********************************************

   If you want to export symbols to the user of the library, you have to
declare whether they are functions or data, and if they are data, you
have to give the size of the data item. This is because the dynamic
linker has to build procedure linkage table entries for any exported
functions, and also moves exported data items away from the library's
data section in which they were declared.

   So to export a function to users of the library, you must use

               global func:function   ; declare it as a function
     func:     push ebp
               ; etc.

   And to export a data item such as an array, you would have to code

               global array:data array.end-array ; give the size too
     array:    resd 128
     .end:

   Be careful: If you export a variable to the library user, by
declaring it as `GLOBAL' and supplying a size, the variable will end up
living in the data section of the main program, rather than in your
library's data section, where you declared it. So you will have to
access your own global variable with the `..got' mechanism rather than
`..gotoff', as if it were external (which, effectively, it has become).

   Equally, if you need to store the address of an exported global in
one of your data sections, you can't do it by means of the standard
sort of code:

     dataptr:  dd global_data_item    ; WRONG

   NASM will interpret this code as an ordinary relocation, in which
`global_data_item' is merely an offset from the beginning of the
`.data' section (or whatever); so this reference will end up pointing
at your data section instead of at the exported global which resides
elsewhere.

   Instead of the above code, then, you must write

     dataptr:  dd global_data_item wrt ..sym

   which makes use of the special `WRT' type `..sym' to instruct NASM
to search the symbol table for a particular symbol at that address,
rather than just relocating by section base.

   Either method will work for functions: referring to one of your
functions by means of

     funcptr:  dd my_function

   will give the user the address of the code you wrote, whereas

     funcptr:  dd my_function wrt ..sym

   will give the address of the procedure linkage table for the
function, which is where the calling program will _believe_ the
function lives.  Either address is a valid way to call the function.


File: nasm.info,  Node: Section 8.2.5,  Next: Section 8.2.6,  Prev: Section 8.2.4,  Up: Section 8.2

8.2.5. Calling Procedures Outside the Library
*********************************************

   Calling procedures outside your shared library has to be done by
means of a _procedure linkage table_, or PLT. The PLT is placed at a
known offset from where the library is loaded, so the library code can
make calls to the PLT in a position-independent way. Within the PLT
there is code to jump to offsets contained in the GOT, so function
calls to other shared libraries or to routines in the main program can
be transparently passed off to their real destinations.

   To call an external routine, you must use another special PIC
relocation type, `WRT ..plt'. This is much easier than the GOT-based
ones: you simply replace calls such as `CALL printf' with the
PLT-relative version `CALL printf WRT ..plt'.


File: nasm.info,  Node: Section 8.2.6,  Next: Chapter 9,  Prev: Section 8.2.5,  Up: Section 8.2

8.2.6. Generating the Library File
**********************************

   Having written some code modules and assembled them to `.o' files,
you then generate your shared library with a command such as

     ld -shared -o library.so module1.o module2.o       # for ELF
     ld -Bshareable -o library.so module1.o module2.o   # for BSD

   For ELF, if your shared library is going to reside in system
directories such as `/usr/lib' or `/lib', it is usually worth using the
`-soname' flag to the linker, to store the final library file name,
with a version number, into the library:

     ld -shared -soname library.so.1 -o library.so.1.2 *.o

   You would then copy `library.so.1.2' into the library directory, and
create `library.so.1' as a symbolic link to it.


File: nasm.info,  Node: Chapter 9,  Next: Section 9.1,  Prev: Section 8.2.6,  Up: Top

Chapter 9: Mixing 16 and 32 Bit Code
************************************

   This chapter tries to cover some of the issues, largely related to
unusual forms of addressing and jump instructions, encountered when
writing operating system code such as protected-mode initialisation
routines, which require code that operates in mixed segment sizes, such
as code in a 16-bit segment trying to modify data in a 32-bit one, or
jumps between different- size segments.

* Menu:

* Section 9.1:: Mixed-Size Jumps
* Section 9.2:: Addressing Between Different-Size Segments
* Section 9.3:: Other Mixed-Size Instructions


File: nasm.info,  Node: Section 9.1,  Next: Section 9.2,  Prev: Chapter 9,  Up: Chapter 9

9.1. Mixed-Size Jumps
*********************

   The most common form of mixed-size instruction is the one used when
writing a 32-bit OS: having done your setup in 16-bit mode, such as
loading the kernel, you then have to boot it by switching into
protected mode and jumping to the 32-bit kernel start address. In a
fully 32-bit OS, this tends to be the _only_ mixed-size instruction you
need, since everything before it can be done in pure 16-bit code, and
everything after it can be pure 32-bit.

   This jump must specify a 48-bit far address, since the target
segment is a 32-bit one. However, it must be assembled in a 16-bit
segment, so just coding, for example,

               jmp 0x1234:0x56789ABC  ; wrong!

   will not work, since the offset part of the address will be
truncated to `0x9ABC' and the jump will be an ordinary 16-bit far one.

   The Linux kernel setup code gets round the inability of `as86' to
generate the required instruction by coding it manually, using `DB'
instructions. NASM can go one better than that, by actually generating
the right instruction itself. Here's how to do it right:

               jmp dword 0x1234:0x56789ABC  ; right

   The `DWORD' prefix (strictly speaking, it should come _after_ the
colon, since it is declaring the _offset_ field to be a doubleword; but
NASM will accept either form, since both are unambiguous) forces the
offset part to be treated as far, in the assumption that you are
deliberately writing a jump from a 16-bit segment to a 32-bit one.

   You can do the reverse operation, jumping from a 32-bit segment to a
16-bit one, by means of the `WORD' prefix:

               jmp word 0x8765:0x4321 ; 32 to 16 bit

   If the `WORD' prefix is specified in 16-bit mode, or the `DWORD'
prefix in 32-bit mode, they will be ignored, since each is explicitly
forcing NASM into a mode it was in anyway.


File: nasm.info,  Node: Section 9.2,  Next: Section 9.3,  Prev: Section 9.1,  Up: Chapter 9

9.2. Addressing Between Different-Size Segments
***********************************************

   If your OS is mixed 16 and 32-bit, or if you are writing a DOS
extender, you are likely to have to deal with some 16-bit segments and
some 32-bit ones. At some point, you will probably end up writing code
in a 16-bit segment which has to access data in a 32-bit segment, or
vice versa.

   If the data you are trying to access in a 32-bit segment lies within
the first 64K of the segment, you may be able to get away with using an
ordinary 16-bit addressing operation for the purpose; but sooner or
later, you will want to do 32-bit addressing from 16-bit mode.

   The easiest way to do this is to make sure you use a register for the
address, since any effective address containing a 32-bit register is
forced to be a 32-bit address. So you can do

               mov eax,offset_into_32_bit_segment_specified_by_fs
               mov dword [fs:eax],0x11223344

   This is fine, but slightly cumbersome (since it wastes an
instruction and a register) if you already know the precise offset you
are aiming at. The x86 architecture does allow 32-bit effective
addresses to specify nothing but a 4-byte offset, so why shouldn't NASM
be able to generate the best instruction for the purpose?

   It can. As in *Note Section 9.1::, you need only prefix the address
with the `DWORD' keyword, and it will be forced to be a 32-bit address:

               mov dword [fs:dword my_offset],0x11223344

   Also as in *Note Section 9.1::, NASM is not fussy about whether the
`DWORD' prefix comes before or after the segment override, so arguably
a nicer-looking way to code the above instruction is

               mov dword [dword fs:my_offset],0x11223344

   Don't confuse the `DWORD' prefix _outside_ the square brackets,
which controls the size of the data stored at the address, with the one
`inside' the square brackets which controls the length of the address
itself. The two can quite easily be different:

               mov word [dword 0x12345678],0x9ABC

   This moves 16 bits of data to an address specified by a 32-bit
offset.

   You can also specify `WORD' or `DWORD' prefixes along with the `FAR'
prefix to indirect far jumps or calls. For example:

               call dword far [fs:word 0x4321]

   This instruction contains an address specified by a 16-bit offset;
it loads a 48-bit far pointer from that (16-bit segment and 32-bit
offset), and calls that address.


File: nasm.info,  Node: Section 9.3,  Next: Chapter 10,  Prev: Section 9.2,  Up: Chapter 9

9.3. Other Mixed-Size Instructions
**********************************

   The other way you might want to access data might be using the string
instructions (`LODSx', `STOSx' and so on) or the `XLATB' instruction.
These instructions, since they take no parameters, might seem to have
no easy way to make them perform 32-bit addressing when assembled in a
16-bit segment.

   This is the purpose of NASM's `a16' and `a32' prefixes. If you are
coding `LODSB' in a 16-bit segment but it is supposed to be accessing a
string in a 32-bit segment, you should load the desired address into
`ESI' and then code

               a32 lodsb

   The prefix forces the addressing size to 32 bits, meaning that
`LODSB' loads from `[DS:ESI]' instead of `[DS:SI]'. To access a string
in a 16-bit segment when coding in a 32-bit one, the corresponding `a16'
prefix can be used.

   The `a16' and `a32' prefixes can be applied to any instruction in
NASM's instruction table, but most of them can generate all the useful
forms without them. The prefixes are necessary only for instructions
with implicit addressing: `CMPSx' (*Note Section A.19::), `SCASx'
(*Note Section A.149::), `LODSx' (*Note Section A.98::), `STOSx' (*Note
Section A.157::), `MOVSx' (*Note Section A.105::), `INSx' (*Note
Section A.80::), `OUTSx' (*Note Section A.112::), and `XLATB' (*Note
Section A.169::). Also, the various push and pop instructions (`PUSHA'
and `POPF' as well as the more usual `PUSH' and `POP') can accept `a16'
or `a32' prefixes to force a particular one of `SP' or `ESP' to be used
as a stack pointer, in case the stack segment in use is a different
size from the code segment.

   `PUSH' and `POP', when applied to segment registers in 32-bit mode,
also have the slightly odd behaviour that they push and pop 4 bytes at
a time, of which the top two are ignored and the bottom two give the
value of the segment register being manipulated. To force the 16-bit
behaviour of segment-register push and pop instructions, you can use the
operand-size prefix `o16':

               o16 push ss
               o16 push ds

   This code saves a doubleword of stack space by fitting two segment
registers into the space which would normally be consumed by pushing
one.

   (You can also use the `o32' prefix to force the 32-bit behaviour when
in 16-bit mode, but this seems less useful.)


File: nasm.info,  Node: Chapter 10,  Next: Section 10.1,  Prev: Section 9.3,  Up: Top

Chapter 10: Troubleshooting
***************************

   This chapter describes some of the common problems that users have
been known to encounter with NASM, and answers them. It also gives
instructions for reporting bugs in NASM if you find a difficulty that
isn't listed here.

* Menu:

* Section 10.1:: Common Problems
* Section 10.2:: Bugs


File: nasm.info,  Node: Section 10.1,  Next: Section 10.1.1,  Prev: Chapter 10,  Up: Chapter 10

10.1. Common Problems
*********************

* Menu:

* Section 10.1.1:: NASM Generates Inefficient Code
* Section 10.1.2:: My Jumps are Out of Range
* Section 10.1.3:: `ORG' Doesn't Work
* Section 10.1.4:: `TIMES' Doesn't Work


File: nasm.info,  Node: Section 10.1.1,  Next: Section 10.1.2,  Prev: Section 10.1,  Up: Section 10.1

10.1.1. NASM Generates Inefficient Code
***************************************

   I get a lot of `bug' reports about NASM generating inefficient, or
even `wrong', code on instructions such as `ADD ESP,8'. This is a
deliberate design feature, connected to predictability of output: NASM,
on seeing `ADD ESP,8', will generate the form of the instruction which
leaves room for a 32-bit offset. You need to code `ADD ESP,BYTE 8' if
you want the space-efficient form of the instruction. This isn't a bug:
at worst it's a misfeature, and that's a matter of opinion only.


File: nasm.info,  Node: Section 10.1.2,  Next: Section 10.1.3,  Prev: Section 10.1.1,  Up: Section 10.1

10.1.2. My Jumps are Out of Range
*********************************

   Similarly, people complain that when they issue conditional jumps
(which are `SHORT' by default) that try to jump too far, NASM reports
`short jump out of range' instead of making the jumps longer.

   This, again, is partly a predictability issue, but in fact has a more
practical reason as well. NASM has no means of being told what type of
processor the code it is generating will be run on; so it cannot decide
for itself that it should generate `Jcc NEAR' type instructions, because
it doesn't know that it's working for a 386 or above. Alternatively, it
could replace the out-of-range short `JNE' instruction with a very
short `JE' instruction that jumps over a `JMP NEAR'; this is a sensible
solution for processors below a 386, but hardly efficient on processors
which have good branch prediction _and_ could have used `JNE NEAR'
instead. So, once again, it's up to the user, not the assembler, to
decide what instructions should be generated.


File: nasm.info,  Node: Section 10.1.3,  Next: Section 10.1.4,  Prev: Section 10.1.2,  Up: Section 10.1

10.1.3. `ORG' Doesn't Work
**************************

   People writing boot sector programs in the `bin' format often
complain that `ORG' doesn't work the way they'd like: in order to place
the `0xAA55' signature word at the end of a 512-byte boot sector, people
who are used to MASM tend to code

               ORG 0
               ; some boot sector code
               ORG 510
               DW 0xAA55

   This is not the intended use of the `ORG' directive in NASM, and will
not work. The correct way to solve this problem in NASM is to use the
`TIMES' directive, like this:

               ORG 0
               ; some boot sector code
               TIMES 510-($-$$) DB 0
               DW 0xAA55

   The `TIMES' directive will insert exactly enough zero bytes into the
output to move the assembly point up to 510. This method also has the
advantage that if you accidentally fill your boot sector too full, NASM
will catch the problem at assembly time and report it, so you won't end
up with a boot sector that you have to disassemble to find out what's
wrong with it.


File: nasm.info,  Node: Section 10.1.4,  Next: Section 10.2,  Prev: Section 10.1.3,  Up: Section 10.1

10.1.4. `TIMES' Doesn't Work
****************************

   The other common problem with the above code is people who write the
`TIMES' line as

               TIMES 510-$ DB 0

   by reasoning that `$' should be a pure number, just like 510, so the
difference between them is also a pure number and can happily be fed to
`TIMES'.

   NASM is a _modular_ assembler: the various component parts are
designed to be easily separable for re-use, so they don't exchange
information unnecessarily. In consequence, the `bin' output format,
even though it has been told by the `ORG' directive that the `.text'
section should start at 0, does not pass that information back to the
expression evaluator. So from the evaluator's point of view, `$' isn't
a pure number: it's an offset from a section base. Therefore the
difference between `$' and 510 is also not a pure number, but involves
a section base. Values involving section bases cannot be passed as
arguments to `TIMES'.

   The solution, as in the previous section, is to code the `TIMES' line
in the form

               TIMES 510-($-$$) DB 0

   in which `$' and `$$' are offsets from the same section base, and so
their difference is a pure number. This will solve the problem and
generate sensible code.


File: nasm.info,  Node: Section 10.2,  Next: Appendix A,  Prev: Section 10.1.4,  Up: Chapter 10

10.2. Bugs
**********

   We have never yet released a version of NASM with any _known_ bugs.
That doesn't usually stop there being plenty we didn't know about,
though.  Any that you find should be reported to `hpa@zytor.com'.

   Please read *Note Section 2.2:: first, and don't report the bug if
it's listed in there as a deliberate feature. (If you think the feature
is badly thought out, feel free to send us reasons why you think it
should be changed, but don't just send us mail saying `This is a bug'
if the documentation says we did it on purpose.) Then read *Note
Section 10.1::, and don't bother reporting the bug if it's listed there.

   If you do report a bug, _please_ give us all of the following
information:

   * What operating system you're running NASM under. DOS, Linux,
     NetBSD, Win16, Win32, VMS (I'd be impressed), whatever.

   * If you're running NASM under DOS or Win32, tell us whether you've
     compiled your own executable from the DOS source archive, or
     whether you were using the standard distribution binaries out of
     the archive. If you were using a locally built executable, try to
     reproduce the problem using one of the standard binaries, as this
     will make it easier for us to reproduce your problem prior to
     fixing it.

   * Which version of NASM you're using, and exactly how you invoked
     it. Give us the precise command line, and the contents of the
     `NASM' environment variable if any.

   * Which versions of any supplementary programs you're using, and how
     you invoked them. If the problem only becomes visible at link
     time, tell us what linker you're using, what version of it you've
     got, and the exact linker command line. If the problem involves
     linking against object files generated by a compiler, tell us what
     compiler, what version, and what command line or options you used.
     (If you're compiling in an IDE, please try to reproduce the
     problem with the command-line version of the compiler.)

   * If at all possible, send us a NASM source file which exhibits the
     problem.  If this causes copyright problems (e.g. you can only
     reproduce the bug in restricted-distribution code) then bear in
     mind the following two points: firstly, we guarantee that any
     source code sent to us for the purposes of debugging NASM will be
     used _only_ for the purposes of debugging NASM, and that we will
     delete all our copies of it as soon as we have found and fixed the
     bug or bugs in question; and secondly, we would prefer _not_ to be
     mailed large chunks of code anyway. The smaller the file, the
     better.  A three-line sample file that does nothing useful
     _except_ demonstrate the problem is much easier to work with than
     a fully fledged ten-thousand- line program. (Of course, some
     errors _do_ only crop up in large files, so this may not be
     possible.)

   * A description of what the problem actually _is_. `It doesn't work'
     is _not_ a helpful description! Please describe exactly what is
     happening that shouldn't be, or what isn't happening that should.
     Examples might be: `NASM generates an error message saying Line 3
     for an error that's actually on Line 5'; `NASM generates an error
     message that I believe it shouldn't be generating at all'; `NASM
     fails to generate an error message that I believe it _should_ be
     generating'; `the object file produced from this source code
     crashes my linker'; `the ninth byte of the output file is 66 and I
     think it should be 77 instead'.

   * If you believe the output file from NASM to be faulty, send it to
     us. That allows us to determine whether our own copy of NASM
     generates the same file, or whether the problem is related to
     portability issues between our development platforms and yours. We
     can handle binary files mailed to us as MIME attachments,
     uuencoded, and even BinHex. Alternatively, we may be able to
     provide an FTP site you can upload the suspect files to; but
     mailing them is easier for us.

   * Any other information or data files that might be helpful. If, for
     example, the problem involves NASM failing to generate an object
     file while TASM can generate an equivalent file without trouble,
     then send us _both_ object files, so we can see what TASM is doing
     differently from us.


File: nasm.info,  Node: Appendix A,  Next: Section A.1,  Prev: Section 10.2,  Up: Top

Appendix A: Intel x86 Instruction Reference
*******************************************

   This appendix provides a complete list of the machine instructions
which NASM will assemble, and a short description of the function of
each one.

   It is not intended to be exhaustive documentation on the fine
details of the instructions' function, such as which exceptions they
can trigger: for such documentation, you should go to Intel's Web site,
`http://www.intel.com/'.

   Instead, this appendix is intended primarily to provide
documentation on the way the instructions may be used within NASM. For
example, looking up `LOOP' will tell you that NASM allows `CX' or `ECX'
to be specified as an optional second argument to the `LOOP'
instruction, to enforce which of the two possible counter registers
should be used if the default is not the one desired.

   The instructions are not quite listed in alphabetical order, since
groups of instructions with similar functions are lumped together in
the same entry. Most of them don't move very far from their alphabetic
position because of this.

* Menu:

* Section A.1:: Key to Operand Specifications
* Section A.2:: Key to Opcode Descriptions
* Section A.3:: Key to Instruction Flags
* Section A.4:: `AAA', `AAS', `AAM', `AAD': ASCII Adjustments
* Section A.5:: `ADC': Add with Carry
* Section A.6:: `ADD': Add Integers
* Section A.7:: `AND': Bitwise AND
* Section A.8:: `ARPL': Adjust RPL Field of Selector
* Section A.9:: `BOUND': Check Array Index against Bounds
* Section A.10:: `BSF', `BSR': Bit Scan
* Section A.11:: `BSWAP': Byte Swap
* Section A.12:: `BT', `BTC', `BTR', `BTS': Bit Test
* Section A.13:: `CALL': Call Subroutine
* Section A.14:: `CBW', `CWD', `CDQ', `CWDE': Sign Extensions
* Section A.15:: `CLC', `CLD', `CLI', `CLTS': Clear Flags
* Section A.16:: `CMC': Complement Carry Flag
* Section A.17:: `CMOVcc': Conditional Move
* Section A.18:: `CMP': Compare Integers
* Section A.19:: `CMPSB', `CMPSW', `CMPSD': Compare Strings
* Section A.20:: `CMPXCHG', `CMPXCHG486': Compare and Exchange
* Section A.21:: `CMPXCHG8B': Compare and Exchange Eight Bytes
* Section A.22:: `CPUID': Get CPU Identification Code
* Section A.23:: `DAA', `DAS': Decimal Adjustments
* Section A.24:: `DEC': Decrement Integer
* Section A.25:: `DIV': Unsigned Integer Divide
* Section A.26:: `EMMS': Empty MMX State
* Section A.27:: `ENTER': Create Stack Frame
* Section A.28:: `F2XM1': Calculate 2**X-1
* Section A.29:: `FABS': Floating-Point Absolute Value
* Section A.30:: `FADD', `FADDP': Floating-Point Addition
* Section A.31:: `FBLD', `FBSTP': BCD Floating-Point Load and Store
* Section A.32:: `FCHS': Floating-Point Change Sign
* Section A.33:: `FCLEX', {FNCLEX}: Clear Floating-Point Exceptions
* Section A.34:: `FCMOVcc': Floating-Point Conditional Move
* Section A.35:: `FCOM', `FCOMP', `FCOMPP', `FCOMI', `FCOMIP': Floating-Point Compare
* Section A.36:: `FCOS': Cosine
* Section A.37:: `FDECSTP': Decrement Floating-Point Stack Pointer
* Section A.38:: `FxDISI', `FxENI': Disable and Enable Floating-Point Interrupts
* Section A.39:: `FDIV', `FDIVP', `FDIVR', `FDIVRP': Floating-Point Division
* Section A.40:: `FFREE': Flag Floating-Point Register as Unused
* Section A.41:: `FIADD': Floating-Point/Integer Addition
* Section A.42:: `FICOM', `FICOMP': Floating-Point/Integer Compare
* Section A.43:: `FIDIV', `FIDIVR': Floating-Point/Integer Division
* Section A.44:: `FILD', `FIST', `FISTP': Floating-Point/Integer Conversion
* Section A.45:: `FIMUL': Floating-Point/Integer Multiplication
* Section A.46:: `FINCSTP': Increment Floating-Point Stack Pointer
* Section A.47:: `FINIT', `FNINIT': Initialise Floating-Point Unit
* Section A.48:: `FISUB': Floating-Point/Integer Subtraction
* Section A.49:: `FLD': Floating-Point Load
* Section A.50:: `FLDxx': Floating-Point Load Constants
* Section A.51:: `FLDCW': Load Floating-Point Control Word
* Section A.52:: `FLDENV': Load Floating-Point Environment
* Section A.53:: `FMUL', `FMULP': Floating-Point Multiply
* Section A.54:: `FNOP': Floating-Point No Operation
* Section A.55:: `FPATAN', `FPTAN': Arctangent and Tangent
* Section A.56:: `FPREM', `FPREM1': Floating-Point Partial Remainder
* Section A.57:: `FRNDINT': Floating-Point Round to Integer
* Section A.58:: `FSAVE', `FRSTOR': Save/Restore Floating-Point State
* Section A.59:: `FSCALE': Scale Floating-Point Value by Power of Two
* Section A.60:: `FSETPM': Set Protected Mode
* Section A.61:: `FSIN', `FSINCOS': Sine and Cosine
* Section A.62:: `FSQRT': Floating-Point Square Root
* Section A.63:: `FST', `FSTP': Floating-Point Store
* Section A.64:: `FSTCW': Store Floating-Point Control Word
* Section A.65:: `FSTENV': Store Floating-Point Environment
* Section A.66:: `FSTSW': Store Floating-Point Status Word
* Section A.67:: `FSUB', `FSUBP', `FSUBR', `FSUBRP': Floating-Point Subtract
* Section A.68:: `FTST': Test `ST0' Against Zero
* Section A.69:: `FUCOMxx': Floating-Point Unordered Compare
* Section A.70:: `FXAM': Examine Class of Value in `ST0'
* Section A.71:: `FXCH': Floating-Point Exchange
* Section A.72:: `FXTRACT': Extract Exponent and Significand
* Section A.73:: `FYL2X', `FYL2XP1': Compute Y times Log2(X) or Log2(X+1)
* Section A.74:: `HLT': Halt Processor
* Section A.75:: `IBTS': Insert Bit String
* Section A.76:: `IDIV': Signed Integer Divide
* Section A.77:: `IMUL': Signed Integer Multiply
* Section A.78:: `IN': Input from I/O Port
* Section A.79:: `INC': Increment Integer
* Section A.80:: `INSB', `INSW', `INSD': Input String from I/O Port
* Section A.81:: `INT': Software Interrupt
* Section A.82:: `INT3', `INT1', `ICEBP', `INT01': Breakpoints
* Section A.83:: `INTO': Interrupt if Overflow
* Section A.84:: `INVD': Invalidate Internal Caches
* Section A.85:: `INVLPG': Invalidate TLB Entry
* Section A.86:: `IRET', `IRETW', `IRETD': Return from Interrupt
* Section A.87:: `JCXZ', `JECXZ': Jump if CX/ECX Zero
* Section A.88:: `JMP': Jump
* Section A.89:: `Jcc': Conditional Branch
* Section A.90:: `LAHF': Load AH from Flags
* Section A.91:: `LAR': Load Access Rights
* Section A.92:: `LDS', `LES', `LFS', `LGS', `LSS': Load Far Pointer
* Section A.93:: `LEA': Load Effective Address
* Section A.94:: `LEAVE': Destroy Stack Frame
* Section A.95:: `LGDT', `LIDT', `LLDT': Load Descriptor Tables
* Section A.96:: `LMSW': Load/Store Machine Status Word
* Section A.97:: `LOADALL', `LOADALL286': Load Processor State
* Section A.98:: `LODSB', `LODSW', `LODSD': Load from String
* Section A.99:: `LOOP', `LOOPE', `LOOPZ', `LOOPNE', `LOOPNZ': Loop with Counter
* Section A.100:: `LSL': Load Segment Limit
* Section A.101:: `LTR': Load Task Register
* Section A.102:: `MOV': Move Data
* Section A.103:: `MOVD': Move Doubleword to/from MMX Register
* Section A.104:: `MOVQ': Move Quadword to/from MMX Register
* Section A.105:: `MOVSB', `MOVSW', `MOVSD': Move String
* Section A.106:: `MOVSX', `MOVZX': Move Data with Sign or Zero Extend
* Section A.107:: `MUL': Unsigned Integer Multiply
* Section A.108:: `NEG', `NOT': Two's and One's Complement
* Section A.109:: `NOP': No Operation
* Section A.110:: `OR': Bitwise OR
* Section A.111:: `OUT': Output Data to I/O Port
* Section A.112:: `OUTSB', `OUTSW', `OUTSD': Output String to I/O Port
* Section A.113:: `PACKSSDW', `PACKSSWB', `PACKUSWB': Pack Data
* Section A.114:: `PADDxx': MMX Packed Addition
* Section A.115:: `PADDSIW': MMX Packed Addition to Implicit Destination
* Section A.116:: `PAND', `PANDN': MMX Bitwise AND and AND-NOT
* Section A.117:: `PAVEB': MMX Packed Average
* Section A.118:: `PCMPxx': MMX Packed Comparison
* Section A.119:: `PDISTIB': MMX Packed Distance and Accumulate with Implied Register
* Section A.120:: `PMACHRIW': MMX Packed Multiply and Accumulate with Rounding
* Section A.121:: `PMADDWD': MMX Packed Multiply and Add
* Section A.122:: `PMAGW': MMX Packed Magnitude
* Section A.123:: `PMULHRW', `PMULHRIW': MMX Packed Multiply High with Rounding
* Section A.124:: `PMULHW', `PMULLW': MMX Packed Multiply
* Section A.125:: `PMVccZB': MMX Packed Conditional Move
* Section A.126:: `POP': Pop Data from Stack
* Section A.127:: `POPAx': Pop All General-Purpose Registers
* Section A.128:: `POPFx': Pop Flags Register
* Section A.129:: `POR': MMX Bitwise OR
* Section A.130:: `PSLLx', `PSRLx', `PSRAx': MMX Bit Shifts
* Section A.131:: `PSUBxx': MMX Packed Subtraction
* Section A.132:: `PSUBSIW': MMX Packed Subtract with Saturation to Implied Destination
* Section A.133:: `PUNPCKxxx': Unpack Data
* Section A.134:: `PUSH': Push Data on Stack
* Section A.135:: `PUSHAx': Push All General-Purpose Registers
* Section A.136:: `PUSHFx': Push Flags Register
* Section A.137:: `PXOR': MMX Bitwise XOR
* Section A.138:: `RCL', `RCR': Bitwise Rotate through Carry Bit
* Section A.139:: `RDMSR': Read Model-Specific Registers
* Section A.140:: `RDPMC': Read Performance-Monitoring Counters
* Section A.141:: `RDTSC': Read Time-Stamp Counter
* Section A.142:: `RET', `RETF', `RETN': Return from Procedure Call
* Section A.143:: `ROL', `ROR': Bitwise Rotate
* Section A.144:: `RSM': Resume from System-Management Mode
* Section A.145:: `SAHF': Store AH to Flags
* Section A.146:: `SAL', `SAR': Bitwise Arithmetic Shifts
* Section A.147:: `SALC': Set AL from Carry Flag
* Section A.148:: `SBB': Subtract with Borrow
* Section A.149:: `SCASB', `SCASW', `SCASD': Scan String
* Section A.150:: `SETcc': Set Register from Condition
* Section A.151:: `SGDT', `SIDT', `SLDT': Store Descriptor Table Pointers
* Section A.152:: `SHL', `SHR': Bitwise Logical Shifts
* Section A.153:: `SHLD', `SHRD': Bitwise Double-Precision Shifts
* Section A.154:: `SMI': System Management Interrupt
* Section A.155:: `SMSW': Store Machine Status Word
* Section A.156:: `STC', `STD', `STI': Set Flags
* Section A.157:: `STOSB', `STOSW', `STOSD': Store Byte to String
* Section A.158:: `STR': Store Task Register
* Section A.159:: `SUB': Subtract Integers
* Section A.160:: `TEST': Test Bits (notional bitwise AND)
* Section A.161:: `UMOV': User Move Data
* Section A.162:: `VERR', `VERW': Verify Segment Readability/Writability
* Section A.163:: `WAIT': Wait for Floating-Point Processor
* Section A.164:: `WBINVD': Write Back and Invalidate Cache
* Section A.165:: `WRMSR': Write Model-Specific Registers
* Section A.166:: `XADD': Exchange and Add
* Section A.167:: `XBTS': Extract Bit String
* Section A.168:: `XCHG': Exchange
* Section A.169:: `XLATB': Translate Byte in Lookup Table
* Section A.170:: `XOR': Bitwise Exclusive OR


File: nasm.info,  Node: Section A.1,  Next: Section A.2,  Prev: Appendix A,  Up: Appendix A

A.1. Key to Operand Specifications
**********************************

   The instruction descriptions in this appendix specify their operands
using the following notation:

   * Registers: `reg8' denotes an 8-bit general purpose register,
     `reg16' denotes a 16-bit general purpose register, and `reg32' a
     32-bit one. `fpureg' denotes one of the eight FPU stack registers,
     `mmxreg' denotes one of the eight 64-bit MMX registers, and
     `segreg' denotes a segment register. In addition, some registers
     (such as `AL', `DX' or `ECX') may be specified explicitly.

   * Immediate operands: `imm' denotes a generic immediate operand.
     `imm8', `imm16' and `imm32' are used when the operand is intended
     to be a specific size. For some of these instructions, NASM needs
     an explicit specifier: for example, `ADD ESP,16' could be
     interpreted as either `ADD r/m32,imm32' or `ADD r/m32,imm8'. NASM
     chooses the former by default, and so you must specify `ADD
     ESP,BYTE 16' for the latter.

   * Memory references: `mem' denotes a generic memory reference;
     `mem8', `mem16', `mem32', `mem64' and `mem80' are used when the
     operand needs to be a specific size. Again, a specifier is needed
     in some cases: `DEC [address]' is ambiguous and will be rejected
     by NASM. You must specify `DEC BYTE [address]', `DEC WORD
     [address]' or `DEC DWORD [address]' instead.

   * Restricted memory references: one form of the `MOV' instruction
     allows a memory address to be specified _without_ allowing the
     normal range of register combinations and effective address
     processing. This is denoted by `memoffs8', `memoffs16' and
     `memoffs32'.

   * Register or memory choices: many instructions can accept either a
     register _or_ a memory reference as an operand. `r/m8' is a
     shorthand for `reg8/mem8'; similarly `r/m16' and `r/m32'. `r/m64'
     is MMX-related, and is a shorthand for `mmxreg/mem64'.


File: nasm.info,  Node: Section A.2,  Next: Section A.2.1,  Prev: Section A.1,  Up: Appendix A

A.2. Key to Opcode Descriptions
*******************************

   This appendix also provides the opcodes which NASM will generate for
each form of each instruction. The opcodes are listed in the following
way:

   * A hex number, such as `3F', indicates a fixed byte containing that
     number.

   * A hex number followed by `+r', such as `C8+r', indicates that one
     of the operands to the instruction is a register, and the
     `register value' of that register should be added to the hex
     number to produce the generated byte. For example, EDX has
     register value 2, so the code `C8+r', when the register operand is
     EDX, generates the hex byte `CA'. Register values for specific
     registers are given in *Note Section A.2.1::.

   * A hex number followed by `+cc', such as `40+cc', indicates that
     the instruction name has a condition code suffix, and the numeric
     representation of the condition code should be added to the hex
     number to produce the generated byte. For example, the code
     `40+cc', when the instruction contains the `NE' condition,
     generates the hex byte `45'. Condition codes and their numeric
     representations are given in *Note Section A.2.2::.

   * A slash followed by a digit, such as `/2', indicates that one of
     the operands to the instruction is a memory address or register
     (denoted `mem' or `r/m', with an optional size). This is to be
     encoded as an effective address, with a ModR/M byte, an optional
     SIB byte, and an optional displacement, and the spare (register)
     field of the ModR/M byte should be the digit given (which will be
     from 0 to 7, so it fits in three bits). The encoding of effective
     addresses is given in *Note Section A.2.3::.

   * The code `/r' combines the above two: it indicates that one of the
     operands is a memory address or `r/m', and another is a register,
     and that an effective address should be generated with the spare
     (register) field in the ModR/M byte being equal to the `register
     value' of the register operand. The encoding of effective
     addresses is given in *Note Section A.2.3::; register values are
     given in *Note Section A.2.1::.

   * The codes `ib', `iw' and `id' indicate that one of the operands to
     the instruction is an immediate value, and that this is to be
     encoded as a byte, little-endian word or little-endian doubleword
     respectively.

   * The codes `rb', `rw' and `rd' indicate that one of the operands to
     the instruction is an immediate value, and that the _difference_
     between this value and the address of the end of the instruction
     is to be encoded as a byte, word or doubleword respectively.
     Where the form `rw/rd' appears, it indicates that either `rw' or
     `rd' should be used according to whether assembly is being
     performed in `BITS 16' or `BITS 32' state respectively.

   * The codes `ow' and `od' indicate that one of the operands to the
     instruction is a reference to the contents of a memory address
     specified as an immediate value: this encoding is used in some
     forms of the `MOV' instruction in place of the standard
     effective-address mechanism. The displacement is encoded as a word
     or doubleword. Again, `ow/od' denotes that `ow' or `od' should be
     chosen according to the `BITS' setting.

   * The codes `o16' and `o32' indicate that the given form of the
     instruction should be assembled with operand size 16 or 32 bits.
     In other words, `o16' indicates a `66' prefix in `BITS 32' state,
     but generates no code in `BITS 16' state; and `o32' indicates a
     `66' prefix in `BITS 16' state but generates nothing in `BITS 32'.

   * The codes `a16' and `a32', similarly to `o16' and `o32', indicate
     the address size of the given form of the instruction.  Where this
     does not match the `BITS' setting, a `67' prefix is required.

* Menu:

* Section A.2.1:: Register Values
* Section A.2.2:: Condition Codes
* Section A.2.3:: Effective Address Encoding: ModR/M and SIB


File: nasm.info,  Node: Section A.2.1,  Next: Section A.2.2,  Prev: Section A.2,  Up: Section A.2

A.2.1. Register Values
**********************

   Where an instruction requires a register value, it is already
implicit in the encoding of the rest of the instruction what type of
register is intended: an 8-bit general-purpose register, a segment
register, a debug register, an MMX register, or whatever. Therefore
there is no problem with registers of different types sharing an
encoding value.

   The encodings for the various classes of register are:

   * 8-bit general registers: `AL' is 0, `CL' is 1, `DL' is 2, `BL' is
     3, `AH' is 4, `CH' is 5, `DH' is 6, and `BH' is 7.

   * 16-bit general registers: `AX' is 0, `CX' is 1, `DX' is 2, `BX' is
     3, `SP' is 4, `BP' is 5, `SI' is 6, and `DI' is 7.

   * 32-bit general registers: `EAX' is 0, `ECX' is 1, `EDX' is 2,
     `EBX' is 3, `ESP' is 4, `EBP' is 5, `ESI' is 6, and `EDI' is 7.

   * Segment registers: `ES' is 0, `CS' is 1, `SS' is 2, `DS' is 3,
     `FS' is 4, and `GS' is 5.

   * {Floating-point registers}: `ST0' is 0, `ST1' is 1, `ST2' is 2,
     `ST3' is 3, `ST4' is 4, `ST5' is 5, `ST6' is 6, and `ST7' is 7.

   * 64-bit MMX registers: `MM0' is 0, `MM1' is 1, `MM2' is 2, `MM3' is
     3, `MM4' is 4, `MM5' is 5, `MM6' is 6, and `MM7' is 7.

   * Control registers: `CR0' is 0, `CR2' is 2, `CR3' is 3, and `CR4'
     is 4.

   * Debug registers: `DR0' is 0, `DR1' is 1, `DR2' is 2, `DR3' is 3,
     `DR6' is 6, and `DR7' is 7.

   * Test registers: `TR3' is 3, `TR4' is 4, `TR5' is 5, `TR6' is 6,
     and `TR7' is 7.

   (Note that wherever a register name contains a number, that number
is also the register value for that register.)


File: nasm.info,  Node: Section A.2.2,  Next: Section A.2.3,  Prev: Section A.2.1,  Up: Section A.2

A.2.2. Condition Codes
**********************

   The available condition codes are given here, along with their
numeric representations as part of opcodes. Many of these condition
codes have synonyms, so several will be listed at a time.

   In the following descriptions, the word `either', when applied to two
possible trigger conditions, is used to mean `either or both'. If
`either but not both' is meant, the phrase `exactly one of' is used.

   * `O' is 0 (trigger if the overflow flag is set); `NO' is 1.

   * `B', `C' and `NAE' are 2 (trigger if the carry flag is set); `AE',
     `NB' and `NC' are 3.

   * `E' and `Z' are 4 (trigger if the zero flag is set); `NE' and `NZ'
     are 5.

   * `BE' and `NA' are 6 (trigger if either of the carry or zero flags
     is set); `A' and `NBE' are 7.

   * `S' is 8 (trigger if the sign flag is set); `NS' is 9.

   * `P' and `PE' are 10 (trigger if the parity flag is set); `NP' and
     `PO' are 11.

   * `L' and `NGE' are 12 (trigger if exactly one of the sign and
     overflow flags is set); `GE' and `NL' are 13.

   * `LE' and `NG' are 14 (trigger if either the zero flag is set, or
     exactly one of the sign and overflow flags is set); `G' and `NLE'
     are 15.

   Note that in all cases, the sense of a condition code may be
reversed by changing the low bit of the numeric representation.


File: nasm.info,  Node: Section A.2.3,  Next: Section A.3,  Prev: Section A.2.2,  Up: Section A.2

A.2.3. Effective Address Encoding: ModR/M and SIB
*************************************************

   An effective address is encoded in up to three parts: a ModR/M byte,
an optional SIB byte, and an optional byte, word or doubleword
displacement field.

   The ModR/M byte consists of three fields: the `mod' field, ranging
from 0 to 3, in the upper two bits of the byte, the `r/m' field,
ranging from 0 to 7, in the lower three bits, and the spare (register)
field in the middle (bit 3 to bit 5). The spare field is not relevant to
the effective address being encoded, and either contains an extension to
the instruction opcode or the register value of another operand.

   The ModR/M system can be used to encode a direct register reference
rather than a memory access. This is always done by setting the `mod'
field to 3 and the `r/m' field to the register value of the register in
question (it must be a general-purpose register, and the size of the
register must already be implicit in the encoding of the rest of the
instruction). In this case, the SIB byte and displacement field are both
absent.

   In 16-bit addressing mode (either `BITS 16' with no `67' prefix, or
`BITS 32' with a `67' prefix), the SIB byte is never used. The general
rules for `mod' and `r/m' (there is an exception, given below) are:

   * The `mod' field gives the length of the displacement field: 0
     means no displacement, 1 means one byte, and 2 means two bytes.

   * The `r/m' field encodes the combination of registers to be added to
     the displacement to give the accessed address: 0 means `BX+SI', 1
     means `BX+DI', 2 means `BP+SI', 3 means `BP+DI', 4 means `SI'
     only, 5 means `DI' only, 6 means `BP' only, and 7 means `BX' only.

   However, there is a special case:

   * If `mod' is 0 and `r/m' is 6, the effective address encoded is not
     `[BP]' as the above rules would suggest, but instead `[disp16]':
     the displacement field is present and is two bytes long, and no
     registers are added to the displacement.

   Therefore the effective address `[BP]' cannot be encoded as
efficiently as `[BX]'; so if you code `[BP]' in a program, NASM adds a
notional 8-bit zero displacement, and sets `mod' to 1, `r/m' to 6, and
the one-byte displacement field to 0.

   In 32-bit addressing mode (either `BITS 16' with a `67' prefix, or
`BITS 32' with no `67' prefix) the general rules (again, there are
exceptions) for `mod' and `r/m' are:

   * The `mod' field gives the length of the displacement field: 0
     means no displacement, 1 means one byte, and 2 means four bytes.

   * If only one register is to be added to the displacement, and it is
     not `ESP', the `r/m' field gives its register value, and the SIB
     byte is absent. If the `r/m' field is 4 (which would encode `ESP'),
     the SIB byte is present and gives the combination and scaling of
     registers to be added to the displacement.

   If the SIB byte is present, it describes the combination of
registers (an optional base register, and an optional index register
scaled by multiplication by 1, 2, 4 or 8) to be added to the
displacement. The SIB byte is divided into the `scale' field, in the
top two bits, the `index' field in the next three, and the `base' field
in the bottom three. The general rules are:

   * The `base' field encodes the register value of the base register.

   * The `index' field encodes the register value of the index register,
     unless it is 4, in which case no index register is used (so `ESP'
     cannot be used as an index register).

   * The `scale' field encodes the multiplier by which the index
     register is scaled before adding it to the base and displacement:
     0 encodes a multiplier of 1, 1 encodes 2, 2 encodes 4 and 3
     encodes 8.

   The exceptions to the 32-bit encoding rules are:

   * If `mod' is 0 and `r/m' is 5, the effective address encoded is not
     `[EBP]' as the above rules would suggest, but instead `[disp32]':
     the displacement field is present and is four bytes long, and no
     registers are added to the displacement.

   * If `mod' is 0, `r/m' is 4 (meaning the SIB byte is present) and
     `base' is 4, the effective address encoded is not `[EBP+index]' as
     the above rules would suggest, but instead `[disp32+index]': the
     displacement field is present and is four bytes long, and there is
     no base register (but the index register is still processed in the
     normal way).


File: nasm.info,  Node: Section A.3,  Next: Section A.4,  Prev: Section A.2.3,  Up: Appendix A

A.3. Key to Instruction Flags
*****************************

   Given along with each instruction in this appendix is a set of flags,
denoting the type of the instruction. The types are as follows:

   * `8086', `186', `286', `386', `486', `PENT' and `P6' denote the
     lowest processor type that supports the instruction. Most
     instructions run on all processors above the given type; those
     that do not are documented. The Pentium II contains no additional
     instructions beyond the P6 (Pentium Pro); from the point of view
     of its instruction set, it can be thought of as a P6 with MMX
     capability.

   * `CYRIX' indicates that the instruction is specific to Cyrix
     processors, for example the extra MMX instructions in the Cyrix
     extended MMX instruction set.

   * `FPU' indicates that the instruction is a floating-point one, and
     will only run on machines with a coprocessor (automatically
     including 486DX, Pentium and above).

   * `MMX' indicates that the instruction is an MMX one, and will run on
     MMX-capable Pentium processors and the Pentium II.

   * `PRIV' indicates that the instruction is a protected-mode
     management instruction. Many of these may only be used in
     protected mode, or only at privilege level zero.

   * `UNDOC' indicates that the instruction is an undocumented one, and
     not part of the official Intel Architecture; it may or may not be
     supported on any given machine.


File: nasm.info,  Node: Section A.4,  Next: Section A.5,  Prev: Section A.3,  Up: Appendix A

A.4. `AAA', `AAS', `AAM', `AAD': ASCII Adjustments
**************************************************

     AAA                           ; 37                   [8086]

     AAS                           ; 3F                   [8086]

     AAD                           ; D5 0A                [8086]
     AAD imm                       ; D5 ib                [8086]

     AAM                           ; D4 0A                [8086]
     AAM imm                       ; D4 ib                [8086]

   These instructions are used in conjunction with the add, subtract,
multiply and divide instructions to perform binary-coded decimal
arithmetic in _unpacked_ (one BCD digit per byte - easy to translate to
and from ASCII, hence the instruction names) form. There are also
packed BCD instructions `DAA' and `DAS': see *Note Section A.23::.

   `AAA' should be used after a one-byte `ADD' instruction whose
destination was the `AL' register: by means of examining the value in
the low nibble of `AL' and also the auxiliary carry flag `AF', it
determines whether the addition has overflowed, and adjusts it (and sets
the carry flag) if so. You can add long BCD strings together by doing
`ADD'/`AAA' on the low digits, then doing `ADC'/`AAA' on each
subsequent digit.

   `AAS' works similarly to `AAA', but is for use after `SUB'
instructions rather than `ADD'.

   `AAM' is for use after you have multiplied two decimal digits
together and left the result in `AL': it divides `AL' by ten and stores
the quotient in `AH', leaving the remainder in `AL'. The divisor 10 can
be changed by specifying an operand to the instruction: a particularly
handy use of this is `AAM 16', causing the two nibbles in `AL' to be
separated into `AH' and `AL'.

   `AAD' performs the inverse operation to `AAM': it multiplies `AH' by
ten, adds it to `AL', and sets `AH' to zero. Again, the multiplier 10
can be changed.


File: nasm.info,  Node: Section A.5,  Next: Section A.6,  Prev: Section A.4,  Up: Appendix A

A.5. `ADC': Add with Carry
**************************

     ADC r/m8,reg8                 ; 10 /r                [8086]
     ADC r/m16,reg16               ; o16 11 /r            [8086]
     ADC r/m32,reg32               ; o32 11 /r            [386]

     ADC reg8,r/m8                 ; 12 /r                [8086]
     ADC reg16,r/m16               ; o16 13 /r            [8086]
     ADC reg32,r/m32               ; o32 13 /r            [386]

     ADC r/m8,imm8                 ; 80 /2 ib             [8086]
     ADC r/m16,imm16               ; o16 81 /2 iw         [8086]
     ADC r/m32,imm32               ; o32 81 /2 id         [386]

     ADC r/m16,imm8                ; o16 83 /2 ib         [8086]
     ADC r/m32,imm8                ; o32 83 /2 ib         [386]

     ADC AL,imm8                   ; 14 ib                [8086]
     ADC AX,imm16                  ; o16 15 iw            [8086]
     ADC EAX,imm32                 ; o32 15 id            [386]

   `ADC' performs integer addition: it adds its two operands together,
plus the value of the carry flag, and leaves the result in its
destination (first) operand. The flags are set according to the result
of the operation: in particular, the carry flag is affected and can be
used by a subsequent `ADC' instruction.

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.

   To add two numbers without also adding the contents of the carry
flag, use `ADD' (*Note Section A.6::).


File: nasm.info,  Node: Section A.6,  Next: Section A.7,  Prev: Section A.5,  Up: Appendix A

A.6. `ADD': Add Integers
************************

     ADD r/m8,reg8                 ; 00 /r                [8086]
     ADD r/m16,reg16               ; o16 01 /r            [8086]
     ADD r/m32,reg32               ; o32 01 /r            [386]

     ADD reg8,r/m8                 ; 02 /r                [8086]
     ADD reg16,r/m16               ; o16 03 /r            [8086]
     ADD reg32,r/m32               ; o32 03 /r            [386]

     ADD r/m8,imm8                 ; 80 /0 ib             [8086]
     ADD r/m16,imm16               ; o16 81 /0 iw         [8086]
     ADD r/m32,imm32               ; o32 81 /0 id         [386]

     ADD r/m16,imm8                ; o16 83 /0 ib         [8086]
     ADD r/m32,imm8                ; o32 83 /0 ib         [386]

     ADD AL,imm8                   ; 04 ib                [8086]
     ADD AX,imm16                  ; o16 05 iw            [8086]
     ADD EAX,imm32                 ; o32 05 id            [386]

   `ADD' performs integer addition: it adds its two operands together,
and leaves the result in its destination (first) operand. The flags are
set according to the result of the operation: in particular, the carry
flag is affected and can be used by a subsequent `ADC' instruction
(*Note Section A.5::).

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.


File: nasm.info,  Node: Section A.7,  Next: Section A.8,  Prev: Section A.6,  Up: Appendix A

A.7. `AND': Bitwise AND
***********************

     AND r/m8,reg8                 ; 20 /r                [8086]
     AND r/m16,reg16               ; o16 21 /r            [8086]
     AND r/m32,reg32               ; o32 21 /r            [386]

     AND reg8,r/m8                 ; 22 /r                [8086]
     AND reg16,r/m16               ; o16 23 /r            [8086]
     AND reg32,r/m32               ; o32 23 /r            [386]

     AND r/m8,imm8                 ; 80 /4 ib             [8086]
     AND r/m16,imm16               ; o16 81 /4 iw         [8086]
     AND r/m32,imm32               ; o32 81 /4 id         [386]

     AND r/m16,imm8                ; o16 83 /4 ib         [8086]
     AND r/m32,imm8                ; o32 83 /4 ib         [386]

     AND AL,imm8                   ; 24 ib                [8086]
     AND AX,imm16                  ; o16 25 iw            [8086]
     AND EAX,imm32                 ; o32 25 id            [386]

   `AND' performs a bitwise AND operation between its two operands (i.e.
each bit of the result is 1 if and only if the corresponding bits of the
two inputs were both 1), and stores the result in the destination
(first) operand.

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.

   The MMX instruction `PAND' (see *Note Section A.116::) performs the
same operation on the 64-bit MMX registers.


File: nasm.info,  Node: Section A.8,  Next: Section A.9,  Prev: Section A.7,  Up: Appendix A

A.8. `ARPL': Adjust RPL Field of Selector
*****************************************

     ARPL r/m16,reg16              ; 63 /r                [286,PRIV]

   `ARPL' expects its two word operands to be segment selectors. It
adjusts the RPL (requested privilege level - stored in the bottom two
bits of the selector) field of the destination (first) operand to
ensure that it is no less (i.e. no more privileged than) the RPL field
of the source operand. The zero flag is set if and only if a change had
to be made.


File: nasm.info,  Node: Section A.9,  Next: Section A.10,  Prev: Section A.8,  Up: Appendix A

A.9. `BOUND': Check Array Index against Bounds
**********************************************

     BOUND reg16,mem               ; o16 62 /r            [186]
     BOUND reg32,mem               ; o32 62 /r            [386]

   `BOUND' expects its second operand to point to an area of memory
containing two signed values of the same size as its first operand (i.e.
two words for the 16-bit form; two doublewords for the 32-bit form). It
performs two signed comparisons: if the value in the register passed as
its first operand is less than the first of the in-memory values, or is
greater than or equal to the second, it throws a BR exception.
Otherwise, it does nothing.


File: nasm.info,  Node: Section A.10,  Next: Section A.11,  Prev: Section A.9,  Up: Appendix A

A.10. `BSF', `BSR': Bit Scan
****************************

     BSF reg16,r/m16               ; o16 0F BC /r         [386]
     BSF reg32,r/m32               ; o32 0F BC /r         [386]

     BSR reg16,r/m16               ; o16 0F BD /r         [386]
     BSR reg32,r/m32               ; o32 0F BD /r         [386]

   `BSF' searches for a set bit in its source (second) operand, starting
from the bottom, and if it finds one, stores the index in its
destination (first) operand. If no set bit is found, the contents of
the destination operand are undefined.

   `BSR' performs the same function, but searches from the top instead,
so it finds the most significant set bit.

   Bit indices are from 0 (least significant) to 15 or 31 (most
significant).


File: nasm.info,  Node: Section A.11,  Next: Section A.12,  Prev: Section A.10,  Up: Appendix A

A.11. `BSWAP': Byte Swap
************************

     BSWAP reg32                   ; o32 0F C8+r          [486]

   `BSWAP' swaps the order of the four bytes of a 32-bit register: bits
0-7 exchange places with bits 24-31, and bits 8-15 swap with bits 16-23.
There is no explicit 16-bit equivalent: to byte-swap `AX', `BX', `CX'
or `DX', `XCHG' can be used.


File: nasm.info,  Node: Section A.12,  Next: Section A.13,  Prev: Section A.11,  Up: Appendix A

A.12. `BT', `BTC', `BTR', `BTS': Bit Test
*****************************************

     BT r/m16,reg16                ; o16 0F A3 /r         [386]
     BT r/m32,reg32                ; o32 0F A3 /r         [386]
     BT r/m16,imm8                 ; o16 0F BA /4 ib      [386]
     BT r/m32,imm8                 ; o32 0F BA /4 ib      [386]

     BTC r/m16,reg16               ; o16 0F BB /r         [386]
     BTC r/m32,reg32               ; o32 0F BB /r         [386]
     BTC r/m16,imm8                ; o16 0F BA /7 ib      [386]
     BTC r/m32,imm8                ; o32 0F BA /7 ib      [386]

     BTR r/m16,reg16               ; o16 0F B3 /r         [386]
     BTR r/m32,reg32               ; o32 0F B3 /r         [386]
     BTR r/m16,imm8                ; o16 0F BA /6 ib      [386]
     BTR r/m32,imm8                ; o32 0F BA /6 ib      [386]

     BTS r/m16,reg16               ; o16 0F AB /r         [386]
     BTS r/m32,reg32               ; o32 0F AB /r         [386]
     BTS r/m16,imm                 ; o16 0F BA /5 ib      [386]
     BTS r/m32,imm                 ; o32 0F BA /5 ib      [386]

   These instructions all test one bit of their first operand, whose
index is given by the second operand, and store the value of that bit
into the carry flag. Bit indices are from 0 (least significant) to 15
or 31 (most significant).

   In addition to storing the original value of the bit into the carry
flag, `BTR' also resets (clears) the bit in the operand itself. `BTS'
sets the bit, and `BTC' complements the bit. `BT' does not modify its
operands.

   The bit offset should be no greater than the size of the operand.


File: nasm.info,  Node: Section A.13,  Next: Section A.14,  Prev: Section A.12,  Up: Appendix A

A.13. `CALL': Call Subroutine
*****************************

     CALL imm                      ; E8 rw/rd             [8086]
     CALL imm:imm16                ; o16 9A iw iw         [8086]
     CALL imm:imm32                ; o32 9A id iw         [386]
     CALL FAR mem16                ; o16 FF /3            [8086]
     CALL FAR mem32                ; o32 FF /3            [386]
     CALL r/m16                    ; o16 FF /2            [8086]
     CALL r/m32                    ; o32 FF /2            [386]

   `CALL' calls a subroutine, by means of pushing the current
instruction pointer (`IP') and optionally `CS' as well on the stack,
and then jumping to a given address.

   `CS' is pushed as well as `IP' if and only if the call is a far
call, i.e. a destination segment address is specified in the
instruction.  The forms involving two colon-separated arguments are far
calls; so are the `CALL FAR mem' forms.

   You can choose between the two immediate far call forms (`CALL
imm:imm') by the use of the `WORD' and `DWORD' keywords: `CALL WORD
0x1234:0x5678') or `CALL DWORD 0x1234:0x56789abc'.

   The `CALL FAR mem' forms execute a far call by loading the
destination address out of memory. The address loaded consists of 16 or
32 bits of offset (depending on the operand size), and 16 bits of
segment. The operand size may be overridden using `CALL WORD FAR mem' or
`CALL DWORD FAR mem'.

   The `CALL r/m' forms execute a near call (within the same segment),
loading the destination address out of memory or out of a register. The
keyword `NEAR' may be specified, for clarity, in these forms, but is
not necessary. Again, operand size can be overridden using `CALL WORD
mem' or `CALL DWORD mem'.

   As a convenience, NASM does not require you to call a far procedure
symbol by coding the cumbersome `CALL SEG routine:routine', but instead
allows the easier synonym `CALL FAR routine'.

   The `CALL r/m' forms given above are near calls; NASM will accept the
`NEAR' keyword (e.g. `CALL NEAR [address]'), even though it is not
strictly necessary.


File: nasm.info,  Node: Section A.14,  Next: Section A.15,  Prev: Section A.13,  Up: Appendix A

A.14. `CBW', `CWD', `CDQ', `CWDE': Sign Extensions
**************************************************

     CBW                           ; o16 98               [8086]
     CWD                           ; o16 99               [8086]
     CDQ                           ; o32 99               [386]
     CWDE                          ; o32 98               [386]

   All these instructions sign-extend a short value into a longer one,
by replicating the top bit of the original value to fill the extended
one.

   `CBW' extends `AL' into `AX' by repeating the top bit of `AL' in
every bit of `AH'. `CWD' extends `AX' into `DX:AX' by repeating the top
bit of `AX' throughout `DX'.  `CWDE' extends `AX' into `EAX', and `CDQ'
extends `EAX' into `EDX:EAX'.


File: nasm.info,  Node: Section A.15,  Next: Section A.16,  Prev: Section A.14,  Up: Appendix A

A.15. `CLC', `CLD', `CLI', `CLTS': Clear Flags
**********************************************

     CLC                           ; F8                   [8086]
     CLD                           ; FC                   [8086]
     CLI                           ; FA                   [8086]
     CLTS                          ; 0F 06                [286,PRIV]

   These instructions clear various flags. `CLC' clears the carry flag;
`CLD' clears the direction flag; `CLI' clears the interrupt flag (thus
disabling interrupts); and `CLTS' clears the task-switched (`TS') flag
in `CR0'.

   To set the carry, direction, or interrupt flags, use the `STC',
`STD' and `STI' instructions (*Note Section A.156::). To invert the
carry flag, use `CMC' (*Note Section A.16::).


File: nasm.info,  Node: Section A.16,  Next: Section A.17,  Prev: Section A.15,  Up: Appendix A

A.16. `CMC': Complement Carry Flag
**********************************

     CMC                           ; F5                   [8086]

   `CMC' changes the value of the carry flag: if it was 0, it sets it to
1, and vice versa.


File: nasm.info,  Node: Section A.17,  Next: Section A.18,  Prev: Section A.16,  Up: Appendix A

A.17. `CMOVcc': Conditional Move
********************************

     CMOVcc reg16,r/m16            ; o16 0F 40+cc /r      [P6]
     CMOVcc reg32,r/m32            ; o32 0F 40+cc /r      [P6]

   `CMOV' moves its source (second) operand into its destination (first)
operand if the given condition code is satisfied; otherwise it does
nothing.

   For a list of condition codes, see *Note Section A.2.2::.

   Although the `CMOV' instructions are flagged `P6' above, they may
not be supported by all Pentium Pro processors; the `CPUID' instruction
(*Note Section A.22::) will return a bit which indicates whether
conditional moves are supported.


File: nasm.info,  Node: Section A.18,  Next: Section A.19,  Prev: Section A.17,  Up: Appendix A

A.18. `CMP': Compare Integers
*****************************

     CMP r/m8,reg8                 ; 38 /r                [8086]
     CMP r/m16,reg16               ; o16 39 /r            [8086]
     CMP r/m32,reg32               ; o32 39 /r            [386]

     CMP reg8,r/m8                 ; 3A /r                [8086]
     CMP reg16,r/m16               ; o16 3B /r            [8086]
     CMP reg32,r/m32               ; o32 3B /r            [386]

     CMP r/m8,imm8                 ; 80 /0 ib             [8086]
     CMP r/m16,imm16               ; o16 81 /0 iw         [8086]
     CMP r/m32,imm32               ; o32 81 /0 id         [386]

     CMP r/m16,imm8                ; o16 83 /0 ib         [8086]
     CMP r/m32,imm8                ; o32 83 /0 ib         [386]

     CMP AL,imm8                   ; 3C ib                [8086]
     CMP AX,imm16                  ; o16 3D iw            [8086]
     CMP EAX,imm32                 ; o32 3D id            [386]

   `CMP' performs a `mental' subtraction of its second operand from its
first operand, and affects the flags as if the subtraction had taken
place, but does not store the result of the subtraction anywhere.

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.


File: nasm.info,  Node: Section A.19,  Next: Section A.20,  Prev: Section A.18,  Up: Appendix A

A.19. `CMPSB', `CMPSW', `CMPSD': Compare Strings
************************************************

     CMPSB                         ; A6                   [8086]
     CMPSW                         ; o16 A7               [8086]
     CMPSD                         ; o32 A7               [386]

   `CMPSB' compares the byte at `[DS:SI]' or `[DS:ESI]' with the byte
at `[ES:DI]' or `[ES:EDI]', and sets the flags accordingly. It then
increments or decrements (depending on the direction flag: increments
if the flag is clear, decrements if it is set) `SI' and `DI' (or `ESI'
and `EDI').

   The registers used are `SI' and `DI' if the address size is 16 bits,
and `ESI' and `EDI' if it is 32 bits. If you need to use an address
size not equal to the current `BITS' setting, you can use an explicit
`a16' or `a32' prefix.

   The segment register used to load from `[SI]' or `[ESI]' can be
overridden by using a segment register name as a prefix (for example,
`es cmpsb'). The use of `ES' for the load from `[DI]' or `[EDI]' cannot
be overridden.

   `CMPSW' and `CMPSD' work in the same way, but they compare a word or
a doubleword instead of a byte, and increment or decrement the
addressing registers by 2 or 4 instead of 1.

   The `REPE' and `REPNE' prefixes (equivalently, `REPZ' and `REPNZ')
may be used to repeat the instruction up to `CX' (or `ECX' - again, the
address size chooses which) times until the first unequal or equal byte
is found.


File: nasm.info,  Node: Section A.20,  Next: Section A.21,  Prev: Section A.19,  Up: Appendix A

A.20. `CMPXCHG', `CMPXCHG486': Compare and Exchange
***************************************************

     CMPXCHG r/m8,reg8             ; 0F B0 /r             [PENT]
     CMPXCHG r/m16,reg16           ; o16 0F B1 /r         [PENT]
     CMPXCHG r/m32,reg32           ; o32 0F B1 /r         [PENT]

     CMPXCHG486 r/m8,reg8          ; 0F A6 /r             [486,UNDOC]
     CMPXCHG486 r/m16,reg16        ; o16 0F A7 /r         [486,UNDOC]
     CMPXCHG486 r/m32,reg32        ; o32 0F A7 /r         [486,UNDOC]

   These two instructions perform exactly the same operation; however,
apparently some (not all) 486 processors support it under a non-standard
opcode, so NASM provides the undocumented `CMPXCHG486' form to generate
the non-standard opcode.

   `CMPXCHG' compares its destination (first) operand to the value in
`AL', `AX' or `EAX' (depending on the size of the instruction). If they
are equal, it copies its source (second) operand into the destination
and sets the zero flag. Otherwise, it clears the zero flag and leaves
the destination alone.

   `CMPXCHG' is intended to be used for atomic operations in
multitasking or multiprocessor environments. To safely update a value
in shared memory, for example, you might load the value into `EAX',
load the updated value into `EBX', and then execute the instruction
`lock cmpxchg [value],ebx'. If `value' has not changed since being
loaded, it is updated with your desired new value, and the zero flag is
set to let you know it has worked. (The `LOCK' prefix prevents another
processor doing anything in the middle of this operation: it guarantees
atomicity.) However, if another processor has modified the value in
between your load and your attempted store, the store does not happen,
and you are notified of the failure by a cleared zero flag, so you can
go round and try again.


File: nasm.info,  Node: Section A.21,  Next: Section A.22,  Prev: Section A.20,  Up: Appendix A

A.21. `CMPXCHG8B': Compare and Exchange Eight Bytes
***************************************************

     CMPXCHG8B mem                 ; 0F C7 /1             [PENT]

   This is a larger and more unwieldy version of `CMPXCHG': it compares
the 64-bit (eight-byte) value stored at `[mem]' with the value in
`EDX:EAX'. If they are equal, it sets the zero flag and stores
`ECX:EBX' into the memory area. If they are unequal, it clears the zero
flag and leaves the memory area untouched.


File: nasm.info,  Node: Section A.22,  Next: Section A.23,  Prev: Section A.21,  Up: Appendix A

A.22. `CPUID': Get CPU Identification Code
******************************************

     CPUID                         ; 0F A2                [PENT]

   `CPUID' returns various information about the processor it is being
executed on. It fills the four registers `EAX', `EBX', `ECX' and `EDX'
with information, which varies depending on the input contents of `EAX'.

   `CPUID' also acts as a barrier to serialise instruction execution:
executing the `CPUID' instruction guarantees that all the effects
(memory modification, flag modification, register modification) of
previous instructions have been completed before the next instruction
gets fetched.

   The information returned is as follows:

   * If `EAX' is zero on input, `EAX' on output holds the maximum
     acceptable input value of `EAX', and `EBX:EDX:ECX' contain the
     string `"GenuineIntel"' (or not, if you have a clone processor).
     That is to say, `EBX' contains `"Genu"' (in NASM's own sense of
     character constants, described in *Note Section 3.4.2::), `EDX'
     contains `"ineI"' and `ECX' contains `"ntel"'.

   * If `EAX' is one on input, `EAX' on output contains version
     information about the processor, and `EDX' contains a set of
     feature flags, showing the presence and absence of various
     features. For example, bit 8 is set if the `CMPXCHG8B' instruction
     (*Note Section A.21::) is supported, bit 15 is set if the
     conditional move instructions (*Note Section A.17:: and *Note
     Section A.34::) are supported, and bit 23 is set if MMX
     instructions are supported.

   * If `EAX' is two on input, `EAX', `EBX', `ECX' and `EDX' all
     contain information about caches and TLBs (Translation Lookahead
     Buffers).

   For more information on the data returned from `CPUID', see the
documentation on Intel's web site.


File: nasm.info,  Node: Section A.23,  Next: Section A.24,  Prev: Section A.22,  Up: Appendix A

A.23. `DAA', `DAS': Decimal Adjustments
***************************************

     DAA                           ; 27                   [8086]
     DAS                           ; 2F                   [8086]

   These instructions are used in conjunction with the add and subtract
instructions to perform binary-coded decimal arithmetic in _packed_
(one BCD digit per nibble) form. For the unpacked equivalents, see
*Note Section A.4::.

   `DAA' should be used after a one-byte `ADD' instruction whose
destination was the `AL' register: by means of examining the value in
the `AL' and also the auxiliary carry flag `AF', it determines whether
either digit of the addition has overflowed, and adjusts it (and sets
the carry and auxiliary-carry flags) if so. You can add long BCD
strings together by doing `ADD'/`DAA' on the low two digits, then doing
`ADC'/`DAA' on each subsequent pair of digits.

   `DAS' works similarly to `DAA', but is for use after `SUB'
instructions rather than `ADD'.


File: nasm.info,  Node: Section A.24,  Next: Section A.25,  Prev: Section A.23,  Up: Appendix A

A.24. `DEC': Decrement Integer
******************************

     DEC reg16                     ; o16 48+r             [8086]
     DEC reg32                     ; o32 48+r             [386]
     DEC r/m8                      ; FE /1                [8086]
     DEC r/m16                     ; o16 FF /1            [8086]
     DEC r/m32                     ; o32 FF /1            [386]

   `DEC' subtracts 1 from its operand. It does _not_ affect the carry
flag: to affect the carry flag, use `SUB something,1' (see *Note
Section A.159::). See also `INC' (*Note Section A.79::).


File: nasm.info,  Node: Section A.25,  Next: Section A.26,  Prev: Section A.24,  Up: Appendix A

A.25. `DIV': Unsigned Integer Divide
************************************

     DIV r/m8                      ; F6 /6                [8086]
     DIV r/m16                     ; o16 F7 /6            [8086]
     DIV r/m32                     ; o32 F7 /6            [386]

   `DIV' performs unsigned integer division. The explicit operand
provided is the divisor; the dividend and destination operands are
implicit, in the following way:

   * For `DIV r/m8', `AX' is divided by the given operand; the quotient
     is stored in `AL' and the remainder in `AH'.

   * For `DIV r/m16', `DX:AX' is divided by the given operand; the
     quotient is stored in `AX' and the remainder in `DX'.

   * For `DIV r/m32', `EDX:EAX' is divided by the given operand; the
     quotient is stored in `EAX' and the remainder in `EDX'.

   Signed integer division is performed by the `IDIV' instruction: see
*Note Section A.76::.


File: nasm.info,  Node: Section A.26,  Next: Section A.27,  Prev: Section A.25,  Up: Appendix A

A.26. `EMMS': Empty MMX State
*****************************

     EMMS                          ; 0F 77                [PENT,MMX]

   `EMMS' sets the FPU tag word (marking which floating-point registers
are available) to all ones, meaning all registers are available for the
FPU to use. It should be used after executing MMX instructions and
before executing any subsequent floating-point operations.


File: nasm.info,  Node: Section A.27,  Next: Section A.28,  Prev: Section A.26,  Up: Appendix A

A.27. `ENTER': Create Stack Frame
*********************************

     ENTER imm,imm                 ; C8 iw ib             [186]

   `ENTER' constructs a stack frame for a high-level language procedure
call. The first operand (the `iw' in the opcode definition above refers
to the first operand) gives the amount of stack space to allocate for
local variables; the second (the `ib' above) gives the nesting level of
the procedure (for languages like Pascal, with nested procedures).

   The function of `ENTER', with a nesting level of zero, is equivalent
to

               PUSH EBP            ; or PUSH BP         in 16 bits
               MOV EBP,ESP         ; or MOV BP,SP       in 16 bits
               SUB ESP,operand1    ; or SUB SP,operand1 in 16 bits

   This creates a stack frame with the procedure parameters accessible
upwards from `EBP', and local variables accessible downwards from `EBP'.

   With a nesting level of one, the stack frame created is 4 (or 2)
bytes bigger, and the value of the final frame pointer `EBP' is
accessible in memory at `[EBP-4]'.

   This allows `ENTER', when called with a nesting level of two, to look
at the stack frame described by the _previous_ value of `EBP', find the
frame pointer at offset -4 from that, and push it along with its new
frame pointer, so that when a level-two procedure is called from within
a level-one procedure, `[EBP-4]' holds the frame pointer of the most
recent level-one procedure call and `[EBP-8]' holds that of the most
recent level-two call. And so on, for nesting levels up to 31.

   Stack frames created by `ENTER' can be destroyed by the `LEAVE'
instruction: see *Note Section A.94::.


File: nasm.info,  Node: Section A.28,  Next: Section A.29,  Prev: Section A.27,  Up: Appendix A

A.28. `F2XM1': Calculate 2**X-1
*******************************

     F2XM1                         ; D9 F0                [8086,FPU]

   `F2XM1' raises 2 to the power of `ST0', subtracts one, and stores
the result back into `ST0'. The initial contents of `ST0' must be a
number in the range -1 to +1.


File: nasm.info,  Node: Section A.29,  Next: Section A.30,  Prev: Section A.28,  Up: Appendix A

A.29. `FABS': Floating-Point Absolute Value
*******************************************

     FABS                          ; D9 E1                [8086,FPU]

   `FABS' computes the absolute value of `ST0', storing the result back
in `ST0'.


File: nasm.info,  Node: Section A.30,  Next: Section A.31,  Prev: Section A.29,  Up: Appendix A

A.30. `FADD', `FADDP': Floating-Point Addition
**********************************************

     FADD mem32                    ; D8 /0                [8086,FPU]
     FADD mem64                    ; DC /0                [8086,FPU]

     FADD fpureg                   ; D8 C0+r              [8086,FPU]
     FADD ST0,fpureg               ; D8 C0+r              [8086,FPU]

     FADD TO fpureg                ; DC C0+r              [8086,FPU]
     FADD fpureg,ST0               ; DC C0+r              [8086,FPU]

     FADDP fpureg                  ; DE C0+r              [8086,FPU]
     FADDP fpureg,ST0              ; DE C0+r              [8086,FPU]

   `FADD', given one operand, adds the operand to `ST0' and stores the
result back in `ST0'. If the operand has the `TO' modifier, the result
is stored in the register given rather than in `ST0'.

   `FADDP' performs the same function as `FADD TO', but pops the
register stack after storing the result.

   The given two-operand forms are synonyms for the one-operand forms.


File: nasm.info,  Node: Section A.31,  Next: Section A.32,  Prev: Section A.30,  Up: Appendix A

A.31. `FBLD', `FBSTP': BCD Floating-Point Load and Store
********************************************************

     FBLD mem80                    ; DF /4                [8086,FPU]
     FBSTP mem80                   ; DF /6                [8086,FPU]

   `FBLD' loads an 80-bit (ten-byte) packed binary-coded decimal number
from the given memory address, converts it to a real, and pushes it on
the register stack. `FBSTP' stores the value of `ST0', in packed BCD,
at the given address and then pops the register stack.


File: nasm.info,  Node: Section A.32,  Next: Section A.33,  Prev: Section A.31,  Up: Appendix A

A.32. `FCHS': Floating-Point Change Sign
****************************************

     FCHS                          ; D9 E0                [8086,FPU]

   `FCHS' negates the number in `ST0': negative numbers become
positive, and vice versa.


File: nasm.info,  Node: Section A.33,  Next: Section A.34,  Prev: Section A.32,  Up: Appendix A

A.33. `FCLEX', {FNCLEX}: Clear Floating-Point Exceptions
********************************************************

     FCLEX                         ; 9B DB E2             [8086,FPU]
     FNCLEX                        ; DB E2                [8086,FPU]

   `FCLEX' clears any floating-point exceptions which may be pending.
`FNCLEX' does the same thing but doesn't wait for previous floating-
point operations (including the _handling_ of pending exceptions) to
finish first.


File: nasm.info,  Node: Section A.34,  Next: Section A.35,  Prev: Section A.33,  Up: Appendix A

A.34. `FCMOVcc': Floating-Point Conditional Move
************************************************

     FCMOVB fpureg                 ; DA C0+r              [P6,FPU]
     FCMOVB ST0,fpureg             ; DA C0+r              [P6,FPU]

     FCMOVBE fpureg                ; DA D0+r              [P6,FPU]
     FCMOVBE ST0,fpureg            ; DA D0+r              [P6,FPU]

     FCMOVE fpureg                 ; DA C8+r              [P6,FPU]
     FCMOVE ST0,fpureg             ; DA C8+r              [P6,FPU]

     FCMOVNB fpureg                ; DB C0+r              [P6,FPU]
     FCMOVNB ST0,fpureg            ; DB C0+r              [P6,FPU]

     FCMOVNBE fpureg               ; DB D0+r              [P6,FPU]
     FCMOVNBE ST0,fpureg           ; DB D0+r              [P6,FPU]

     FCMOVNE fpureg                ; DB C8+r              [P6,FPU]
     FCMOVNE ST0,fpureg            ; DB C8+r              [P6,FPU]

     FCMOVNU fpureg                ; DB D8+r              [P6,FPU]
     FCMOVNU ST0,fpureg            ; DB D8+r              [P6,FPU]

     FCMOVU fpureg                 ; DA D8+r              [P6,FPU]
     FCMOVU ST0,fpureg             ; DA D8+r              [P6,FPU]

   The `FCMOV' instructions perform conditional move operations: each of
them moves the contents of the given register into `ST0' if its
condition is satisfied, and does nothing if not.

   The conditions are not the same as the standard condition codes used
with conditional jump instructions. The conditions `B', `BE', `NB',
`NBE', `E' and `NE' are exactly as normal, but none of the other
standard ones are supported. Instead, the condition `U' and its
counterpart `NU' are provided; the `U' condition is satisfied if the
last two floating-point numbers compared were _unordered_, i.e. they
were not equal but neither one could be said to be greater than the
other, for example if they were NaNs. (The flag state which signals
this is the setting of the parity flag: so the `U' condition is
notionally equivalent to `PE', and `NU' is equivalent to `PO'.)

   The `FCMOV' conditions test the main processor's status flags, not
the FPU status flags, so using `FCMOV' directly after `FCOM' will not
work. Instead, you should either use `FCOMI' which writes directly to
the main CPU flags word, or use `FSTSW' to extract the FPU flags.

   Although the `FCMOV' instructions are flagged `P6' above, they may
not be supported by all Pentium Pro processors; the `CPUID' instruction
(*Note Section A.22::) will return a bit which indicates whether
conditional moves are supported.


File: nasm.info,  Node: Section A.35,  Next: Section A.36,  Prev: Section A.34,  Up: Appendix A

A.35. `FCOM', `FCOMP', `FCOMPP', `FCOMI', `FCOMIP': Floating-Point Compare
**************************************************************************

     FCOM mem32                    ; D8 /2                [8086,FPU]
     FCOM mem64                    ; DC /2                [8086,FPU]
     FCOM fpureg                   ; D8 D0+r              [8086,FPU]
     FCOM ST0,fpureg               ; D8 D0+r              [8086,FPU]

     FCOMP mem32                   ; D8 /3                [8086,FPU]
     FCOMP mem64                   ; DC /3                [8086,FPU]
     FCOMP fpureg                  ; D8 D8+r              [8086,FPU]
     FCOMP ST0,fpureg              ; D8 D8+r              [8086,FPU]

     FCOMPP                        ; DE D9                [8086,FPU]

     FCOMI fpureg                  ; DB F0+r              [P6,FPU]
     FCOMI ST0,fpureg              ; DB F0+r              [P6,FPU]

     FCOMIP fpureg                 ; DF F0+r              [P6,FPU]
     FCOMIP ST0,fpureg             ; DF F0+r              [P6,FPU]

   `FCOM' compares `ST0' with the given operand, and sets the FPU flags
accordingly. `ST0' is treated as the left-hand side of the comparison,
so that the carry flag is set (for a `less-than' result) if `ST0' is
less than the given operand.

   `FCOMP' does the same as `FCOM', but pops the register stack
afterwards. `FCOMPP' compares `ST0' with `ST1' and then pops the
register stack twice.

   `FCOMI' and `FCOMIP' work like the corresponding forms of `FCOM' and
`FCOMP', but write their results directly to the CPU flags register
rather than the FPU status word, so they can be immediately followed by
conditional jump or conditional move instructions.

   The `FCOM' instructions differ from the `FUCOM' instructions (*Note
Section A.69::) only in the way they handle quiet NaNs: `FUCOM' will
handle them silently and set the condition code flags to an `unordered'
result, whereas `FCOM' will generate an exception.


File: nasm.info,  Node: Section A.36,  Next: Section A.37,  Prev: Section A.35,  Up: Appendix A

A.36. `FCOS': Cosine
********************

     FCOS                          ; D9 FF                [386,FPU]

   `FCOS' computes the cosine of `ST0' (in radians), and stores the
result in `ST0'. See also `FSINCOS' (*Note Section A.61::).


File: nasm.info,  Node: Section A.37,  Next: Section A.38,  Prev: Section A.36,  Up: Appendix A

A.37. `FDECSTP': Decrement Floating-Point Stack Pointer
*******************************************************

     FDECSTP                       ; D9 F6                [8086,FPU]

   `FDECSTP' decrements the `top' field in the floating-point status
word. This has the effect of rotating the FPU register stack by one, as
if the contents of `ST7' had been pushed on the stack. See also
`FINCSTP' (*Note Section A.46::).


File: nasm.info,  Node: Section A.38,  Next: Section A.39,  Prev: Section A.37,  Up: Appendix A

A.38. `FxDISI', `FxENI': Disable and Enable Floating-Point Interrupts
*********************************************************************

     FDISI                         ; 9B DB E1             [8086,FPU]
     FNDISI                        ; DB E1                [8086,FPU]

     FENI                          ; 9B DB E0             [8086,FPU]
     FNENI                         ; DB E0                [8086,FPU]

   `FDISI' and `FENI' disable and enable floating-point interrupts.
These instructions are only meaningful on original 8087 processors: the
287 and above treat them as no-operation instructions.

   `FNDISI' and `FNENI' do the same thing as `FDISI' and `FENI'
respectively, but without waiting for the floating-point processor to
finish what it was doing first.


File: nasm.info,  Node: Section A.39,  Next: Section A.40,  Prev: Section A.38,  Up: Appendix A

A.39. `FDIV', `FDIVP', `FDIVR', `FDIVRP': Floating-Point Division
*****************************************************************

     FDIV mem32                    ; D8 /6                [8086,FPU]
     FDIV mem64                    ; DC /6                [8086,FPU]

     FDIV fpureg                   ; D8 F0+r              [8086,FPU]
     FDIV ST0,fpureg               ; D8 F0+r              [8086,FPU]

     FDIV TO fpureg                ; DC F8+r              [8086,FPU]
     FDIV fpureg,ST0               ; DC F8+r              [8086,FPU]

     FDIVR mem32                   ; D8 /0                [8086,FPU]
     FDIVR mem64                   ; DC /0                [8086,FPU]

     FDIVR fpureg                  ; D8 F8+r              [8086,FPU]
     FDIVR ST0,fpureg              ; D8 F8+r              [8086,FPU]

     FDIVR TO fpureg               ; DC F0+r              [8086,FPU]
     FDIVR fpureg,ST0              ; DC F0+r              [8086,FPU]

     FDIVP fpureg                  ; DE F8+r              [8086,FPU]
     FDIVP fpureg,ST0              ; DE F8+r              [8086,FPU]

     FDIVRP fpureg                 ; DE F0+r              [8086,FPU]
     FDIVRP fpureg,ST0             ; DE F0+r              [8086,FPU]

   `FDIV' divides `ST0' by the given operand and stores the result back
in `ST0', unless the `TO' qualifier is given, in which case it divides
the given operand by `ST0' and stores the result in the operand.

   `FDIVR' does the same thing, but does the division the other way up:
so if `TO' is not given, it divides the given operand by `ST0' and
stores the result in `ST0', whereas if `TO' is given it divides `ST0'
by its operand and stores the result in the operand.

   `FDIVP' operates like `FDIV TO', but pops the register stack once it
has finished. `FDIVRP' operates like `FDIVR TO', but pops the register
stack once it has finished.


File: nasm.info,  Node: Section A.40,  Next: Section A.41,  Prev: Section A.39,  Up: Appendix A

A.40. `FFREE': Flag Floating-Point Register as Unused
*****************************************************

     FFREE fpureg                  ; DD C0+r              [8086,FPU]

   `FFREE' marks the given register as being empty.


File: nasm.info,  Node: Section A.41,  Next: Section A.42,  Prev: Section A.40,  Up: Appendix A

A.41. `FIADD': Floating-Point/Integer Addition
**********************************************

     FIADD mem16                   ; DE /0                [8086,FPU]
     FIADD mem32                   ; DA /0                [8086,FPU]

   `FIADD' adds the 16-bit or 32-bit integer stored in the given memory
location to `ST0', storing the result in `ST0'.


File: nasm.info,  Node: Section A.42,  Next: Section A.43,  Prev: Section A.41,  Up: Appendix A

A.42. `FICOM', `FICOMP': Floating-Point/Integer Compare
*******************************************************

     FICOM mem16                   ; DE /2                [8086,FPU]
     FICOM mem32                   ; DA /2                [8086,FPU]

     FICOMP mem16                  ; DE /3                [8086,FPU]
     FICOMP mem32                  ; DA /3                [8086,FPU]

   `FICOM' compares `ST0' with the 16-bit or 32-bit integer stored in
the given memory location, and sets the FPU flags accordingly.
`FICOMP' does the same, but pops the register stack afterwards.


File: nasm.info,  Node: Section A.43,  Next: Section A.44,  Prev: Section A.42,  Up: Appendix A

A.43. `FIDIV', `FIDIVR': Floating-Point/Integer Division
********************************************************

     FIDIV mem16                   ; DE /6                [8086,FPU]
     FIDIV mem32                   ; DA /6                [8086,FPU]

     FIDIVR mem16                  ; DE /0                [8086,FPU]
     FIDIVR mem32                  ; DA /0                [8086,FPU]

   `FIDIV' divides `ST0' by the 16-bit or 32-bit integer stored in the
given memory location, and stores the result in `ST0'.  `FIDIVR' does
the division the other way up: it divides the integer by `ST0', but
still stores the result in `ST0'.


File: nasm.info,  Node: Section A.44,  Next: Section A.45,  Prev: Section A.43,  Up: Appendix A

A.44. `FILD', `FIST', `FISTP': Floating-Point/Integer Conversion
****************************************************************

     FILD mem16                    ; DF /0                [8086,FPU]
     FILD mem32                    ; DB /0                [8086,FPU]
     FILD mem64                    ; DF /5                [8086,FPU]

     FIST mem16                    ; DF /2                [8086,FPU]
     FIST mem32                    ; DB /2                [8086,FPU]

     FISTP mem16                   ; DF /3                [8086,FPU]
     FISTP mem32                   ; DB /3                [8086,FPU]
     FISTP mem64                   ; DF /0                [8086,FPU]

   `FILD' loads an integer out of a memory location, converts it to a
real, and pushes it on the FPU register stack. `FIST' converts `ST0' to
an integer and stores that in memory; `FISTP' does the same as `FIST',
but pops the register stack afterwards.


File: nasm.info,  Node: Section A.45,  Next: Section A.46,  Prev: Section A.44,  Up: Appendix A

A.45. `FIMUL': Floating-Point/Integer Multiplication
****************************************************

     FIMUL mem16                   ; DE /1                [8086,FPU]
     FIMUL mem32                   ; DA /1                [8086,FPU]

   `FIMUL' multiplies `ST0' by the 16-bit or 32-bit integer stored in
the given memory location, and stores the result in `ST0'.


File: nasm.info,  Node: Section A.46,  Next: Section A.47,  Prev: Section A.45,  Up: Appendix A

A.46. `FINCSTP': Increment Floating-Point Stack Pointer
*******************************************************

     FINCSTP                       ; D9 F7                [8086,FPU]

   `FINCSTP' increments the `top' field in the floating-point status
word. This has the effect of rotating the FPU register stack by one, as
if the register stack had been popped; however, unlike the popping of
the stack performed by many FPU instructions, it does not flag the new
`ST7' (previously `ST0') as empty. See also `FDECSTP' (*Note Section
A.37::).


File: nasm.info,  Node: Section A.47,  Next: Section A.48,  Prev: Section A.46,  Up: Appendix A

A.47. `FINIT', `FNINIT': Initialise Floating-Point Unit
*******************************************************

     FINIT                         ; 9B DB E3             [8086,FPU]
     FNINIT                        ; DB E3                [8086,FPU]

   `FINIT' initialises the FPU to its default state. It flags all
registers as empty, though it does not actually change their values.
`FNINIT' does the same, without first waiting for pending exceptions to
clear.


File: nasm.info,  Node: Section A.48,  Next: Section A.49,  Prev: Section A.47,  Up: Appendix A

A.48. `FISUB': Floating-Point/Integer Subtraction
*************************************************

     FISUB mem16                   ; DE /4                [8086,FPU]
     FISUB mem32                   ; DA /4                [8086,FPU]

     FISUBR mem16                  ; DE /5                [8086,FPU]
     FISUBR mem32                  ; DA /5                [8086,FPU]

   `FISUB' subtracts the 16-bit or 32-bit integer stored in the given
memory location from `ST0', and stores the result in `ST0'.  `FISUBR'
does the subtraction the other way round, i.e. it subtracts `ST0' from
the given integer, but still stores the result in `ST0'.


File: nasm.info,  Node: Section A.49,  Next: Section A.50,  Prev: Section A.48,  Up: Appendix A

A.49. `FLD': Floating-Point Load
********************************

     FLD mem32                     ; D9 /0                [8086,FPU]
     FLD mem64                     ; DD /0                [8086,FPU]
     FLD mem80                     ; DB /5                [8086,FPU]
     FLD fpureg                    ; D9 C0+r              [8086,FPU]

   `FLD' loads a floating-point value out of the given register or
memory location, and pushes it on the FPU register stack.


File: nasm.info,  Node: Section A.50,  Next: Section A.51,  Prev: Section A.49,  Up: Appendix A

A.50. `FLDxx': Floating-Point Load Constants
********************************************

     FLD1                          ; D9 E8                [8086,FPU]
     FLDL2E                        ; D9 EA                [8086,FPU]
     FLDL2T                        ; D9 E9                [8086,FPU]
     FLDLG2                        ; D9 EC                [8086,FPU]
     FLDLN2                        ; D9 ED                [8086,FPU]
     FLDPI                         ; D9 EB                [8086,FPU]
     FLDZ                          ; D9 EE                [8086,FPU]

   These instructions push specific standard constants on the FPU
register stack. `FLD1' pushes the value 1; `FLDL2E' pushes the base-2
logarithm of e; `FLDL2T' pushes the base-2 log of 10; `FLDLG2' pushes
the base-10 log of 2; `FLDLN2' pushes the base-e log of 2; `FLDPI'
pushes pi; and `FLDZ' pushes zero.


File: nasm.info,  Node: Section A.51,  Next: Section A.52,  Prev: Section A.50,  Up: Appendix A

A.51. `FLDCW': Load Floating-Point Control Word
***********************************************

     FLDCW mem16                   ; D9 /5                [8086,FPU]

   `FLDCW' loads a 16-bit value out of memory and stores it into the FPU
control word (governing things like the rounding mode, the precision,
and the exception masks). See also `FSTCW' (*Note Section A.64::).


File: nasm.info,  Node: Section A.52,  Next: Section A.53,  Prev: Section A.51,  Up: Appendix A

A.52. `FLDENV': Load Floating-Point Environment
***********************************************

     FLDENV mem                    ; D9 /4                [8086,FPU]

   `FLDENV' loads the FPU operating environment (control word, status
word, tag word, instruction pointer, data pointer and last opcode) from
memory. The memory area is 14 or 28 bytes long, depending on the CPU
mode at the time. See also `FSTENV' (*Note Section A.65::).


File: nasm.info,  Node: Section A.53,  Next: Section A.54,  Prev: Section A.52,  Up: Appendix A

A.53. `FMUL', `FMULP': Floating-Point Multiply
**********************************************

     FMUL mem32                    ; D8 /1                [8086,FPU]
     FMUL mem64                    ; DC /1                [8086,FPU]

     FMUL fpureg                   ; D8 C8+r              [8086,FPU]
     FMUL ST0,fpureg               ; D8 C8+r              [8086,FPU]

     FMUL TO fpureg                ; DC C8+r              [8086,FPU]
     FMUL fpureg,ST0               ; DC C8+r              [8086,FPU]

     FMULP fpureg                  ; DE C8+r              [8086,FPU]
     FMULP fpureg,ST0              ; DE C8+r              [8086,FPU]

   `FMUL' multiplies `ST0' by the given operand, and stores the result
in `ST0', unless the `TO' qualifier is used in which case it stores the
result in the operand. `FMULP' performs the same operation as `FMUL
TO', and then pops the register stack.


File: nasm.info,  Node: Section A.54,  Next: Section A.55,  Prev: Section A.53,  Up: Appendix A

A.54. `FNOP': Floating-Point No Operation
*****************************************

     FNOP                          ; D9 D0                [8086,FPU]

   `FNOP' does nothing.


File: nasm.info,  Node: Section A.55,  Next: Section A.56,  Prev: Section A.54,  Up: Appendix A

A.55. `FPATAN', `FPTAN': Arctangent and Tangent
***********************************************

     FPATAN                        ; D9 F3                [8086,FPU]
     FPTAN                         ; D9 F2                [8086,FPU]

   `FPATAN' computes the arctangent, in radians, of the result of
dividing `ST1' by `ST0', stores the result in `ST1', and pops the
register stack. It works like the C `atan2' function, in that changing
the sign of both `ST0' and `ST1' changes the output value by pi (so it
performs true rectangular-to-polar coordinate conversion, with `ST1'
being the Y coordinate and `ST0' being the X coordinate, not merely an
arctangent).

   `FPTAN' computes the tangent of the value in `ST0' (in radians), and
stores the result back into `ST0'.


File: nasm.info,  Node: Section A.56,  Next: Section A.57,  Prev: Section A.55,  Up: Appendix A

A.56. `FPREM', `FPREM1': Floating-Point Partial Remainder
*********************************************************

     FPREM                         ; D9 F8                [8086,FPU]
     FPREM1                        ; D9 F5                [386,FPU]

   These instructions both produce the remainder obtained by dividing
`ST0' by `ST1'. This is calculated, notionally, by dividing `ST0' by
`ST1', rounding the result to an integer, multiplying by `ST1' again,
and computing the value which would need to be added back on to the
result to get back to the original value in `ST0'.

   The two instructions differ in the way the notional round-to-integer
operation is performed. `FPREM' does it by rounding towards zero, so
that the remainder it returns always has the same sign as the original
value in `ST0'; `FPREM1' does it by rounding to the nearest integer, so
that the remainder always has at most half the magnitude of `ST1'.

   Both instructions calculate _partial_ remainders, meaning that they
may not manage to provide the final result, but might leave intermediate
results in `ST0' instead. If this happens, they will set the C2 flag in
the FPU status word; therefore, to calculate a remainder, you should
repeatedly execute `FPREM' or `FPREM1' until C2 becomes clear.


File: nasm.info,  Node: Section A.57,  Next: Section A.58,  Prev: Section A.56,  Up: Appendix A

A.57. `FRNDINT': Floating-Point Round to Integer
************************************************

     FRNDINT                       ; D9 FC                [8086,FPU]

   `FRNDINT' rounds the contents of `ST0' to an integer, according to
the current rounding mode set in the FPU control word, and stores the
result back in `ST0'.


File: nasm.info,  Node: Section A.58,  Next: Section A.59,  Prev: Section A.57,  Up: Appendix A

A.58. `FSAVE', `FRSTOR': Save/Restore Floating-Point State
**********************************************************

     FSAVE mem                     ; 9B DD /6             [8086,FPU]
     FNSAVE mem                    ; DD /6                [8086,FPU]

     FRSTOR mem                    ; DD /4                [8086,FPU]

   `FSAVE' saves the entire floating-point unit state, including all the
information saved by `FSTENV' (*Note Section A.65::) plus the contents
of all the registers, to a 94 or 108 byte area of memory (depending on
the CPU mode). `FRSTOR' restores the floating-point state from the same
area of memory.

   `FNSAVE' does the same as `FSAVE', without first waiting for pending
floating-point exceptions to clear.


File: nasm.info,  Node: Section A.59,  Next: Section A.60,  Prev: Section A.58,  Up: Appendix A

A.59. `FSCALE': Scale Floating-Point Value by Power of Two
**********************************************************

     FSCALE                        ; D9 FD                [8086,FPU]

   `FSCALE' scales a number by a power of two: it rounds `ST1' towards
zero to obtain an integer, then multiplies `ST0' by two to the power of
that integer, and stores the result in `ST0'.


File: nasm.info,  Node: Section A.60,  Next: Section A.61,  Prev: Section A.59,  Up: Appendix A

A.60. `FSETPM': Set Protected Mode
**********************************

     FSETPM                        ; DB E4                [286,FPU]

   This instruction initalises protected mode on the 287 floating-point
coprocessor. It is only meaningful on that processor: the 387 and above
treat the instruction as a no-operation.


File: nasm.info,  Node: Section A.61,  Next: Section A.62,  Prev: Section A.60,  Up: Appendix A

A.61. `FSIN', `FSINCOS': Sine and Cosine
****************************************

     FSIN                          ; D9 FE                [386,FPU]
     FSINCOS                       ; D9 FB                [386,FPU]

   `FSIN' calculates the sine of `ST0' (in radians) and stores the
result in `ST0'. `FSINCOS' does the same, but then pushes the cosine of
the same value on the register stack, so that the sine ends up in `ST1'
and the cosine in `ST0'. `FSINCOS' is faster than executing `FSIN' and
`FCOS' (see *Note Section A.36::) in succession.


File: nasm.info,  Node: Section A.62,  Next: Section A.63,  Prev: Section A.61,  Up: Appendix A

A.62. `FSQRT': Floating-Point Square Root
*****************************************

     FSQRT                         ; D9 FA                [8086,FPU]

   `FSQRT' calculates the square root of `ST0' and stores the result in
`ST0'.


File: nasm.info,  Node: Section A.63,  Next: Section A.64,  Prev: Section A.62,  Up: Appendix A

A.63. `FST', `FSTP': Floating-Point Store
*****************************************

     FST mem32                     ; D9 /2                [8086,FPU]
     FST mem64                     ; DD /2                [8086,FPU]
     FST fpureg                    ; DD D0+r              [8086,FPU]

     FSTP mem32                    ; D9 /3                [8086,FPU]
     FSTP mem64                    ; DD /3                [8086,FPU]
     FSTP mem80                    ; DB /0                [8086,FPU]
     FSTP fpureg                   ; DD D8+r              [8086,FPU]

   `FST' stores the value in `ST0' into the given memory location or
other FPU register. `FSTP' does the same, but then pops the register
stack.


File: nasm.info,  Node: Section A.64,  Next: Section A.65,  Prev: Section A.63,  Up: Appendix A

A.64. `FSTCW': Store Floating-Point Control Word
************************************************

     FSTCW mem16                   ; 9B D9 /0             [8086,FPU]
     FNSTCW mem16                  ; D9 /0                [8086,FPU]

   `FSTCW' stores the FPU control word (governing things like the
rounding mode, the precision, and the exception masks) into a 2-byte
memory area. See also `FLDCW' (*Note Section A.51::).

   `FNSTCW' does the same thing as `FSTCW', without first waiting for
pending floating-point exceptions to clear.


File: nasm.info,  Node: Section A.65,  Next: Section A.66,  Prev: Section A.64,  Up: Appendix A

A.65. `FSTENV': Store Floating-Point Environment
************************************************

     FSTENV mem                    ; 9B D9 /6             [8086,FPU]
     FNSTENV mem                   ; D9 /6                [8086,FPU]

   `FSTENV' stores the FPU operating environment (control word, status
word, tag word, instruction pointer, data pointer and last opcode) into
memory. The memory area is 14 or 28 bytes long, depending on the CPU
mode at the time. See also `FLDENV' (*Note Section A.52::).

   `FNSTENV' does the same thing as `FSTENV', without first waiting for
pending floating-point exceptions to clear.


File: nasm.info,  Node: Section A.66,  Next: Section A.67,  Prev: Section A.65,  Up: Appendix A

A.66. `FSTSW': Store Floating-Point Status Word
***********************************************

     FSTSW mem16                   ; 9B DD /0             [8086,FPU]
     FSTSW AX                      ; 9B DF E0             [286,FPU]

     FNSTSW mem16                  ; DD /0                [8086,FPU]
     FNSTSW AX                     ; DF E0                [286,FPU]

   `FSTSW' stores the FPU status word into `AX' or into a 2-byte memory
area.

   `FNSTSW' does the same thing as `FSTSW', without first waiting for
pending floating-point exceptions to clear.


File: nasm.info,  Node: Section A.67,  Next: Section A.68,  Prev: Section A.66,  Up: Appendix A

A.67. `FSUB', `FSUBP', `FSUBR', `FSUBRP': Floating-Point Subtract
*****************************************************************

     FSUB mem32                    ; D8 /4                [8086,FPU]
     FSUB mem64                    ; DC /4                [8086,FPU]

     FSUB fpureg                   ; D8 E0+r              [8086,FPU]
     FSUB ST0,fpureg               ; D8 E0+r              [8086,FPU]

     FSUB TO fpureg                ; DC E8+r              [8086,FPU]
     FSUB fpureg,ST0               ; DC E8+r              [8086,FPU]

     FSUBR mem32                   ; D8 /5                [8086,FPU]
     FSUBR mem64                   ; DC /5                [8086,FPU]

     FSUBR fpureg                  ; D8 E8+r              [8086,FPU]
     FSUBR ST0,fpureg              ; D8 E8+r              [8086,FPU]

     FSUBR TO fpureg               ; DC E0+r              [8086,FPU]
     FSUBR fpureg,ST0              ; DC E0+r              [8086,FPU]

     FSUBP fpureg                  ; DE E8+r              [8086,FPU]
     FSUBP fpureg,ST0              ; DE E8+r              [8086,FPU]

     FSUBRP fpureg                 ; DE E0+r              [8086,FPU]
     FSUBRP fpureg,ST0             ; DE E0+r              [8086,FPU]

   `FSUB' subtracts the given operand from `ST0' and stores the result
back in `ST0', unless the `TO' qualifier is given, in which case it
subtracts `ST0' from the given operand and stores the result in the
operand.

   `FSUBR' does the same thing, but does the subtraction the other way
up: so if `TO' is not given, it subtracts `ST0' from the given operand
and stores the result in `ST0', whereas if `TO' is given it subtracts
its operand from `ST0' and stores the result in the operand.

   `FSUBP' operates like `FSUB TO', but pops the register stack once it
has finished. `FSUBRP' operates like `FSUBR TO', but pops the register
stack once it has finished.


File: nasm.info,  Node: Section A.68,  Next: Section A.69,  Prev: Section A.67,  Up: Appendix A

A.68. `FTST': Test `ST0' Against Zero
*************************************

     FTST                          ; D9 E4                [8086,FPU]

   `FTST' compares `ST0' with zero and sets the FPU flags accordingly.
`ST0' is treated as the left-hand side of the comparison, so that a
`less-than' result is generated if `ST0' is negative.


File: nasm.info,  Node: Section A.69,  Next: Section A.70,  Prev: Section A.68,  Up: Appendix A

A.69. `FUCOMxx': Floating-Point Unordered Compare
*************************************************

     FUCOM fpureg                  ; DD E0+r              [386,FPU]
     FUCOM ST0,fpureg              ; DD E0+r              [386,FPU]

     FUCOMP fpureg                 ; DD E8+r              [386,FPU]
     FUCOMP ST0,fpureg             ; DD E8+r              [386,FPU]

     FUCOMPP                       ; DA E9                [386,FPU]

     FUCOMI fpureg                 ; DB E8+r              [P6,FPU]
     FUCOMI ST0,fpureg             ; DB E8+r              [P6,FPU]

     FUCOMIP fpureg                ; DF E8+r              [P6,FPU]
     FUCOMIP ST0,fpureg            ; DF E8+r              [P6,FPU]

   `FUCOM' compares `ST0' with the given operand, and sets the FPU
flags accordingly. `ST0' is treated as the left-hand side of the
comparison, so that the carry flag is set (for a `less-than' result) if
`ST0' is less than the given operand.

   `FUCOMP' does the same as `FUCOM', but pops the register stack
afterwards. `FUCOMPP' compares `ST0' with `ST1' and then pops the
register stack twice.

   `FUCOMI' and `FUCOMIP' work like the corresponding forms of `FUCOM'
and `FUCOMP', but write their results directly to the CPU flags
register rather than the FPU status word, so they can be immediately
followed by conditional jump or conditional move instructions.

   The `FUCOM' instructions differ from the `FCOM' instructions (*Note
Section A.35::) only in the way they handle quiet NaNs: `FUCOM' will
handle them silently and set the condition code flags to an `unordered'
result, whereas `FCOM' will generate an exception.


File: nasm.info,  Node: Section A.70,  Next: Section A.71,  Prev: Section A.69,  Up: Appendix A

A.70. `FXAM': Examine Class of Value in `ST0'
*********************************************

     FXAM                          ; D9 E5                [8086,FPU]

   `FXAM' sets the FPU flags C3, C2 and C0 depending on the type of
value stored in `ST0': 000 (respectively) for an unsupported format,
001 for a NaN, 010 for a normal finite number, 011 for an infinity, 100
for a zero, 101 for an empty register, and 110 for a denormal. It also
sets the C1 flag to the sign of the number.


File: nasm.info,  Node: Section A.71,  Next: Section A.72,  Prev: Section A.70,  Up: Appendix A

A.71. `FXCH': Floating-Point Exchange
*************************************

     FXCH                          ; D9 C9                [8086,FPU]
     FXCH fpureg                   ; D9 C8+r              [8086,FPU]
     FXCH fpureg,ST0               ; D9 C8+r              [8086,FPU]
     FXCH ST0,fpureg               ; D9 C8+r              [8086,FPU]

   `FXCH' exchanges `ST0' with a given FPU register. The no-operand
form exchanges `ST0' with `ST1'.


File: nasm.info,  Node: Section A.72,  Next: Section A.73,  Prev: Section A.71,  Up: Appendix A

A.72. `FXTRACT': Extract Exponent and Significand
*************************************************

     FXTRACT                       ; D9 F4                [8086,FPU]

   `FXTRACT' separates the number in `ST0' into its exponent and
significand (mantissa), stores the exponent back into `ST0', and then
pushes the significand on the register stack (so that the significand
ends up in `ST0', and the exponent in `ST1').


File: nasm.info,  Node: Section A.73,  Next: Section A.74,  Prev: Section A.72,  Up: Appendix A

A.73. `FYL2X', `FYL2XP1': Compute Y times Log2(X) or Log2(X+1)
**************************************************************

     FYL2X                         ; D9 F1                [8086,FPU]
     FYL2XP1                       ; D9 F9                [8086,FPU]

   `FYL2X' multiplies `ST1' by the base-2 logarithm of `ST0', stores
the result in `ST1', and pops the register stack (so that the result
ends up in `ST0'). `ST0' must be non-zero and positive.

   `FYL2XP1' works the same way, but replacing the base-2 log of `ST0'
with that of `ST0' plus one. This time, `ST0' must have magnitude no
greater than 1 minus half the square root of two.


File: nasm.info,  Node: Section A.74,  Next: Section A.75,  Prev: Section A.73,  Up: Appendix A

A.74. `HLT': Halt Processor
***************************

     HLT                           ; F4                   [8086]

   `HLT' puts the processor into a halted state, where it will perform
no more operations until restarted by an interrupt or a reset.


File: nasm.info,  Node: Section A.75,  Next: Section A.76,  Prev: Section A.74,  Up: Appendix A

A.75. `IBTS': Insert Bit String
*******************************

     IBTS r/m16,reg16              ; o16 0F A7 /r         [386,UNDOC]
     IBTS r/m32,reg32              ; o32 0F A7 /r         [386,UNDOC]

   No clear documentation seems to be available for this instruction:
the best I've been able to find reads `Takes a string of bits from the
second operand and puts them in the first operand'. It is present only
in early 386 processors, and conflicts with the opcodes for
`CMPXCHG486'. NASM supports it only for completeness. Its counterpart
is `XBTS' (see *Note Section A.167::).


File: nasm.info,  Node: Section A.76,  Next: Section A.77,  Prev: Section A.75,  Up: Appendix A

A.76. `IDIV': Signed Integer Divide
***********************************

     IDIV r/m8                     ; F6 /7                [8086]
     IDIV r/m16                    ; o16 F7 /7            [8086]
     IDIV r/m32                    ; o32 F7 /7            [386]

   `IDIV' performs signed integer division. The explicit operand
provided is the divisor; the dividend and destination operands are
implicit, in the following way:

   * For `IDIV r/m8', `AX' is divided by the given operand; the
     quotient is stored in `AL' and the remainder in `AH'.

   * For `IDIV r/m16', `DX:AX' is divided by the given operand; the
     quotient is stored in `AX' and the remainder in `DX'.

   * For `IDIV r/m32', `EDX:EAX' is divided by the given operand; the
     quotient is stored in `EAX' and the remainder in `EDX'.

   Unsigned integer division is performed by the `DIV' instruction: see
*Note Section A.25::.


File: nasm.info,  Node: Section A.77,  Next: Section A.78,  Prev: Section A.76,  Up: Appendix A

A.77. `IMUL': Signed Integer Multiply
*************************************

     IMUL r/m8                     ; F6 /5                [8086]
     IMUL r/m16                    ; o16 F7 /5            [8086]
     IMUL r/m32                    ; o32 F7 /5            [386]

     IMUL reg16,r/m16              ; o16 0F AF /r         [386]
     IMUL reg32,r/m32              ; o32 0F AF /r         [386]

     IMUL reg16,imm8               ; o16 6B /r ib         [286]
     IMUL reg16,imm16              ; o16 69 /r iw         [286]
     IMUL reg32,imm8               ; o32 6B /r ib         [386]
     IMUL reg32,imm32              ; o32 69 /r id         [386]

     IMUL reg16,r/m16,imm8         ; o16 6B /r ib         [286]
     IMUL reg16,r/m16,imm16        ; o16 69 /r iw         [286]
     IMUL reg32,r/m32,imm8         ; o32 6B /r ib         [386]
     IMUL reg32,r/m32,imm32        ; o32 69 /r id         [386]

   `IMUL' performs signed integer multiplication. For the single-operand
form, the other operand and destination are implicit, in the following
way:

   * For `IMUL r/m8', `AL' is multiplied by the given operand; the
     product is stored in `AX'.

   * For `IMUL r/m16', `AX' is multiplied by the given operand; the
     product is stored in `DX:AX'.

   * For `IMUL r/m32', `EAX' is multiplied by the given operand; the
     product is stored in `EDX:EAX'.

   The two-operand form multiplies its two operands and stores the
result in the destination (first) operand. The three-operand form
multiplies its last two operands and stores the result in the first
operand.

   The two-operand form is in fact a shorthand for the three-operand
form, as can be seen by examining the opcode descriptions: in the
two-operand form, the code `/r' takes both its register and `r/m' parts
from the same operand (the first one).

   In the forms with an 8-bit immediate operand and another longer
source operand, the immediate operand is considered to be signed, and
is sign- extended to the length of the other source operand. In these
cases, the `BYTE' qualifier is necessary to force NASM to generate this
form of the instruction.

   Unsigned integer multiplication is performed by the `MUL'
instruction: see *Note Section A.107::.


File: nasm.info,  Node: Section A.78,  Next: Section A.79,  Prev: Section A.77,  Up: Appendix A

A.78. `IN': Input from I/O Port
*******************************

     IN AL,imm8                    ; E4 ib                [8086]
     IN AX,imm8                    ; o16 E5 ib            [8086]
     IN EAX,imm8                   ; o32 E5 ib            [386]
     IN AL,DX                      ; EC                   [8086]
     IN AX,DX                      ; o16 ED               [8086]
     IN EAX,DX                     ; o32 ED               [386]

   `IN' reads a byte, word or doubleword from the specified I/O port,
and stores it in the given destination register. The port number may be
specified as an immediate value if it is between 0 and 255, and
otherwise must be stored in `DX'. See also `OUT' (*Note Section
A.111::).


File: nasm.info,  Node: Section A.79,  Next: Section A.80,  Prev: Section A.78,  Up: Appendix A

A.79. `INC': Increment Integer
******************************

     INC reg16                     ; o16 40+r             [8086]
     INC reg32                     ; o32 40+r             [386]
     INC r/m8                      ; FE /0                [8086]
     INC r/m16                     ; o16 FF /0            [8086]
     INC r/m32                     ; o32 FF /0            [386]

   `INC' adds 1 to its operand. It does _not_ affect the carry flag: to
affect the carry flag, use `ADD something,1' (see *Note Section A.6::).
See also `DEC' (*Note Section A.24::).


File: nasm.info,  Node: Section A.80,  Next: Section A.81,  Prev: Section A.79,  Up: Appendix A

A.80. `INSB', `INSW', `INSD': Input String from I/O Port
********************************************************

     INSB                          ; 6C                   [186]
     INSW                          ; o16 6D               [186]
     INSD                          ; o32 6D               [386]

   `INSB' inputs a byte from the I/O port specified in `DX' and stores
it at `[ES:DI]' or `[ES:EDI]'. It then increments or decrements
(depending on the direction flag: increments if the flag is clear,
decrements if it is set) `DI' or `EDI'.

   The register used is `DI' if the address size is 16 bits, and `EDI'
if it is 32 bits. If you need to use an address size not equal to the
current `BITS' setting, you can use an explicit `a16' or `a32' prefix.

   Segment override prefixes have no effect for this instruction: the
use of `ES' for the load from `[DI]' or `[EDI]' cannot be overridden.

   `INSW' and `INSD' work in the same way, but they input a word or a
doubleword instead of a byte, and increment or decrement the addressing
register by 2 or 4 instead of 1.

   The `REP' prefix may be used to repeat the instruction `CX' (or
`ECX' - again, the address size chooses which) times.

   See also `OUTSB', `OUTSW' and `OUTSD' (*Note Section A.112::).


File: nasm.info,  Node: Section A.81,  Next: Section A.82,  Prev: Section A.80,  Up: Appendix A

A.81. `INT': Software Interrupt
*******************************

     INT imm8                      ; CD ib                [8086]

   `INT' causes a software interrupt through a specified vector number
from 0 to 255.

   The code generated by the `INT' instruction is always two bytes long:
although there are short forms for some `INT' instructions, NASM does
not generate them when it sees the `INT' mnemonic. In order to generate
single-byte breakpoint instructions, use the `INT3' or `INT1'
instructions (see *Note Section A.82::) instead.


File: nasm.info,  Node: Section A.82,  Next: Section A.83,  Prev: Section A.81,  Up: Appendix A

A.82. `INT3', `INT1', `ICEBP', `INT01': Breakpoints
***************************************************

     INT1                          ; F1                   [P6]
     ICEBP                         ; F1                   [P6]
     INT01                         ; F1                   [P6]

     INT3                          ; CC                   [8086]

   `INT1' and `INT3' are short one-byte forms of the instructions `INT
1' and `INT 3' (see *Note Section A.81::). They perform a similar
function to their longer counterparts, but take up less code space.
They are used as breakpoints by debuggers.

   `INT1', and its alternative synonyms `INT01' and `ICEBP', is an
instruction used by in-circuit emulators (ICEs). It is present, though
not documented, on some processors down to the 286, but is only
documented for the Pentium Pro. `INT3' is the instruction normally used
as a breakpoint by debuggers.

   `INT3' is not precisely equivalent to `INT 3': the short form, since
it is designed to be used as a breakpoint, bypasses the normal IOPL
checks in virtual-8086 mode, and also does not go through interrupt
redirection.


File: nasm.info,  Node: Section A.83,  Next: Section A.84,  Prev: Section A.82,  Up: Appendix A

A.83. `INTO': Interrupt if Overflow
***********************************

     INTO                          ; CE                   [8086]

   `INTO' performs an `INT 4' software interrupt (see *Note Section
A.81::) if and only if the overflow flag is set.


File: nasm.info,  Node: Section A.84,  Next: Section A.85,  Prev: Section A.83,  Up: Appendix A

A.84. `INVD': Invalidate Internal Caches
****************************************

     INVD                          ; 0F 08                [486]

   `INVD' invalidates and empties the processor's internal caches, and
causes the processor to instruct external caches to do the same. It does
not write the contents of the caches back to memory first: any modified
data held in the caches will be lost. To write the data back first, use
`WBINVD' (*Note Section A.164::).


File: nasm.info,  Node: Section A.85,  Next: Section A.86,  Prev: Section A.84,  Up: Appendix A

A.85. `INVLPG': Invalidate TLB Entry
************************************

     INVLPG mem                    ; 0F 01 /0             [486]

   `INVLPG' invalidates the translation lookahead buffer (TLB) entry
associated with the supplied memory address.


File: nasm.info,  Node: Section A.86,  Next: Section A.87,  Prev: Section A.85,  Up: Appendix A

A.86. `IRET', `IRETW', `IRETD': Return from Interrupt
*****************************************************

     IRET                          ; CF                   [8086]
     IRETW                         ; o16 CF               [8086]
     IRETD                         ; o32 CF               [386]

   `IRET' returns from an interrupt (hardware or software) by means of
popping `IP' (or `EIP'), `CS' and the flags off the stack and then
continuing execution from the new `CS:IP'.

   `IRETW' pops `IP', `CS' and the flags as 2 bytes each, taking 6
bytes off the stack in total. `IRETD' pops `EIP' as 4 bytes, pops a
further 4 bytes of which the top two are discarded and the bottom two
go into `CS', and pops the flags as 4 bytes as well, taking 12 bytes
off the stack.

   `IRET' is a shorthand for either `IRETW' or `IRETD', depending on
the default `BITS' setting at the time.


File: nasm.info,  Node: Section A.87,  Next: Section A.88,  Prev: Section A.86,  Up: Appendix A

A.87. `JCXZ', `JECXZ': Jump if CX/ECX Zero
******************************************

     JCXZ imm                      ; o16 E3 rb            [8086]
     JECXZ imm                     ; o32 E3 rb            [386]

   `JCXZ' performs a short jump (with maximum range 128 bytes) if and
only if the contents of the `CX' register is 0. `JECXZ' does the same
thing, but with `ECX'.


File: nasm.info,  Node: Section A.88,  Next: Section A.89,  Prev: Section A.87,  Up: Appendix A

A.88. `JMP': Jump
*****************

     JMP imm                       ; E9 rw/rd             [8086]
     JMP SHORT imm                 ; EB rb                [8086]
     JMP imm:imm16                 ; o16 EA iw iw         [8086]
     JMP imm:imm32                 ; o32 EA id iw         [386]
     JMP FAR mem                   ; o16 FF /5            [8086]
     JMP FAR mem                   ; o32 FF /5            [386]
     JMP r/m16                     ; o16 FF /4            [8086]
     JMP r/m32                     ; o32 FF /4            [386]

   `JMP' jumps to a given address. The address may be specified as an
absolute segment and offset, or as a relative jump within the current
segment.

   `JMP SHORT imm' has a maximum range of 128 bytes, since the
displacement is specified as only 8 bits, but takes up less code space.
NASM does not choose when to generate `JMP SHORT' for you: you must
explicitly code `SHORT' every time you want a short jump.

   You can choose between the two immediate far jump forms (`JMP
imm:imm') by the use of the `WORD' and `DWORD' keywords: `JMP WORD
0x1234:0x5678') or `JMP DWORD 0x1234:0x56789abc'.

   The `JMP FAR mem' forms execute a far jump by loading the destination
address out of memory. The address loaded consists of 16 or 32 bits of
offset (depending on the operand size), and 16 bits of segment. The
operand size may be overridden using `JMP WORD FAR mem' or `JMP DWORD
FAR mem'.

   The `JMP r/m' forms execute a near jump (within the same segment),
loading the destination address out of memory or out of a register. The
keyword `NEAR' may be specified, for clarity, in these forms, but is
not necessary. Again, operand size can be overridden using `JMP WORD
mem' or `JMP DWORD mem'.

   As a convenience, NASM does not require you to jump to a far symbol
by coding the cumbersome `JMP SEG routine:routine', but instead allows
the easier synonym `JMP FAR routine'.

   The `CALL r/m' forms given above are near calls; NASM will accept the
`NEAR' keyword (e.g. `CALL NEAR [address]'), even though it is not
strictly necessary.


File: nasm.info,  Node: Section A.89,  Next: Section A.90,  Prev: Section A.88,  Up: Appendix A

A.89. `Jcc': Conditional Branch
*******************************

     Jcc imm                       ; 70+cc rb             [8086]
     Jcc NEAR imm                  ; 0F 80+cc rw/rd       [386]

   The conditional jump instructions execute a near (same segment) jump
if and only if their conditions are satisfied. For example, `JNZ' jumps
only if the zero flag is not set.

   The ordinary form of the instructions has only a 128-byte range; the
`NEAR' form is a 386 extension to the instruction set, and can span the
full size of a segment. NASM will not override your choice of jump
instruction: if you want `Jcc NEAR', you have to use the `NEAR' keyword.

   The `SHORT' keyword is allowed on the first form of the instruction,
for clarity, but is not necessary.


File: nasm.info,  Node: Section A.90,  Next: Section A.91,  Prev: Section A.89,  Up: Appendix A

A.90. `LAHF': Load AH from Flags
********************************

     LAHF                          ; 9F                   [8086]

   `LAHF' sets the `AH' register according to the contents of the low
byte of the flags word. See also `SAHF' (*Note Section A.145::).


File: nasm.info,  Node: Section A.91,  Next: Section A.92,  Prev: Section A.90,  Up: Appendix A

A.91. `LAR': Load Access Rights
*******************************

     LAR reg16,r/m16               ; o16 0F 02 /r         [286,PRIV]
     LAR reg32,r/m32               ; o32 0F 02 /r         [286,PRIV]

   `LAR' takes the segment selector specified by its source (second)
operand, finds the corresponding segment descriptor in the GDT or LDT,
and loads the access-rights byte of the descriptor into its destination
(first) operand.


File: nasm.info,  Node: Section A.92,  Next: Section A.93,  Prev: Section A.91,  Up: Appendix A

A.92. `LDS', `LES', `LFS', `LGS', `LSS': Load Far Pointer
*********************************************************

     LDS reg16,mem                 ; o16 C5 /r            [8086]
     LDS reg32,mem                 ; o32 C5 /r            [8086]

     LES reg16,mem                 ; o16 C4 /r            [8086]
     LES reg32,mem                 ; o32 C4 /r            [8086]

     LFS reg16,mem                 ; o16 0F B4 /r         [386]
     LFS reg32,mem                 ; o32 0F B4 /r         [386]

     LGS reg16,mem                 ; o16 0F B5 /r         [386]
     LGS reg32,mem                 ; o32 0F B5 /r         [386]

     LSS reg16,mem                 ; o16 0F B2 /r         [386]
     LSS reg32,mem                 ; o32 0F B2 /r         [386]

   These instructions load an entire far pointer (16 or 32 bits of
offset, plus 16 bits of segment) out of memory in one go. `LDS', for
example, loads 16 or 32 bits from the given memory address into the
given register (depending on the size of the register), then loads the
_next_ 16 bits from memory into `DS'. `LES', `LFS', `LGS' and `LSS'
work in the same way but use the other segment registers.


File: nasm.info,  Node: Section A.93,  Next: Section A.94,  Prev: Section A.92,  Up: Appendix A

A.93. `LEA': Load Effective Address
***********************************

     LEA reg16,mem                 ; o16 8D /r            [8086]
     LEA reg32,mem                 ; o32 8D /r            [8086]

   `LEA', despite its syntax, does not access memory. It calculates the
effective address specified by its second operand as if it were going to
load or store data from it, but instead it stores the calculated address
into the register specified by its first operand. This can be used to
perform quite complex calculations (e.g. `LEA EAX,[EBX+ECX*4+100]') in
one instruction.

   `LEA', despite being a purely arithmetic instruction which accesses
no memory, still requires square brackets around its second operand, as
if it were a memory reference.


File: nasm.info,  Node: Section A.94,  Next: Section A.95,  Prev: Section A.93,  Up: Appendix A

A.94. `LEAVE': Destroy Stack Frame
**********************************

     LEAVE                         ; C9                   [186]

   `LEAVE' destroys a stack frame of the form created by the `ENTER'
instruction (see *Note Section A.27::). It is functionally equivalent to
`MOV ESP,EBP' followed by `POP EBP' (or `MOV SP,BP' followed by `POP
BP' in 16-bit mode).


File: nasm.info,  Node: Section A.95,  Next: Section A.96,  Prev: Section A.94,  Up: Appendix A

A.95. `LGDT', `LIDT', `LLDT': Load Descriptor Tables
****************************************************

     LGDT mem                      ; 0F 01 /2             [286,PRIV]
     LIDT mem                      ; 0F 01 /3             [286,PRIV]
     LLDT r/m16                    ; 0F 00 /2             [286,PRIV]

   `LGDT' and `LIDT' both take a 6-byte memory area as an operand: they
load a 32-bit linear address and a 16-bit size limit from that area (in
the opposite order) into the GDTR (global descriptor table register) or
IDTR (interrupt descriptor table register). These are the only
instructions which directly use _linear_ addresses, rather than
segment/offset pairs.

   `LLDT' takes a segment selector as an operand. The processor looks up
that selector in the GDT and stores the limit and base address given
there into the LDTR (local descriptor table register).

   See also `SGDT', `SIDT' and `SLDT' (*Note Section A.151::).


File: nasm.info,  Node: Section A.96,  Next: Section A.97,  Prev: Section A.95,  Up: Appendix A

A.96. `LMSW': Load/Store Machine Status Word
********************************************

     LMSW r/m16                    ; 0F 01 /6             [286,PRIV]

   `LMSW' loads the bottom four bits of the source operand into the
bottom four bits of the `CR0' control register (or the Machine Status
Word, on 286 processors). See also `SMSW' (*Note Section A.155::).


File: nasm.info,  Node: Section A.97,  Next: Section A.98,  Prev: Section A.96,  Up: Appendix A

A.97. `LOADALL', `LOADALL286': Load Processor State
***************************************************

     LOADALL                       ; 0F 07                [386,UNDOC]
     LOADALL286                    ; 0F 05                [286,UNDOC]

   This instruction, in its two different-opcode forms, is apparently
supported on most 286 processors, some 386 and possibly some 486. The
opcode differs between the 286 and the 386.

   The function of the instruction is to load all information relating
to the state of the processor out of a block of memory: on the 286,
this block is located implicitly at absolute address `0x800', and on
the 386 and 486 it is at `[ES:EDI]'.


File: nasm.info,  Node: Section A.98,  Next: Section A.99,  Prev: Section A.97,  Up: Appendix A

A.98. `LODSB', `LODSW', `LODSD': Load from String
*************************************************

     LODSB                         ; AC                   [8086]
     LODSW                         ; o16 AD               [8086]
     LODSD                         ; o32 AD               [386]

   `LODSB' loads a byte from `[DS:SI]' or `[DS:ESI]' into `AL'. It then
increments or decrements (depending on the direction flag: increments
if the flag is clear, decrements if it is set) `SI' or `ESI'.

   The register used is `SI' if the address size is 16 bits, and `ESI'
if it is 32 bits. If you need to use an address size not equal to the
current `BITS' setting, you can use an explicit `a16' or `a32' prefix.

   The segment register used to load from `[SI]' or `[ESI]' can be
overridden by using a segment register name as a prefix (for example,
`es lodsb').

   `LODSW' and `LODSD' work in the same way, but they load a word or a
doubleword instead of a byte, and increment or decrement the addressing
registers by 2 or 4 instead of 1.


File: nasm.info,  Node: Section A.99,  Next: Section A.100,  Prev: Section A.98,  Up: Appendix A

A.99. `LOOP', `LOOPE', `LOOPZ', `LOOPNE', `LOOPNZ': Loop with Counter
*********************************************************************

     LOOP imm                      ; E2 rb                [8086]
     LOOP imm,CX                   ; a16 E2 rb            [8086]
     LOOP imm,ECX                  ; a32 E2 rb            [386]

     LOOPE imm                     ; E1 rb                [8086]
     LOOPE imm,CX                  ; a16 E1 rb            [8086]
     LOOPE imm,ECX                 ; a32 E1 rb            [386]
     LOOPZ imm                     ; E1 rb                [8086]
     LOOPZ imm,CX                  ; a16 E1 rb            [8086]
     LOOPZ imm,ECX                 ; a32 E1 rb            [386]

     LOOPNE imm                    ; E0 rb                [8086]
     LOOPNE imm,CX                 ; a16 E0 rb            [8086]
     LOOPNE imm,ECX                ; a32 E0 rb            [386]
     LOOPNZ imm                    ; E0 rb                [8086]
     LOOPNZ imm,CX                 ; a16 E0 rb            [8086]
     LOOPNZ imm,ECX                ; a32 E0 rb            [386]

   `LOOP' decrements its counter register (either `CX' or `ECX' - if
one is not specified explicitly, the `BITS' setting dictates which is
used) by one, and if the counter does not become zero as a result of
this operation, it jumps to the given label. The jump has a range of 128
bytes.

   `LOOPE' (or its synonym `LOOPZ') adds the additional condition that
it only jumps if the counter is nonzero _and_ the zero flag is set.
Similarly, `LOOPNE' (and `LOOPNZ') jumps only if the counter is nonzero
and the zero flag is clear.


File: nasm.info,  Node: Section A.100,  Next: Section A.101,  Prev: Section A.99,  Up: Appendix A

A.100. `LSL': Load Segment Limit
********************************

     LSL reg16,r/m16               ; o16 0F 03 /r         [286,PRIV]
     LSL reg32,r/m32               ; o32 0F 03 /r         [286,PRIV]

   `LSL' is given a segment selector in its source (second) operand; it
computes the segment limit value by loading the segment limit field from
the associated segment descriptor in the GDT or LDT. (This involves
shifting left by 12 bits if the segment limit is page-granular, and not
if it is byte-granular; so you end up with a byte limit in either
case.) The segment limit obtained is then loaded into the destination
(first) operand.


File: nasm.info,  Node: Section A.101,  Next: Section A.102,  Prev: Section A.100,  Up: Appendix A

A.101. `LTR': Load Task Register
********************************

     LTR r/m16                     ; 0F 00 /3             [286,PRIV]

   `LTR' looks up the segment base and limit in the GDT or LDT
descriptor specified by the segment selector given as its operand, and
loads them into the Task Register.


File: nasm.info,  Node: Section A.102,  Next: Section A.103,  Prev: Section A.101,  Up: Appendix A

A.102. `MOV': Move Data
***********************

     MOV r/m8,reg8                 ; 88 /r                [8086]
     MOV r/m16,reg16               ; o16 89 /r            [8086]
     MOV r/m32,reg32               ; o32 89 /r            [386]
     MOV reg8,r/m8                 ; 8A /r                [8086]
     MOV reg16,r/m16               ; o16 8B /r            [8086]
     MOV reg32,r/m32               ; o32 8B /r            [386]

     MOV reg8,imm8                 ; B0+r ib              [8086]
     MOV reg16,imm16               ; o16 B8+r iw          [8086]
     MOV reg32,imm32               ; o32 B8+r id          [386]
     MOV r/m8,imm8                 ; C6 /0 ib             [8086]
     MOV r/m16,imm16               ; o16 C7 /0 iw         [8086]
     MOV r/m32,imm32               ; o32 C7 /0 id         [386]

     MOV AL,memoffs8               ; A0 ow/od             [8086]
     MOV AX,memoffs16              ; o16 A1 ow/od         [8086]
     MOV EAX,memoffs32             ; o32 A1 ow/od         [386]
     MOV memoffs8,AL               ; A2 ow/od             [8086]
     MOV memoffs16,AX              ; o16 A3 ow/od         [8086]
     MOV memoffs32,EAX             ; o32 A3 ow/od         [386]

     MOV r/m16,segreg              ; o16 8C /r            [8086]
     MOV r/m32,segreg              ; o32 8C /r            [386]
     MOV segreg,r/m16              ; o16 8E /r            [8086]
     MOV segreg,r/m32              ; o32 8E /r            [386]

     MOV reg32,CR0/2/3/4           ; 0F 20 /r             [386]
     MOV reg32,DR0/1/2/3/6/7       ; 0F 21 /r             [386]
     MOV reg32,TR3/4/5/6/7         ; 0F 24 /r             [386]
     MOV CR0/2/3/4,reg32           ; 0F 22 /r             [386]
     MOV DR0/1/2/3/6/7,reg32       ; 0F 23 /r             [386]
     MOV TR3/4/5/6/7,reg32         ; 0F 26 /r             [386]

   `MOV' copies the contents of its source (second) operand into its
destination (first) operand.

   In all forms of the `MOV' instruction, the two operands are the same
size, except for moving between a segment register and an `r/m32'
operand. These instructions are treated exactly like the corresponding
16- bit equivalent (so that, for example, `MOV DS,EAX' functions
identically to `MOV DS,AX' but saves a prefix when in 32-bit mode),
except that when a segment register is moved into a 32-bit destination,
the top two bytes of the result are undefined.

   `MOV' may not use `CS' as a destination.

   `CR4' is only a supported register on the Pentium and above.


File: nasm.info,  Node: Section A.103,  Next: Section A.104,  Prev: Section A.102,  Up: Appendix A

A.103. `MOVD': Move Doubleword to/from MMX Register
***************************************************

     MOVD mmxreg,r/m32             ; 0F 6E /r             [PENT,MMX]
     MOVD r/m32,mmxreg             ; 0F 7E /r             [PENT,MMX]

   `MOVD' copies 32 bits from its source (second) operand into its
destination (first) operand. When the destination is a 64-bit MMX
register, the top 32 bits are set to zero.


File: nasm.info,  Node: Section A.104,  Next: Section A.105,  Prev: Section A.103,  Up: Appendix A

A.104. `MOVQ': Move Quadword to/from MMX Register
*************************************************

     MOVQ mmxreg,r/m64             ; 0F 6F /r             [PENT,MMX]
     MOVQ r/m64,mmxreg             ; 0F 7F /r             [PENT,MMX]

   `MOVQ' copies 64 bits from its source (second) operand into its
destination (first) operand.


File: nasm.info,  Node: Section A.105,  Next: Section A.106,  Prev: Section A.104,  Up: Appendix A

A.105. `MOVSB', `MOVSW', `MOVSD': Move String
*********************************************

     MOVSB                         ; A4                   [8086]
     MOVSW                         ; o16 A5               [8086]
     MOVSD                         ; o32 A5               [386]

   `MOVSB' copies the byte at `[ES:DI]' or `[ES:EDI]' to `[DS:SI]' or
`[DS:ESI]'. It then increments or decrements (depending on the
direction flag: increments if the flag is clear, decrements if it is
set) `SI' and `DI' (or `ESI' and `EDI').

   The registers used are `SI' and `DI' if the address size is 16 bits,
and `ESI' and `EDI' if it is 32 bits. If you need to use an address
size not equal to the current `BITS' setting, you can use an explicit
`a16' or `a32' prefix.

   The segment register used to load from `[SI]' or `[ESI]' can be
overridden by using a segment register name as a prefix (for example,
`es movsb'). The use of `ES' for the store to `[DI]' or `[EDI]' cannot
be overridden.

   `MOVSW' and `MOVSD' work in the same way, but they copy a word or a
doubleword instead of a byte, and increment or decrement the addressing
registers by 2 or 4 instead of 1.

   The `REP' prefix may be used to repeat the instruction `CX' (or
`ECX' - again, the address size chooses which) times.


File: nasm.info,  Node: Section A.106,  Next: Section A.107,  Prev: Section A.105,  Up: Appendix A

A.106. `MOVSX', `MOVZX': Move Data with Sign or Zero Extend
***********************************************************

     MOVSX reg16,r/m8              ; o16 0F BE /r         [386]
     MOVSX reg32,r/m8              ; o32 0F BE /r         [386]
     MOVSX reg32,r/m16             ; o32 0F BF /r         [386]

     MOVZX reg16,r/m8              ; o16 0F B6 /r         [386]
     MOVZX reg32,r/m8              ; o32 0F B6 /r         [386]
     MOVZX reg32,r/m16             ; o32 0F B7 /r         [386]

   `MOVSX' sign-extends its source (second) operand to the length of its
destination (first) operand, and copies the result into the destination
operand. `MOVZX' does the same, but zero-extends rather than sign-
extending.


File: nasm.info,  Node: Section A.107,  Next: Section A.108,  Prev: Section A.106,  Up: Appendix A

A.107. `MUL': Unsigned Integer Multiply
***************************************

     MUL r/m8                      ; F6 /4                [8086]
     MUL r/m16                     ; o16 F7 /4            [8086]
     MUL r/m32                     ; o32 F7 /4            [386]

   `MUL' performs unsigned integer multiplication. The other operand to
the multiplication, and the destination operand, are implicit, in the
following way:

   * For `MUL r/m8', `AL' is multiplied by the given operand; the
     product is stored in `AX'.

   * For `MUL r/m16', `AX' is multiplied by the given operand; the
     product is stored in `DX:AX'.

   * For `MUL r/m32', `EAX' is multiplied by the given operand; the
     product is stored in `EDX:EAX'.

   Signed integer multiplication is performed by the `IMUL' instruction:
see *Note Section A.77::.


File: nasm.info,  Node: Section A.108,  Next: Section A.109,  Prev: Section A.107,  Up: Appendix A

A.108. `NEG', `NOT': Two's and One's Complement
***********************************************

     NEG r/m8                      ; F6 /3                [8086]
     NEG r/m16                     ; o16 F7 /3            [8086]
     NEG r/m32                     ; o32 F7 /3            [386]

     NOT r/m8                      ; F6 /2                [8086]
     NOT r/m16                     ; o16 F7 /2            [8086]
     NOT r/m32                     ; o32 F7 /2            [386]

   `NEG' replaces the contents of its operand by the two's complement
negation (invert all the bits and then add one) of the original value.
`NOT', similarly, performs one's complement (inverts all the bits).


File: nasm.info,  Node: Section A.109,  Next: Section A.110,  Prev: Section A.108,  Up: Appendix A

A.109. `NOP': No Operation
**************************

     NOP                           ; 90                   [8086]

   `NOP' performs no operation. Its opcode is the same as that generated
by `XCHG AX,AX' or `XCHG EAX,EAX' (depending on the processor mode; see
*Note Section A.168::).


File: nasm.info,  Node: Section A.110,  Next: Section A.111,  Prev: Section A.109,  Up: Appendix A

A.110. `OR': Bitwise OR
***********************

     OR r/m8,reg8                  ; 08 /r                [8086]
     OR r/m16,reg16                ; o16 09 /r            [8086]
     OR r/m32,reg32                ; o32 09 /r            [386]

     OR reg8,r/m8                  ; 0A /r                [8086]
     OR reg16,r/m16                ; o16 0B /r            [8086]
     OR reg32,r/m32                ; o32 0B /r            [386]

     OR r/m8,imm8                  ; 80 /1 ib             [8086]
     OR r/m16,imm16                ; o16 81 /1 iw         [8086]
     OR r/m32,imm32                ; o32 81 /1 id         [386]

     OR r/m16,imm8                 ; o16 83 /1 ib         [8086]
     OR r/m32,imm8                 ; o32 83 /1 ib         [386]

     OR AL,imm8                    ; 0C ib                [8086]
     OR AX,imm16                   ; o16 0D iw            [8086]
     OR EAX,imm32                  ; o32 0D id            [386]

   `OR' performs a bitwise OR operation between its two operands (i.e.
each bit of the result is 1 if and only if at least one of the
corresponding bits of the two inputs was 1), and stores the result in
the destination (first) operand.

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.

   The MMX instruction `POR' (see *Note Section A.129::) performs the
same operation on the 64-bit MMX registers.


File: nasm.info,  Node: Section A.111,  Next: Section A.112,  Prev: Section A.110,  Up: Appendix A

A.111. `OUT': Output Data to I/O Port
*************************************

     OUT imm8,AL                   ; E6 ib                [8086]
     OUT imm8,AX                   ; o16 E7 ib            [8086]
     OUT imm8,EAX                  ; o32 E7 ib            [386]
     OUT DX,AL                     ; EE                   [8086]
     OUT DX,AX                     ; o16 EF               [8086]
     OUT DX,EAX                    ; o32 EF               [386]

   `IN' writes the contents of the given source register to the
specified I/O port. The port number may be specified as an immediate
value if it is between 0 and 255, and otherwise must be stored in `DX'.
See also `IN' (*Note Section A.78::).


File: nasm.info,  Node: Section A.112,  Next: Section A.113,  Prev: Section A.111,  Up: Appendix A

A.112. `OUTSB', `OUTSW', `OUTSD': Output String to I/O Port
***********************************************************

     OUTSB                         ; 6E                   [186]

     OUTSW                         ; o16 6F               [186]

     OUTSD                         ; o32 6F               [386]

   `OUTSB' loads a byte from `[DS:SI]' or `[DS:ESI]' and writes it to
the I/O port specified in `DX'. It then increments or decrements
(depending on the direction flag: increments if the flag is clear,
decrements if it is set) `SI' or `ESI'.

   The register used is `SI' if the address size is 16 bits, and `ESI'
if it is 32 bits. If you need to use an address size not equal to the
current `BITS' setting, you can use an explicit `a16' or `a32' prefix.

   The segment register used to load from `[SI]' or `[ESI]' can be
overridden by using a segment register name as a prefix (for example,
`es outsb').

   `OUTSW' and `OUTSD' work in the same way, but they output a word or
a doubleword instead of a byte, and increment or decrement the
addressing registers by 2 or 4 instead of 1.

   The `REP' prefix may be used to repeat the instruction `CX' (or
`ECX' - again, the address size chooses which) times.


File: nasm.info,  Node: Section A.113,  Next: Section A.114,  Prev: Section A.112,  Up: Appendix A

A.113. `PACKSSDW', `PACKSSWB', `PACKUSWB': Pack Data
****************************************************

     PACKSSDW mmxreg,r/m64         ; 0F 6B /r             [PENT,MMX]
     PACKSSWB mmxreg,r/m64         ; 0F 63 /r             [PENT,MMX]
     PACKUSWB mmxreg,r/m64         ; 0F 67 /r             [PENT,MMX]

   All these instructions start by forming a notional 128-bit word by
placing the source (second) operand on the left of the destination
(first) operand.  `PACKSSDW' then splits this 128-bit word into four
doublewords, converts each to a word, and loads them side by side into
the destination register; `PACKSSWB' and `PACKUSWB' both split the
128-bit word into eight words, converts each to a byte, and loads
_those_ side by side into the destination register.

   `PACKSSDW' and `PACKSSWB' perform signed saturation when reducing
the length of numbers: if the number is too large to fit into the
reduced space, they replace it by the largest signed number (`7FFFh' or
`7Fh') that _will_ fit, and if it is too small then they replace it by
the smallest signed number (`8000h' or `80h') that will fit. `PACKUSWB'
performs unsigned saturation: it treats its input as unsigned, and
replaces it by the largest unsigned number that will fit.


File: nasm.info,  Node: Section A.114,  Next: Section A.115,  Prev: Section A.113,  Up: Appendix A

A.114. `PADDxx': MMX Packed Addition
************************************

     PADDB mmxreg,r/m64            ; 0F FC /r             [PENT,MMX]
     PADDW mmxreg,r/m64            ; 0F FD /r             [PENT,MMX]
     PADDD mmxreg,r/m64            ; 0F FE /r             [PENT,MMX]

     PADDSB mmxreg,r/m64           ; 0F EC /r             [PENT,MMX]
     PADDSW mmxreg,r/m64           ; 0F ED /r             [PENT,MMX]

     PADDUSB mmxreg,r/m64          ; 0F DC /r             [PENT,MMX]
     PADDUSW mmxreg,r/m64          ; 0F DD /r             [PENT,MMX]

   `PADDxx' all perform packed addition between their two 64-bit
operands, storing the result in the destination (first) operand. The
`PADDxB' forms treat the 64-bit operands as vectors of eight bytes, and
add each byte individually; `PADDxW' treat the operands as vectors of
four words; and `PADDD' treats its operands as vectors of two
doublewords.

   `PADDSB' and `PADDSW' perform signed saturation on the sum of each
pair of bytes or words: if the result of an addition is too large or
too small to fit into a signed byte or word result, it is clipped
(saturated) to the largest or smallest value which _will_ fit.
`PADDUSB' and `PADDUSW' similarly perform unsigned saturation, clipping
to `0FFh' or `0FFFFh' if the result is larger than that.


File: nasm.info,  Node: Section A.115,  Next: Section A.116,  Prev: Section A.114,  Up: Appendix A

A.115. `PADDSIW': MMX Packed Addition to Implicit Destination
*************************************************************

     PADDSIW mmxreg,r/m64          ; 0F 51 /r             [CYRIX,MMX]

   `PADDSIW', specific to the Cyrix extensions to the MMX instruction
set, performs the same function as `PADDSW', except that the result is
not placed in the register specified by the first operand, but instead
in the register whose number differs from the first operand only in the
last bit. So `PADDSIW MM0,MM2' would put the result in `MM1', but
`PADDSIW MM1,MM2' would put the result in `MM0'.


File: nasm.info,  Node: Section A.116,  Next: Section A.117,  Prev: Section A.115,  Up: Appendix A

A.116. `PAND', `PANDN': MMX Bitwise AND and AND-NOT
***************************************************

     PAND mmxreg,r/m64             ; 0F DB /r             [PENT,MMX]
     PANDN mmxreg,r/m64            ; 0F DF /r             [PENT,MMX]

   `PAND' performs a bitwise AND operation between its two operands
(i.e.  each bit of the result is 1 if and only if the corresponding
bits of the two inputs were both 1), and stores the result in the
destination (first) operand.

   `PANDN' performs the same operation, but performs a one's complement
operation on the destination (first) operand first.


File: nasm.info,  Node: Section A.117,  Next: Section A.118,  Prev: Section A.116,  Up: Appendix A

A.117. `PAVEB': MMX Packed Average
**********************************

     PAVEB mmxreg,r/m64            ; 0F 50 /r             [CYRIX,MMX]

   `PAVEB', specific to the Cyrix MMX extensions, treats its two
operands as vectors of eight unsigned bytes, and calculates the average
of the corresponding bytes in the operands. The resulting vector of
eight averages is stored in the first operand.


File: nasm.info,  Node: Section A.118,  Next: Section A.119,  Prev: Section A.117,  Up: Appendix A

A.118. `PCMPxx': MMX Packed Comparison
**************************************

     PCMPEQB mmxreg,r/m64          ; 0F 74 /r             [PENT,MMX]
     PCMPEQW mmxreg,r/m64          ; 0F 75 /r             [PENT,MMX]
     PCMPEQD mmxreg,r/m64          ; 0F 76 /r             [PENT,MMX]

     PCMPGTB mmxreg,r/m64          ; 0F 64 /r             [PENT,MMX]
     PCMPGTW mmxreg,r/m64          ; 0F 65 /r             [PENT,MMX]
     PCMPGTD mmxreg,r/m64          ; 0F 66 /r             [PENT,MMX]

   The `PCMPxx' instructions all treat their operands as vectors of
bytes, words, or doublewords; corresponding elements of the source and
destination are compared, and the corresponding element of the
destination (first) operand is set to all zeros or all ones depending
on the result of the comparison.

   `PCMPxxB' treats the operands as vectors of eight bytes, `PCMPxxW'
treats them as vectors of four words, and `PCMPxxD' as two doublewords.

   `PCMPEQx' sets the corresponding element of the destination operand
to all ones if the two elements compared are equal; `PCMPGTx' sets the
destination element to all ones if the element of the first
(destination) operand is greater (treated as a signed integer) than
that of the second (source) operand.


File: nasm.info,  Node: Section A.119,  Next: Section A.120,  Prev: Section A.118,  Up: Appendix A

A.119. `PDISTIB': MMX Packed Distance and Accumulate with Implied Register
**************************************************************************

     PDISTIB mmxreg,mem64          ; 0F 54 /r             [CYRIX,MMX]

   `PDISTIB', specific to the Cyrix MMX extensions, treats its two input
operands as vectors of eight unsigned bytes. For each byte position, it
finds the absolute difference between the bytes in that position in the
two input operands, and adds that value to the byte in the same
position in the implied output register. The addition is saturated to
an unsigned byte in the same way as `PADDUSB'.

   The implied output register is found in the same way as `PADDSIW'
(*Note Section A.115::).

   Note that `PDISTIB' cannot take a register as its second source
operand.


File: nasm.info,  Node: Section A.120,  Next: Section A.121,  Prev: Section A.119,  Up: Appendix A

A.120. `PMACHRIW': MMX Packed Multiply and Accumulate with Rounding
*******************************************************************

     PMACHRIW mmxreg,mem64         ; 0F 5E /r             [CYRIX,MMX]

   `PMACHRIW' acts almost identically to `PMULHRIW' (*Note Section
A.123::), but instead of _storing_ its result in the implied
destination register, it _adds_ its result, as four packed words, to
the implied destination register. No saturation is done: the addition
can wrap around.

   Note that `PMACHRIW' cannot take a register as its second source
operand.


File: nasm.info,  Node: Section A.121,  Next: Section A.122,  Prev: Section A.120,  Up: Appendix A

A.121. `PMADDWD': MMX Packed Multiply and Add
*********************************************

     PMADDWD mmxreg,r/m64          ; 0F F5 /r             [PENT,MMX]

   `PMADDWD' treats its two inputs as vectors of four signed words. It
multiplies corresponding elements of the two operands, giving four
signed doubleword results. The top two of these are added and placed in
the top 32 bits of the destination (first) operand; the bottom two are
added and placed in the bottom 32 bits.


File: nasm.info,  Node: Section A.122,  Next: Section A.123,  Prev: Section A.121,  Up: Appendix A

A.122. `PMAGW': MMX Packed Magnitude
************************************

     PMAGW mmxreg,r/m64            ; 0F 52 /r             [CYRIX,MMX]

   `PMAGW', specific to the Cyrix MMX extensions, treats both its
operands as vectors of four signed words. It compares the absolute
values of the words in corresponding positions, and sets each word of
the destination (first) operand to whichever of the two words in that
position had the larger absolute value.


File: nasm.info,  Node: Section A.123,  Next: Section A.124,  Prev: Section A.122,  Up: Appendix A

A.123. `PMULHRW', `PMULHRIW': MMX Packed Multiply High with Rounding
********************************************************************

     PMULHRW mmxreg,r/m64          ; 0F 59 /r             [CYRIX,MMX]
     PMULHRIW mmxreg,r/m64         ; 0F 5D /r             [CYRIX,MMX]

   These instructions, specific to the Cyrix MMX extensions, treat their
operands as vectors of four signed words. Words in corresponding
positions are multiplied, to give a 32-bit value in which bits 30 and
31 are guaranteed equal. Bits 30 to 15 of this value (bit mask
`0x7FFF8000') are taken and stored in the corresponding position of the
destination operand, after first rounding the low bit (equivalent to
adding `0x4000' before extracting bits 30 to 15).

   For `PMULHRW', the destination operand is the first operand; for
`PMULHRIW' the destination operand is implied by the first operand in
the manner of `PADDSIW' (*Note Section A.115::).


File: nasm.info,  Node: Section A.124,  Next: Section A.125,  Prev: Section A.123,  Up: Appendix A

A.124. `PMULHW', `PMULLW': MMX Packed Multiply
**********************************************

     PMULHW mmxreg,r/m64           ; 0F E5 /r             [PENT,MMX]
     PMULLW mmxreg,r/m64           ; 0F D5 /r             [PENT,MMX]

   `PMULxW' treats its two inputs as vectors of four signed words. It
multiplies corresponding elements of the two operands, giving four
signed doubleword results.

   `PMULHW' then stores the top 16 bits of each doubleword in the
destination (first) operand; `PMULLW' stores the bottom 16 bits of each
doubleword in the destination operand.


File: nasm.info,  Node: Section A.125,  Next: Section A.126,  Prev: Section A.124,  Up: Appendix A

A.125. `PMVccZB': MMX Packed Conditional Move
*********************************************

     PMVZB mmxreg,mem64            ; 0F 58 /r             [CYRIX,MMX]
     PMVNZB mmxreg,mem64           ; 0F 5A /r             [CYRIX,MMX]
     PMVLZB mmxreg,mem64           ; 0F 5B /r             [CYRIX,MMX]
     PMVGEZB mmxreg,mem64          ; 0F 5C /r             [CYRIX,MMX]

   These instructions, specific to the Cyrix MMX extensions, perform
parallel conditional moves. The two input operands are treated as
vectors of eight bytes. Each byte of the destination (first) operand is
either written from the corresponding byte of the source (second)
operand, or left alone, depending on the value of the byte in the
_implied_ operand (specified in the same way as `PADDSIW', in *Note
Section A.115::).

   `PMVZB' performs each move if the corresponding byte in the implied
operand is zero. `PMVNZB' moves if the byte is non-zero. `PMVLZB' moves
if the byte is less than zero, and `PMVGEZB' moves if the byte is
greater than or equal to zero.

   Note that these instructions cannot take a register as their second
source operand.


File: nasm.info,  Node: Section A.126,  Next: Section A.127,  Prev: Section A.125,  Up: Appendix A

A.126. `POP': Pop Data from Stack
*********************************

     POP reg16                     ; o16 58+r             [8086]
     POP reg32                     ; o32 58+r             [386]

     POP r/m16                     ; o16 8F /0            [8086]
     POP r/m32                     ; o32 8F /0            [386]

     POP CS                        ; 0F                   [8086,UNDOC]
     POP DS                        ; 1F                   [8086]
     POP ES                        ; 07                   [8086]
     POP SS                        ; 17                   [8086]
     POP FS                        ; 0F A1                [386]
     POP GS                        ; 0F A9                [386]

   `POP' loads a value from the stack (from `[SS:SP]' or `[SS:ESP]')
and then increments the stack pointer.

   The address-size attribute of the instruction determines whether `SP'
or `ESP' is used as the stack pointer: to deliberately override the
default given by the `BITS' setting, you can use an `a16' or `a32'
prefix.

   The operand-size attribute of the instruction determines whether the
stack pointer is incremented by 2 or 4: this means that segment
register pops in `BITS 32' mode will pop 4 bytes off the stack and
discard the upper two of them. If you need to override that, you can
use an `o16' or `o32' prefix.

   The above opcode listings give two forms for general-purpose
register pop instructions: for example, `POP BX' has the two forms `5B'
and `8F C3'. NASM will always generate the shorter form when given `POP
BX'. NDISASM will disassemble both.

   `POP CS' is not a documented instruction, and is not supported on any
processor above the 8086 (since they use `0Fh' as an opcode prefix for
instruction set extensions). However, at least some 8086 processors do
support it, and so NASM generates it for completeness.


File: nasm.info,  Node: Section A.127,  Next: Section A.128,  Prev: Section A.126,  Up: Appendix A

A.127. `POPAx': Pop All General-Purpose Registers
*************************************************

     POPA                          ; 61                   [186]
     POPAW                         ; o16 61               [186]
     POPAD                         ; o32 61               [386]

   `POPAW' pops a word from the stack into each of, successively, `DI',
`SI', `BP', nothing (it discards a word from the stack which was a
placeholder for `SP'), `BX', `DX', `CX' and `AX'. It is intended to
reverse the operation of `PUSHAW' (see *Note Section A.135::), but it
ignores the value for `SP' that was pushed on the stack by `PUSHAW'.

   `POPAD' pops twice as much data, and places the results in `EDI',
`ESI', `EBP', nothing (placeholder for `ESP'), `EBX', `EDX', `ECX' and
`EAX'. It reverses the operation of `PUSHAD'.

   `POPA' is an alias mnemonic for either `POPAW' or `POPAD', depending
on the current `BITS' setting.

   Note that the registers are popped in reverse order of their numeric
values in opcodes (see *Note Section A.2.1::).


File: nasm.info,  Node: Section A.128,  Next: Section A.129,  Prev: Section A.127,  Up: Appendix A

A.128. `POPFx': Pop Flags Register
**********************************

     POPF                          ; 9D                   [186]
     POPFW                         ; o16 9D               [186]
     POPFD                         ; o32 9D               [386]

   `POPFW' pops a word from the stack and stores it in the bottom 16
bits of the flags register (or the whole flags register, on processors
below a 386). `POPFD' pops a doubleword and stores it in the entire
flags register.

   `POPF' is an alias mnemonic for either `POPFW' or `POPFD', depending
on the current `BITS' setting.

   See also `PUSHF' (*Note Section A.136::).


File: nasm.info,  Node: Section A.129,  Next: Section A.130,  Prev: Section A.128,  Up: Appendix A

A.129. `POR': MMX Bitwise OR
****************************

     POR mmxreg,r/m64              ; 0F EB /r             [PENT,MMX]

   `POR' performs a bitwise OR operation between its two operands (i.e.
each bit of the result is 1 if and only if at least one of the
corresponding bits of the two inputs was 1), and stores the result in
the destination (first) operand.


File: nasm.info,  Node: Section A.130,  Next: Section A.131,  Prev: Section A.129,  Up: Appendix A

A.130. `PSLLx', `PSRLx', `PSRAx': MMX Bit Shifts
************************************************

     PSLLW mmxreg,r/m64            ; 0F F1 /r             [PENT,MMX]
     PSLLW mmxreg,imm8             ; 0F 71 /6 ib          [PENT,MMX]

     PSLLD mmxreg,r/m64            ; 0F F2 /r             [PENT,MMX]
     PSLLD mmxreg,imm8             ; 0F 72 /6 ib          [PENT,MMX]

     PSLLQ mmxreg,r/m64            ; 0F F3 /r             [PENT,MMX]
     PSLLQ mmxreg,imm8             ; 0F 73 /6 ib          [PENT,MMX]

     PSRAW mmxreg,r/m64            ; 0F E1 /r             [PENT,MMX]
     PSRAW mmxreg,imm8             ; 0F 71 /4 ib          [PENT,MMX]

     PSRAD mmxreg,r/m64            ; 0F E2 /r             [PENT,MMX]
     PSRAD mmxreg,imm8             ; 0F 72 /4 ib          [PENT,MMX]

     PSRLW mmxreg,r/m64            ; 0F D1 /r             [PENT,MMX]
     PSRLW mmxreg,imm8             ; 0F 71 /2 ib          [PENT,MMX]

     PSRLD mmxreg,r/m64            ; 0F D2 /r             [PENT,MMX]
     PSRLD mmxreg,imm8             ; 0F 72 /2 ib          [PENT,MMX]

     PSRLQ mmxreg,r/m64            ; 0F D3 /r             [PENT,MMX]
     PSRLQ mmxreg,imm8             ; 0F 73 /2 ib          [PENT,MMX]

   `PSxxQ' perform simple bit shifts on the 64-bit MMX registers: the
destination (first) operand is shifted left or right by the number of
bits given in the source (second) operand, and the vacated bits are
filled in with zeros (for a logical shift) or copies of the original
sign bit (for an arithmetic right shift).

   `PSxxW' and `PSxxD' perform packed bit shifts: the destination
operand is treated as a vector of four words or two doublewords, and
each element is shifted individually, so bits shifted out of one
element do not interfere with empty bits coming into the next.

   `PSLLx' and `PSRLx' perform logical shifts: the vacated bits at one
end of the shifted number are filled with zeros. `PSRAx' performs an
arithmetic right shift: the vacated bits at the top of the shifted
number are filled with copies of the original top (sign) bit.


File: nasm.info,  Node: Section A.131,  Next: Section A.132,  Prev: Section A.130,  Up: Appendix A

A.131. `PSUBxx': MMX Packed Subtraction
***************************************

     PSUBB mmxreg,r/m64            ; 0F F8 /r             [PENT,MMX]
     PSUBW mmxreg,r/m64            ; 0F F9 /r             [PENT,MMX]
     PSUBD mmxreg,r/m64            ; 0F FA /r             [PENT,MMX]

     PSUBSB mmxreg,r/m64           ; 0F E8 /r             [PENT,MMX]
     PSUBSW mmxreg,r/m64           ; 0F E9 /r             [PENT,MMX]

     PSUBUSB mmxreg,r/m64          ; 0F D8 /r             [PENT,MMX]
     PSUBUSW mmxreg,r/m64          ; 0F D9 /r             [PENT,MMX]

   `PSUBxx' all perform packed subtraction between their two 64-bit
operands, storing the result in the destination (first) operand. The
`PSUBxB' forms treat the 64-bit operands as vectors of eight bytes, and
subtract each byte individually; `PSUBxW' treat the operands as vectors
of four words; and `PSUBD' treats its operands as vectors of two
doublewords.

   In all cases, the elements of the operand on the right are
subtracted from the corresponding elements of the operand on the left,
not the other way round.

   `PSUBSB' and `PSUBSW' perform signed saturation on the sum of each
pair of bytes or words: if the result of a subtraction is too large or
too small to fit into a signed byte or word result, it is clipped
(saturated) to the largest or smallest value which _will_ fit.
`PSUBUSB' and `PSUBUSW' similarly perform unsigned saturation, clipping
to `0FFh' or `0FFFFh' if the result is larger than that.


File: nasm.info,  Node: Section A.132,  Next: Section A.133,  Prev: Section A.131,  Up: Appendix A

A.132. `PSUBSIW': MMX Packed Subtract with Saturation to Implied Destination
****************************************************************************

     PSUBSIW mmxreg,r/m64          ; 0F 55 /r             [CYRIX,MMX]

   `PSUBSIW', specific to the Cyrix extensions to the MMX instruction
set, performs the same function as `PSUBSW', except that the result is
not placed in the register specified by the first operand, but instead
in the implied destination register, specified as for `PADDSIW' (*Note
Section A.115::).


File: nasm.info,  Node: Section A.133,  Next: Section A.134,  Prev: Section A.132,  Up: Appendix A

A.133. `PUNPCKxxx': Unpack Data
*******************************

     PUNPCKHBW mmxreg,r/m64        ; 0F 68 /r             [PENT,MMX]
     PUNPCKHWD mmxreg,r/m64        ; 0F 69 /r             [PENT,MMX]
     PUNPCKHDQ mmxreg,r/m64        ; 0F 6A /r             [PENT,MMX]

     PUNPCKLBW mmxreg,r/m64        ; 0F 60 /r             [PENT,MMX]
     PUNPCKLWD mmxreg,r/m64        ; 0F 61 /r             [PENT,MMX]
     PUNPCKLDQ mmxreg,r/m64        ; 0F 62 /r             [PENT,MMX]

   `PUNPCKxx' all treat their operands as vectors, and produce a new
vector generated by interleaving elements from the two inputs. The
`PUNPCKHxx' instructions start by throwing away the bottom half of each
input operand, and the `PUNPCKLxx' instructions throw away the top half.

   The remaining elements, totalling 64 bits, are then interleaved into
the destination, alternating elements from the second (source) operand
and the first (destination) operand: so the leftmost element in the
result always comes from the second operand, and the rightmost from the
destination.

   `PUNPCKxBW' works a byte at a time, `PUNPCKxWD' a word at a time,
and `PUNPCKxDQ' a doubleword at a time.

   So, for example, if the first operand held `0x7A6A5A4A3A2A1A0A' and
the second held `0x7B6B5B4B3B2B1B0B', then:

   * `PUNPCKHBW' would return `0x7B7A6B6A5B5A4B4A'.

   * `PUNPCKHWD' would return `0x7B6B7A6A5B4B5A4A'.

   * `PUNPCKHDQ' would return `0x7B6B5B4B7A6A5A4A'.

   * `PUNPCKLBW' would return `0x3B3A2B2A1B1A0B0A'.

   * `PUNPCKLWD' would return `0x3B2B3A2A1B0B1A0A'.

   * `PUNPCKLDQ' would return `0x3B2B1B0B3A2A1A0A'.


File: nasm.info,  Node: Section A.134,  Next: Section A.135,  Prev: Section A.133,  Up: Appendix A

A.134. `PUSH': Push Data on Stack
*********************************

     PUSH reg16                    ; o16 50+r             [8086]
     PUSH reg32                    ; o32 50+r             [386]

     PUSH r/m16                    ; o16 FF /6            [8086]
     PUSH r/m32                    ; o32 FF /6            [386]

     PUSH CS                       ; 0E                   [8086]
     PUSH DS                       ; 1E                   [8086]
     PUSH ES                       ; 06                   [8086]
     PUSH SS                       ; 16                   [8086]
     PUSH FS                       ; 0F A0                [386]
     PUSH GS                       ; 0F A8                [386]

     PUSH imm8                     ; 6A ib                [286]
     PUSH imm16                    ; o16 68 iw            [286]
     PUSH imm32                    ; o32 68 id            [386]

   `PUSH' decrements the stack pointer (`SP' or `ESP') by 2 or 4, and
then stores the given value at `[SS:SP]' or `[SS:ESP]'.

   The address-size attribute of the instruction determines whether `SP'
or `ESP' is used as the stack pointer: to deliberately override the
default given by the `BITS' setting, you can use an `a16' or `a32'
prefix.

   The operand-size attribute of the instruction determines whether the
stack pointer is decremented by 2 or 4: this means that segment
register pushes in `BITS 32' mode will push 4 bytes on the stack, of
which the upper two are undefined. If you need to override that, you
can use an `o16' or `o32' prefix.

   The above opcode listings give two forms for general-purpose
register push instructions: for example, `PUSH BX' has the two forms
`53' and `FF F3'. NASM will always generate the shorter form when given
`PUSH BX'. NDISASM will disassemble both.

   Unlike the undocumented and barely supported `POP CS', `PUSH CS' is
a perfectly valid and sensible instruction, supported on all processors.

   The instruction `PUSH SP' may be used to distinguish an 8086 from
later processors: on an 8086, the value of `SP' stored is the value it
has _after_ the push instruction, whereas on later processors it is the
value _before_ the push instruction.


File: nasm.info,  Node: Section A.135,  Next: Section A.136,  Prev: Section A.134,  Up: Appendix A

A.135. `PUSHAx': Push All General-Purpose Registers
***************************************************

     PUSHA                         ; 60                   [186]
     PUSHAD                        ; o32 60               [386]
     PUSHAW                        ; o16 60               [186]

   `PUSHAW' pushes, in succession, `AX', `CX', `DX', `BX', `SP', `BP',
`SI' and `DI' on the stack, decrementing the stack pointer by a total
of 16.

   `PUSHAD' pushes, in succession, `EAX', `ECX', `EDX', `EBX', `ESP',
`EBP', `ESI' and `EDI' on the stack, decrementing the stack pointer by
a total of 32.

   In both cases, the value of `SP' or `ESP' pushed is its _original_
value, as it had before the instruction was executed.

   `PUSHA' is an alias mnemonic for either `PUSHAW' or `PUSHAD',
depending on the current `BITS' setting.

   Note that the registers are pushed in order of their numeric values
in opcodes (see *Note Section A.2.1::).

   See also `POPA' (*Note Section A.127::).


File: nasm.info,  Node: Section A.136,  Next: Section A.137,  Prev: Section A.135,  Up: Appendix A

A.136. `PUSHFx': Push Flags Register
************************************

     PUSHF                         ; 9C                   [186]
     PUSHFD                        ; o32 9C               [386]
     PUSHFW                        ; o16 9C               [186]

   `PUSHFW' pops a word from the stack and stores it in the bottom 16
bits of the flags register (or the whole flags register, on processors
below a 386). `PUSHFD' pops a doubleword and stores it in the entire
flags register.

   `PUSHF' is an alias mnemonic for either `PUSHFW' or `PUSHFD',
depending on the current `BITS' setting.

   See also `POPF' (*Note Section A.128::).


File: nasm.info,  Node: Section A.137,  Next: Section A.138,  Prev: Section A.136,  Up: Appendix A

A.137. `PXOR': MMX Bitwise XOR
******************************

     PXOR mmxreg,r/m64             ; 0F EF /r             [PENT,MMX]

   `PXOR' performs a bitwise XOR operation between its two operands
(i.e.  each bit of the result is 1 if and only if exactly one of the
corresponding bits of the two inputs was 1), and stores the result in
the destination (first) operand.


File: nasm.info,  Node: Section A.138,  Next: Section A.139,  Prev: Section A.137,  Up: Appendix A

A.138. `RCL', `RCR': Bitwise Rotate through Carry Bit
*****************************************************

     RCL r/m8,1                    ; D0 /2                [8086]
     RCL r/m8,CL                   ; D2 /2                [8086]
     RCL r/m8,imm8                 ; C0 /2 ib             [286]
     RCL r/m16,1                   ; o16 D1 /2            [8086]
     RCL r/m16,CL                  ; o16 D3 /2            [8086]
     RCL r/m16,imm8                ; o16 C1 /2 ib         [286]
     RCL r/m32,1                   ; o32 D1 /2            [386]
     RCL r/m32,CL                  ; o32 D3 /2            [386]
     RCL r/m32,imm8                ; o32 C1 /2 ib         [386]

     RCR r/m8,1                    ; D0 /3                [8086]
     RCR r/m8,CL                   ; D2 /3                [8086]
     RCR r/m8,imm8                 ; C0 /3 ib             [286]
     RCR r/m16,1                   ; o16 D1 /3            [8086]
     RCR r/m16,CL                  ; o16 D3 /3            [8086]
     RCR r/m16,imm8                ; o16 C1 /3 ib         [286]
     RCR r/m32,1                   ; o32 D1 /3            [386]
     RCR r/m32,CL                  ; o32 D3 /3            [386]
     RCR r/m32,imm8                ; o32 C1 /3 ib         [386]

   `RCL' and `RCR' perform a 9-bit, 17-bit or 33-bit bitwise rotation
operation, involving the given source/destination (first) operand and
the carry bit. Thus, for example, in the operation `RCR AL,1', a 9-bit
rotation is performed in which `AL' is shifted left by 1, the top bit
of `AL' moves into the carry flag, and the original value of the carry
flag is placed in the low bit of `AL'.

   The number of bits to rotate by is given by the second operand. Only
the bottom five bits of the rotation count are considered by processors
above the 8086.

   You can force the longer (286 and upwards, beginning with a `C1'
byte) form of `RCL foo,1' by using a `BYTE' prefix: `RCL foo,BYTE 1'.
Similarly with `RCR'.


File: nasm.info,  Node: Section A.139,  Next: Section A.140,  Prev: Section A.138,  Up: Appendix A

A.139. `RDMSR': Read Model-Specific Registers
*********************************************

     RDMSR                         ; 0F 32                [PENT]

   `RDMSR' reads the processor Model-Specific Register (MSR) whose index
is stored in `ECX', and stores the result in `EDX:EAX'. See also
`WRMSR' (*Note Section A.165::).


File: nasm.info,  Node: Section A.140,  Next: Section A.141,  Prev: Section A.139,  Up: Appendix A

A.140. `RDPMC': Read Performance-Monitoring Counters
****************************************************

     RDPMC                         ; 0F 33                [P6]

   `RDPMC' reads the processor performance-monitoring counter whose
index is stored in `ECX', and stores the result in `EDX:EAX'.


File: nasm.info,  Node: Section A.141,  Next: Section A.142,  Prev: Section A.140,  Up: Appendix A

A.141. `RDTSC': Read Time-Stamp Counter
***************************************

     RDTSC                         ; 0F 31                [PENT]

   `RDTSC' reads the processor's time-stamp counter into `EDX:EAX'.


File: nasm.info,  Node: Section A.142,  Next: Section A.143,  Prev: Section A.141,  Up: Appendix A

A.142. `RET', `RETF', `RETN': Return from Procedure Call
********************************************************

     RET                           ; C3                   [8086]
     RET imm16                     ; C2 iw                [8086]

     RETF                          ; CB                   [8086]
     RETF imm16                    ; CA iw                [8086]

     RETN                          ; C3                   [8086]
     RETN imm16                    ; C2 iw                [8086]

   `RET', and its exact synonym `RETN', pop `IP' or `EIP' from the
stack and transfer control to the new address. Optionally, if a numeric
second operand is provided, they increment the stack pointer by a
further `imm16' bytes after popping the return address.

   `RETF' executes a far return: after popping `IP'/`EIP', it then pops
`CS', and _then_ increments the stack pointer by the optional argument
if present.


File: nasm.info,  Node: Section A.143,  Next: Section A.144,  Prev: Section A.142,  Up: Appendix A

A.143. `ROL', `ROR': Bitwise Rotate
***********************************

     ROL r/m8,1                    ; D0 /0                [8086]
     ROL r/m8,CL                   ; D2 /0                [8086]
     ROL r/m8,imm8                 ; C0 /0 ib             [286]
     ROL r/m16,1                   ; o16 D1 /0            [8086]
     ROL r/m16,CL                  ; o16 D3 /0            [8086]
     ROL r/m16,imm8                ; o16 C1 /0 ib         [286]
     ROL r/m32,1                   ; o32 D1 /0            [386]
     ROL r/m32,CL                  ; o32 D3 /0            [386]
     ROL r/m32,imm8                ; o32 C1 /0 ib         [386]

     ROR r/m8,1                    ; D0 /1                [8086]
     ROR r/m8,CL                   ; D2 /1                [8086]
     ROR r/m8,imm8                 ; C0 /1 ib             [286]
     ROR r/m16,1                   ; o16 D1 /1            [8086]
     ROR r/m16,CL                  ; o16 D3 /1            [8086]
     ROR r/m16,imm8                ; o16 C1 /1 ib         [286]
     ROR r/m32,1                   ; o32 D1 /1            [386]
     ROR r/m32,CL                  ; o32 D3 /1            [386]
     ROR r/m32,imm8                ; o32 C1 /1 ib         [386]

   `ROL' and `ROR' perform a bitwise rotation operation on the given
source/destination (first) operand. Thus, for example, in the operation
`ROR AL,1', an 8-bit rotation is performed in which `AL' is shifted
left by 1 and the original top bit of `AL' moves round into the low bit.

   The number of bits to rotate by is given by the second operand. Only
the bottom 3, 4 or 5 bits (depending on the source operand size) of the
rotation count are considered by processors above the 8086.

   You can force the longer (286 and upwards, beginning with a `C1'
byte) form of `ROL foo,1' by using a `BYTE' prefix: `ROL foo,BYTE 1'.
Similarly with `ROR'.


File: nasm.info,  Node: Section A.144,  Next: Section A.145,  Prev: Section A.143,  Up: Appendix A

A.144. `RSM': Resume from System-Management Mode
************************************************

     RSM                           ; 0F AA                [PENT]

   `RSM' returns the processor to its normal operating mode when it was
in System-Management Mode.


File: nasm.info,  Node: Section A.145,  Next: Section A.146,  Prev: Section A.144,  Up: Appendix A

A.145. `SAHF': Store AH to Flags
********************************

     SAHF                          ; 9E                   [8086]

   `SAHF' sets the low byte of the flags word according to the contents
of the `AH' register. See also `LAHF' (*Note Section A.90::).


File: nasm.info,  Node: Section A.146,  Next: Section A.147,  Prev: Section A.145,  Up: Appendix A

A.146. `SAL', `SAR': Bitwise Arithmetic Shifts
**********************************************

     SAL r/m8,1                    ; D0 /4                [8086]
     SAL r/m8,CL                   ; D2 /4                [8086]
     SAL r/m8,imm8                 ; C0 /4 ib             [286]
     SAL r/m16,1                   ; o16 D1 /4            [8086]
     SAL r/m16,CL                  ; o16 D3 /4            [8086]
     SAL r/m16,imm8                ; o16 C1 /4 ib         [286]
     SAL r/m32,1                   ; o32 D1 /4            [386]
     SAL r/m32,CL                  ; o32 D3 /4            [386]
     SAL r/m32,imm8                ; o32 C1 /4 ib         [386]

     SAR r/m8,1                    ; D0 /0                [8086]
     SAR r/m8,CL                   ; D2 /0                [8086]
     SAR r/m8,imm8                 ; C0 /0 ib             [286]
     SAR r/m16,1                   ; o16 D1 /0            [8086]
     SAR r/m16,CL                  ; o16 D3 /0            [8086]
     SAR r/m16,imm8                ; o16 C1 /0 ib         [286]
     SAR r/m32,1                   ; o32 D1 /0            [386]
     SAR r/m32,CL                  ; o32 D3 /0            [386]
     SAR r/m32,imm8                ; o32 C1 /0 ib         [386]

   `SAL' and `SAR' perform an arithmetic shift operation on the given
source/destination (first) operand. The vacated bits are filled with
zero for `SAL', and with copies of the original high bit of the source
operand for `SAR'.

   `SAL' is a synonym for `SHL' (see *Note Section A.152::). NASM will
assemble either one to the same code, but NDISASM will always
disassemble that code as `SHL'.

   The number of bits to shift by is given by the second operand. Only
the bottom 3, 4 or 5 bits (depending on the source operand size) of the
shift count are considered by processors above the 8086.

   You can force the longer (286 and upwards, beginning with a `C1'
byte) form of `SAL foo,1' by using a `BYTE' prefix: `SAL foo,BYTE 1'.
Similarly with `SAR'.


File: nasm.info,  Node: Section A.147,  Next: Section A.148,  Prev: Section A.146,  Up: Appendix A

A.147. `SALC': Set AL from Carry Flag
*************************************

     SALC                          ; D6                   [8086,UNDOC]

   `SALC' is an early undocumented instruction similar in concept to
`SETcc' (*Note Section A.150::). Its function is to set `AL' to zero if
the carry flag is clear, or to `0xFF' if it is set.


File: nasm.info,  Node: Section A.148,  Next: Section A.149,  Prev: Section A.147,  Up: Appendix A

A.148. `SBB': Subtract with Borrow
**********************************

     SBB r/m8,reg8                 ; 18 /r                [8086]
     SBB r/m16,reg16               ; o16 19 /r            [8086]
     SBB r/m32,reg32               ; o32 19 /r            [386]

     SBB reg8,r/m8                 ; 1A /r                [8086]
     SBB reg16,r/m16               ; o16 1B /r            [8086]
     SBB reg32,r/m32               ; o32 1B /r            [386]

     SBB r/m8,imm8                 ; 80 /3 ib             [8086]
     SBB r/m16,imm16               ; o16 81 /3 iw         [8086]
     SBB r/m32,imm32               ; o32 81 /3 id         [386]

     SBB r/m16,imm8                ; o16 83 /3 ib         [8086]
     SBB r/m32,imm8                ; o32 83 /3 ib         [8086]

     SBB AL,imm8                   ; 1C ib                [8086]
     SBB AX,imm16                  ; o16 1D iw            [8086]
     SBB EAX,imm32                 ; o32 1D id            [386]

   `SBB' performs integer subtraction: it subtracts its second operand,
plus the value of the carry flag, from its first, and leaves the result
in its destination (first) operand. The flags are set according to the
result of the operation: in particular, the carry flag is affected and
can be used by a subsequent `SBB' instruction.

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.

   To subtract one number from another without also subtracting the
contents of the carry flag, use `SUB' (*Note Section A.159::).


File: nasm.info,  Node: Section A.149,  Next: Section A.150,  Prev: Section A.148,  Up: Appendix A

A.149. `SCASB', `SCASW', `SCASD': Scan String
*********************************************

     SCASB                         ; AE                   [8086]
     SCASW                         ; o16 AF               [8086]
     SCASD                         ; o32 AF               [386]

   `SCASB' compares the byte in `AL' with the byte at `[ES:DI]' or
`[ES:EDI]', and sets the flags accordingly. It then increments or
decrements (depending on the direction flag: increments if the flag is
clear, decrements if it is set) `DI' (or `EDI').

   The register used is `DI' if the address size is 16 bits, and `EDI'
if it is 32 bits. If you need to use an address size not equal to the
current `BITS' setting, you can use an explicit `a16' or `a32' prefix.

   Segment override prefixes have no effect for this instruction: the
use of `ES' for the load from `[DI]' or `[EDI]' cannot be overridden.

   `SCASW' and `SCASD' work in the same way, but they compare a word to
`AX' or a doubleword to `EAX' instead of a byte to `AL', and increment
or decrement the addressing registers by 2 or 4 instead of 1.

   The `REPE' and `REPNE' prefixes (equivalently, `REPZ' and `REPNZ')
may be used to repeat the instruction up to `CX' (or `ECX' - again, the
address size chooses which) times until the first unequal or equal byte
is found.


File: nasm.info,  Node: Section A.150,  Next: Section A.151,  Prev: Section A.149,  Up: Appendix A

A.150. `SETcc': Set Register from Condition
*******************************************

     SETcc r/m8                    ; 0F 90+cc /2          [386]

   `SETcc' sets the given 8-bit operand to zero if its condition is not
satisfied, and to 1 if it is.


File: nasm.info,  Node: Section A.151,  Next: Section A.152,  Prev: Section A.150,  Up: Appendix A

A.151. `SGDT', `SIDT', `SLDT': Store Descriptor Table Pointers
**************************************************************

     SGDT mem                      ; 0F 01 /0             [286,PRIV]
     SIDT mem                      ; 0F 01 /1             [286,PRIV]
     SLDT r/m16                    ; 0F 00 /0             [286,PRIV]

   `SGDT' and `SIDT' both take a 6-byte memory area as an operand: they
store the contents of the GDTR (global descriptor table register) or
IDTR (interrupt descriptor table register) into that area as a 32-bit
linear address and a 16-bit size limit from that area (in that order).
These are the only instructions which directly use _linear_ addresses,
rather than segment/offset pairs.

   `SLDT' stores the segment selector corresponding to the LDT (local
descriptor table) into the given operand.

   See also `LGDT', `LIDT' and `LLDT' (*Note Section A.95::).


File: nasm.info,  Node: Section A.152,  Next: Section A.153,  Prev: Section A.151,  Up: Appendix A

A.152. `SHL', `SHR': Bitwise Logical Shifts
*******************************************

     SHL r/m8,1                    ; D0 /4                [8086]
     SHL r/m8,CL                   ; D2 /4                [8086]
     SHL r/m8,imm8                 ; C0 /4 ib             [286]
     SHL r/m16,1                   ; o16 D1 /4            [8086]
     SHL r/m16,CL                  ; o16 D3 /4            [8086]
     SHL r/m16,imm8                ; o16 C1 /4 ib         [286]
     SHL r/m32,1                   ; o32 D1 /4            [386]
     SHL r/m32,CL                  ; o32 D3 /4            [386]
     SHL r/m32,imm8                ; o32 C1 /4 ib         [386]

     SHR r/m8,1                    ; D0 /5                [8086]
     SHR r/m8,CL                   ; D2 /5                [8086]
     SHR r/m8,imm8                 ; C0 /5 ib             [286]
     SHR r/m16,1                   ; o16 D1 /5            [8086]
     SHR r/m16,CL                  ; o16 D3 /5            [8086]
     SHR r/m16,imm8                ; o16 C1 /5 ib         [286]
     SHR r/m32,1                   ; o32 D1 /5            [386]
     SHR r/m32,CL                  ; o32 D3 /5            [386]
     SHR r/m32,imm8                ; o32 C1 /5 ib         [386]

   `SHL' and `SHR' perform a logical shift operation on the given
source/destination (first) operand. The vacated bits are filled with
zero.

   A synonym for `SHL' is `SAL' (see *Note Section A.146::). NASM will
assemble either one to the same code, but NDISASM will always
disassemble that code as `SHL'.

   The number of bits to shift by is given by the second operand. Only
the bottom 3, 4 or 5 bits (depending on the source operand size) of the
shift count are considered by processors above the 8086.

   You can force the longer (286 and upwards, beginning with a `C1'
byte) form of `SHL foo,1' by using a `BYTE' prefix: `SHL foo,BYTE 1'.
Similarly with `SHR'.


File: nasm.info,  Node: Section A.153,  Next: Section A.154,  Prev: Section A.152,  Up: Appendix A

A.153. `SHLD', `SHRD': Bitwise Double-Precision Shifts
******************************************************

     SHLD r/m16,reg16,imm8         ; o16 0F A4 /r ib      [386]
     SHLD r/m16,reg32,imm8         ; o32 0F A4 /r ib      [386]
     SHLD r/m16,reg16,CL           ; o16 0F A5 /r         [386]
     SHLD r/m16,reg32,CL           ; o32 0F A5 /r         [386]

     SHRD r/m16,reg16,imm8         ; o16 0F AC /r ib      [386]
     SHRD r/m32,reg32,imm8         ; o32 0F AC /r ib      [386]
     SHRD r/m16,reg16,CL           ; o16 0F AD /r         [386]
     SHRD r/m32,reg32,CL           ; o32 0F AD /r         [386]

   `SHLD' performs a double-precision left shift. It notionally places
its second operand to the right of its first, then shifts the entire bit
string thus generated to the left by a number of bits specified in the
third operand. It then updates only the _first_ operand according to
the result of this. The second operand is not modified.

   `SHRD' performs the corresponding right shift: it notionally places
the second operand to the _left_ of the first, shifts the whole bit
string right, and updates only the first operand.

   For example, if `EAX' holds `0x01234567' and `EBX' holds
`0x89ABCDEF', then the instruction `SHLD EAX,EBX,4' would update `EAX'
to hold `0x12345678'. Under the same conditions, `SHRD EAX,EBX,4' would
update `EAX' to hold `0xF0123456'.

   The number of bits to shift by is given by the third operand. Only
the bottom 5 bits of the shift count are considered.


File: nasm.info,  Node: Section A.154,  Next: Section A.155,  Prev: Section A.153,  Up: Appendix A

A.154. `SMI': System Management Interrupt
*****************************************

     SMI                           ; F1                   [386,UNDOC]

   This is an opcode apparently supported by some AMD processors (which
is why it can generate the same opcode as `INT1'), and places the
machine into system-management mode, a special debugging mode.


File: nasm.info,  Node: Section A.155,  Next: Section A.156,  Prev: Section A.154,  Up: Appendix A

A.155. `SMSW': Store Machine Status Word
****************************************

     SMSW r/m16                    ; 0F 01 /4             [286,PRIV]

   `SMSW' stores the bottom half of the `CR0' control register (or the
Machine Status Word, on 286 processors) into the destination operand.
See also `LMSW' (*Note Section A.96::).


File: nasm.info,  Node: Section A.156,  Next: Section A.157,  Prev: Section A.155,  Up: Appendix A

A.156. `STC', `STD', `STI': Set Flags
*************************************

     STC                           ; F9                   [8086]
     STD                           ; FD                   [8086]
     STI                           ; FB                   [8086]

   These instructions set various flags. `STC' sets the carry flag;
`STD' sets the direction flag; and `STI' sets the interrupt flag (thus
enabling interrupts).

   To clear the carry, direction, or interrupt flags, use the `CLC',
`CLD' and `CLI' instructions (*Note Section A.15::). To invert the
carry flag, use `CMC' (*Note Section A.16::).


File: nasm.info,  Node: Section A.157,  Next: Section A.158,  Prev: Section A.156,  Up: Appendix A

A.157. `STOSB', `STOSW', `STOSD': Store Byte to String
******************************************************

     STOSB                         ; AA                   [8086]
     STOSW                         ; o16 AB               [8086]
     STOSD                         ; o32 AB               [386]

   `STOSB' stores the byte in `AL' at `[ES:DI]' or `[ES:EDI]', and sets
the flags accordingly. It then increments or decrements (depending on
the direction flag: increments if the flag is clear, decrements if it
is set) `DI' (or `EDI').

   The register used is `DI' if the address size is 16 bits, and `EDI'
if it is 32 bits. If you need to use an address size not equal to the
current `BITS' setting, you can use an explicit `a16' or `a32' prefix.

   Segment override prefixes have no effect for this instruction: the
use of `ES' for the store to `[DI]' or `[EDI]' cannot be overridden.

   `STOSW' and `STOSD' work in the same way, but they store the word in
`AX' or the doubleword in `EAX' instead of the byte in `AL', and
increment or decrement the addressing registers by 2 or 4 instead of 1.

   The `REP' prefix may be used to repeat the instruction `CX' (or
`ECX' - again, the address size chooses which) times.


File: nasm.info,  Node: Section A.158,  Next: Section A.159,  Prev: Section A.157,  Up: Appendix A

A.158. `STR': Store Task Register
*********************************

     STR r/m16                     ; 0F 00 /1             [286,PRIV]

   `STR' stores the segment selector corresponding to the contents of
the Task Register into its operand.


File: nasm.info,  Node: Section A.159,  Next: Section A.160,  Prev: Section A.158,  Up: Appendix A

A.159. `SUB': Subtract Integers
*******************************

     SUB r/m8,reg8                 ; 28 /r                [8086]
     SUB r/m16,reg16               ; o16 29 /r            [8086]
     SUB r/m32,reg32               ; o32 29 /r            [386]

     SUB reg8,r/m8                 ; 2A /r                [8086]
     SUB reg16,r/m16               ; o16 2B /r            [8086]
     SUB reg32,r/m32               ; o32 2B /r            [386]

     SUB r/m8,imm8                 ; 80 /5 ib             [8086]
     SUB r/m16,imm16               ; o16 81 /5 iw         [8086]
     SUB r/m32,imm32               ; o32 81 /5 id         [386]

     SUB r/m16,imm8                ; o16 83 /5 ib         [8086]
     SUB r/m32,imm8                ; o32 83 /5 ib         [386]

     SUB AL,imm8                   ; 2C ib                [8086]
     SUB AX,imm16                  ; o16 2D iw            [8086]
     SUB EAX,imm32                 ; o32 2D id            [386]

   `SUB' performs integer subtraction: it subtracts its second operand
from its first, and leaves the result in its destination (first)
operand.  The flags are set according to the result of the operation:
in particular, the carry flag is affected and can be used by a
subsequent `SBB' instruction (*Note Section A.148::).

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.


File: nasm.info,  Node: Section A.160,  Next: Section A.161,  Prev: Section A.159,  Up: Appendix A

A.160. `TEST': Test Bits (notional bitwise AND)
***********************************************

     TEST r/m8,reg8                ; 84 /r                [8086]
     TEST r/m16,reg16              ; o16 85 /r            [8086]
     TEST r/m32,reg32              ; o32 85 /r            [386]

     TEST r/m8,imm8                ; F6 /7 ib             [8086]
     TEST r/m16,imm16              ; o16 F7 /7 iw         [8086]
     TEST r/m32,imm32              ; o32 F7 /7 id         [386]

     TEST AL,imm8                  ; A8 ib                [8086]
     TEST AX,imm16                 ; o16 A9 iw            [8086]
     TEST EAX,imm32                ; o32 A9 id            [386]

   `TEST' performs a `mental' bitwise AND of its two operands, and
affects the flags as if the operation had taken place, but does not
store the result of the operation anywhere.


File: nasm.info,  Node: Section A.161,  Next: Section A.162,  Prev: Section A.160,  Up: Appendix A

A.161. `UMOV': User Move Data
*****************************

     UMOV r/m8,reg8                ; 0F 10 /r             [386,UNDOC]
     UMOV r/m16,reg16              ; o16 0F 11 /r         [386,UNDOC]
     UMOV r/m32,reg32              ; o32 0F 11 /r         [386,UNDOC]

     UMOV reg8,r/m8                ; 0F 12 /r             [386,UNDOC]
     UMOV reg16,r/m16              ; o16 0F 13 /r         [386,UNDOC]
     UMOV reg32,r/m32              ; o32 0F 13 /r         [386,UNDOC]

   This undocumented instruction is used by in-circuit emulators to
access user memory (as opposed to host memory). It is used just like an
ordinary memory/register or register/register `MOV' instruction, but
accesses user space.


File: nasm.info,  Node: Section A.162,  Next: Section A.163,  Prev: Section A.161,  Up: Appendix A

A.162. `VERR', `VERW': Verify Segment Readability/Writability
*************************************************************

     VERR r/m16                    ; 0F 00 /4             [286,PRIV]

     VERW r/m16                    ; 0F 00 /5             [286,PRIV]

   `VERR' sets the zero flag if the segment specified by the selector in
its operand can be read from at the current privilege level. `VERW'
sets the zero flag if the segment can be written.


File: nasm.info,  Node: Section A.163,  Next: Section A.164,  Prev: Section A.162,  Up: Appendix A

A.163. `WAIT': Wait for Floating-Point Processor
************************************************

     WAIT                          ; 9B                   [8086]

   `WAIT', on 8086 systems with a separate 8087 FPU, waits for the FPU
to have finished any operation it is engaged in before continuing main
processor operations, so that (for example) an FPU store to main memory
can be guaranteed to have completed before the CPU tries to read the
result back out.

   On higher processors, `WAIT' is unnecessary for this purpose, and it
has the alternative purpose of ensuring that any pending unmasked FPU
exceptions have happened before execution continues.


File: nasm.info,  Node: Section A.164,  Next: Section A.165,  Prev: Section A.163,  Up: Appendix A

A.164. `WBINVD': Write Back and Invalidate Cache
************************************************

     WBINVD                        ; 0F 09                [486]

   `WBINVD' invalidates and empties the processor's internal caches, and
causes the processor to instruct external caches to do the same. It
writes the contents of the caches back to memory first, so no data is
lost. To flush the caches quickly without bothering to write the data
back first, use `INVD' (*Note Section A.84::).


File: nasm.info,  Node: Section A.165,  Next: Section A.166,  Prev: Section A.164,  Up: Appendix A

A.165. `WRMSR': Write Model-Specific Registers
**********************************************

     WRMSR                         ; 0F 30                [PENT]

   `WRMSR' writes the value in `EDX:EAX' to the processor Model-
Specific Register (MSR) whose index is stored in `ECX'. See also
`RDMSR' (*Note Section A.139::).


File: nasm.info,  Node: Section A.166,  Next: Section A.167,  Prev: Section A.165,  Up: Appendix A

A.166. `XADD': Exchange and Add
*******************************

     XADD r/m8,reg8                ; 0F C0 /r             [486]
     XADD r/m16,reg16              ; o16 0F C1 /r         [486]
     XADD r/m32,reg32              ; o32 0F C1 /r         [486]

   `XADD' exchanges the values in its two operands, and then adds them
together and writes the result into the destination (first) operand.
This instruction can be used with a `LOCK' prefix for multi-processor
synchronisation purposes.


File: nasm.info,  Node: Section A.167,  Next: Section A.168,  Prev: Section A.166,  Up: Appendix A

A.167. `XBTS': Extract Bit String
*********************************

     XBTS reg16,r/m16              ; o16 0F A6 /r         [386,UNDOC]
     XBTS reg32,r/m32              ; o32 0F A6 /r         [386,UNDOC]

   No clear documentation seems to be available for this instruction:
the best I've been able to find reads `Takes a string of bits from the
first operand and puts them in the second operand'. It is present only
in early 386 processors, and conflicts with the opcodes for
`CMPXCHG486'. NASM supports it only for completeness. Its counterpart
is `IBTS' (see *Note Section A.75::).


File: nasm.info,  Node: Section A.168,  Next: Section A.169,  Prev: Section A.167,  Up: Appendix A

A.168. `XCHG': Exchange
***********************

     XCHG reg8,r/m8                ; 86 /r                [8086]
     XCHG reg16,r/m8               ; o16 87 /r            [8086]
     XCHG reg32,r/m32              ; o32 87 /r            [386]

     XCHG r/m8,reg8                ; 86 /r                [8086]
     XCHG r/m16,reg16              ; o16 87 /r            [8086]
     XCHG r/m32,reg32              ; o32 87 /r            [386]

     XCHG AX,reg16                 ; o16 90+r             [8086]
     XCHG EAX,reg32                ; o32 90+r             [386]
     XCHG reg16,AX                 ; o16 90+r             [8086]
     XCHG reg32,EAX                ; o32 90+r             [386]

   `XCHG' exchanges the values in its two operands. It can be used with
a `LOCK' prefix for purposes of multi-processor synchronisation.

   `XCHG AX,AX' or `XCHG EAX,EAX' (depending on the `BITS' setting)
generates the opcode `90h', and so is a synonym for `NOP' (*Note
Section A.109::).


File: nasm.info,  Node: Section A.169,  Next: Section A.170,  Prev: Section A.168,  Up: Appendix A

A.169. `XLATB': Translate Byte in Lookup Table
**********************************************

     XLATB                         ; D7                   [8086]

   `XLATB' adds the value in `AL', treated as an unsigned byte, to `BX'
or `EBX', and loads the byte from the resulting address (in the segment
specified by `DS') back into `AL'.

   The base register used is `BX' if the address size is 16 bits, and
`EBX' if it is 32 bits. If you need to use an address size not equal to
the current `BITS' setting, you can use an explicit `a16' or `a32'
prefix.

   The segment register used to load from `[BX+AL]' or `[EBX+AL]' can
be overridden by using a segment register name as a prefix (for
example, `es xlatb').


File: nasm.info,  Node: Section A.170,  Prev: Section A.169,  Up: Appendix A

A.170. `XOR': Bitwise Exclusive OR
**********************************

     XOR r/m8,reg8                 ; 30 /r                [8086]
     XOR r/m16,reg16               ; o16 31 /r            [8086]
     XOR r/m32,reg32               ; o32 31 /r            [386]

     XOR reg8,r/m8                 ; 32 /r                [8086]
     XOR reg16,r/m16               ; o16 33 /r            [8086]
     XOR reg32,r/m32               ; o32 33 /r            [386]

     XOR r/m8,imm8                 ; 80 /6 ib             [8086]
     XOR r/m16,imm16               ; o16 81 /6 iw         [8086]
     XOR r/m32,imm32               ; o32 81 /6 id         [386]

     XOR r/m16,imm8                ; o16 83 /6 ib         [8086]
     XOR r/m32,imm8                ; o32 83 /6 ib         [386]

     XOR AL,imm8                   ; 34 ib                [8086]
     XOR AX,imm16                  ; o16 35 iw            [8086]
     XOR EAX,imm32                 ; o32 35 id            [386]

   `XOR' performs a bitwise XOR operation between its two operands (i.e.
each bit of the result is 1 if and only if exactly one of the
corresponding bits of the two inputs was 1), and stores the result in
the destination (first) operand.

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.

   The MMX instruction `PXOR' (see *Note Section A.137::) performs the
same operation on the 64-bit MMX registers.


File: nasm.info,  Node: Index,  Up: Top

Index
*****

* Menu:

* `!=' operator                            (1): Section 4.3.3.
* `$' Here token                           (1): Section 3.5.
* `$$' token                               (1): Section 3.5.
* `$$' token                               (2): Section 6.5.2.
* `%' operator                             (1): Section 3.5.6.
* `%$' and `%$$' prefixes                  (1): Section 4.6.2.
* `%%' operator                            (1): Section 3.5.6.
* `%%' operator                            (2): Section 4.2.2.
* `%+1' and `%-1' syntax                   (1): Section 4.2.8.
* `%0' parameter count                     (1): Section 4.2.4.
* `%0' parameter count                     (2): Section 4.2.5.
* `&' operator                             (1): Section 3.5.3.
* `&&' operator                            (1): Section 4.3.3.
* `*' operator                             (1): Section 3.5.6.
* `+' modifier                             (1): Section 4.2.3.
* `+' operator, binary                     (1): Section 3.5.5.
* `+' operator, unary                      (1): Section 3.5.7.
* `-' operator, binary                     (1): Section 3.5.5.
* `-' operator, unary                      (1): Section 3.5.7.
* `..@' symbol prefix                      (1): Section 3.8.
* `..@' symbol prefix                      (2): Section 4.2.2.
* `/' operator                             (1): Section 3.5.6.
* `//' operator                            (1): Section 3.5.6.
* `<' operator                             (1): Section 4.3.3.
* `<<' operator                            (1): Section 3.5.4.
* `<=' operator                            (1): Section 4.3.3.
* `<>' operator                            (1): Section 4.3.3.
* `=' operator                             (1): Section 4.3.3.
* `==' operator                            (1): Section 4.3.3.
* `>' operator                             (1): Section 4.3.3.
* `>=' operator                            (1): Section 4.3.3.
* `>>' operator                            (1): Section 3.5.4.
* `?' MASM syntax                          (1): Section 3.2.2.
* `^' operator                             (1): Section 3.5.2.
* `^^' operator                            (1): Section 4.3.3.
* `|' operator                             (1): Section 3.5.1.
* `||' operator                            (1): Section 4.3.3.
* `~' operator                             (1): Section 3.5.7.
* `-a' option                              (1): Section 2.1.11.
* `a16'                                    (1): Section 9.3.
* `a16'                                    (2): Section A.19.
* `a16'                                    (3): Section A.80.
* `a16'                                    (4): Section A.98.
* `a16'                                    (5): Section A.105.
* `a16'                                    (6): Section A.112.
* `a16'                                    (7): Section A.126.
* `a16'                                    (8): Section A.134.
* `a16'                                    (9): Section A.149.
* `a16'                                    (A): Section A.157.
* `a16'                                    (B): Section A.169.
* `a32'                                    (1): Section 9.3.
* `a32'                                    (2): Section A.19.
* `a32'                                    (3): Section A.80.
* `a32'                                    (4): Section A.98.
* `a32'                                    (5): Section A.105.
* `a32'                                    (6): Section A.112.
* `a32'                                    (7): Section A.126.
* `a32'                                    (8): Section A.134.
* `a32'                                    (9): Section A.149.
* `a32'                                    (A): Section A.157.
* `a32'                                    (B): Section A.169.
* `a86'                                    (1): Section 1.1.1.
* `a86'                                    (2): Section 2.2.
* `a86'                                    (3): Section 2.2.2.
* `a86'                                    (4): Section 2.2.6.
* `AAA'                                    (1): Section A.4.
* `AAD'                                    (1): Section A.4.
* `AAM'                                    (1): Section A.4.
* `AAS'                                    (1): Section A.4.
* `ABSOLUTE'                               (1): Section 5.3.
* `ABSOLUTE'                               (2): Section 6.2.1.
* `ADC'                                    (1): Section A.5.
* `ADD'                                    (1): Section A.6.
* addition                                 (1): Section 3.5.5.
* addressing, mixed-size                   (1): Section 9.2.
* address-size prefixes                    (1): Section 3.1.
* algebra                                  (1): Section 3.3.
* `ALIGN'                                  (1): Section 4.7.5.
* `ALIGN'                                  (2): Section 6.1.2.
* `ALIGN'                                  (3): Section 6.2.1.
* `ALIGNB'                                 (1): Section 4.7.5.
* alignment, in `bin' sections             (1): Section 6.1.2.
* alignment, in `elf' sections             (1): Section 6.5.1.
* alignment, in `obj' sections             (1): Section 6.2.1.
* alignment, in `win32' sections           (1): Section 6.3.1.
* alignment, of `elf' common variables     (1): Section 6.5.4.
* `alloc'                                  (1): Section 6.5.1.
* `alt.lang.asm'                           (1): Section 1.1.1.
* `alt.lang.asm'                           (2): Section 1.2.
* ambiguity                                (1): Section 2.2.3.
* `AND'                                    (1): Section A.7.
* `a.out', BSD version                     (1): Section 6.7.
* `a.out', Linux version                   (1): Section 6.6.
* `aout'                                   (1): Section 2.1.1.
* `aout'                                   (2): Section 6.6.
* `aoutb'                                  (1): Section 6.7.
* `aoutb'                                  (2): Section 8.2.
* `arg'                                    (1): Section 7.4.5.
* `arg'                                    (2): Section 8.1.4.
* `ARPL'                                   (1): Section A.8.
* `as86'                                   (1): Section 1.1.1.
* `as86'                                   (2): Section 2.1.1.
* `as86'                                   (3): Section 6.8.
* assembler directives                     (1): Chapter 5.
* assembly passes                          (1): Section 3.7.
* assembly-time options                    (1): Section 2.1.8.
* `%assign'                                (1): Section 4.1.3.
* `ASSUME'                                 (1): Section 2.2.4.
* `AT'                                     (1): Section 4.7.4.
* Autoconf                                 (1): Section 1.3.2.
* `autoexec.bat'                           (1): Section 1.3.1.
* `bin'                                    (1): Section 2.1.1.
* `bin'                                    (2): Section 2.1.2.
* `bin'                                    (3): Section 6.1.
* binary                                   (1): Section 3.4.1.
* binary files                             (1): Section 3.2.3.
* 16-bit mode, versus 32-bit mode          (1): Section 5.1.
* bit shift                                (1): Section 3.5.4.
* `BITS'                                   (1): Section 5.1.
* `BITS'                                   (2): Section 6.1.
* bitwise AND                              (1): Section 3.5.3.
* bitwise OR                               (1): Section 3.5.1.
* bitwise XOR                              (1): Section 3.5.2.
* block IFs                                (1): Section 4.6.5.
* boot loader                              (1): Section 6.1.
* boot sector                              (1): Section 10.1.3.
* Borland, Pascal                          (1): Section 7.5.
* Borland, Win32 compilers                 (1): Section 6.2.
* `BOUND'                                  (1): Section A.9.
* braces, after `%' sign                   (1): Section 4.2.7.
* braces, around macro parameters          (1): Section 4.2.
* BSD                                      (1): Section 8.2.
* `BSF'                                    (1): Section A.10.
* `BSR'                                    (1): Section A.10.
* `.bss'                                   (1): Section 6.1.
* `.bss'                                   (2): Section 6.5.1.
* `.bss'                                   (3): Section 6.6.
* `.bss'                                   (4): Section 6.7.
* `.bss'                                   (5): Section 6.8.
* `.bss'                                   (6): Section 6.9.
* `BSWAP'                                  (1): Section A.11.
* `BT'                                     (1): Section A.12.
* `BTC'                                    (1): Section A.12.
* `BTR'                                    (1): Section A.12.
* `BTS'                                    (1): Section A.12.
* bugs                                     (1): Section 10.2.
* `BYTE'                                   (1): Section 10.1.1.
* C calling convention                     (1): Section 7.4.3.
* C calling convention                     (2): Section 8.1.2.
* C symbol names                           (1): Section 7.4.1.
* `CALL'                                   (1): Section A.13.
* `CALL FAR'                               (1): Section 3.6.
* case sensitivity                         (1): Section 2.2.1.
* case sensitivity                         (2): Section 4.1.1.
* case sensitivity                         (3): Section 4.1.3.
* case sensitivity                         (4): Section 4.2.
* case sensitivity                         (5): Section 4.3.4.
* case sensitivity                         (6): Section 6.2.3.
* `CBW'                                    (1): Section A.14.
* `CDQ'                                    (1): Section A.14.
* changing sections                        (1): Section 5.2.
* character constant                       (1): Section 3.2.1.
* character constant                       (2): Section 3.4.2.
* circular references                      (1): Section 4.1.1.
* `CLASS'                                  (1): Section 6.2.1.
* `CLC'                                    (1): Section A.15.
* `CLD'                                    (1): Section A.15.
* `%clear'                                 (1): Section 4.7.
* `CLI'                                    (1): Section A.15.
* `CLTS'                                   (1): Section A.15.
* `c16.mac'                                (1): Section 7.4.5.
* `c16.mac'                                (2): Section 7.5.3.
* `c32.mac'                                (1): Section 8.1.4.
* `CMC'                                    (1): Section A.16.
* `CMOVcc'                                 (1): Section A.17.
* `CMP'                                    (1): Section A.18.
* `CMPSB'                                  (1): Section A.19.
* `CMPSD'                                  (1): Section A.19.
* `CMPSW'                                  (1): Section A.19.
* `CMPXCHG'                                (1): Section A.20.
* `CMPXCHG486'                             (1): Section A.20.
* `CMPXCHG8B'                              (1): Section A.21.
* `coff'                                   (1): Section 2.1.1.
* `coff'                                   (2): Section 6.4.
* colon                                    (1): Section 3.1.
* `.COM'                                   (1): Section 6.1.
* `.COM'                                   (2): Section 7.2.
* command-line                             (1): Section 2.1.
* command-line                             (2): Chapter 6.
* commas in macro parameters               (1): Section 4.2.3.
* `COMMON'                                 (1): Section 5.6.
* `COMMON'                                 (2): Section 6.2.1.
* `COMMON', `elf' extensions to            (1): Section 6.5.4.
* `COMMON', `obj' extensions to            (1): Section 6.2.8.
* Common Object File Format                (1): Section 6.4.
* common variables                         (1): Section 5.6.
* common variables, alignment in `elf'     (1): Section 6.5.4.
* common variables, element size           (1): Section 6.2.8.
* `comp.archives.msdos.announce'           (1): Section 1.2.
* `comp.lang.asm.x86'                      (1): Section 1.1.1.
* `comp.lang.asm.x86'                      (2): Section 1.2.
* `comp.os.linux.announce'                 (1): Section 1.2.
* `comp.os.msdos.programmer'               (1): Section 7.3.
* concatenating macro parameters           (1): Section 4.2.7.
* condition codes                          (1): Section A.2.2.
* condition codes as macro parameters      (1): Section 4.2.8.
* conditional assembly                     (1): Section 4.3.
* conditional jump                         (1): Section A.89.
* conditional jumps                        (1): Section 10.1.2.
* conditional-return macro                 (1): Section 4.2.8.
* `configure'                              (1): Section 1.3.2.
* constants                                (1): Section 3.4.
* context stack                            (1): Section 4.6.
* context stack                            (2): Section 4.6.5.
* context-local labels                     (1): Section 4.6.2.
* context-local single-line macros         (1): Section 4.6.3.
* control registers                        (1): Section A.2.1.
* counting macro parameters                (1): Section 4.2.5.
* `CPUID'                                  (1): Section 3.4.2.
* `CPUID'                                  (2): Section A.22.
* creating contexts                        (1): Section 4.6.1.
* critical expression                      (1): Section 3.2.2.
* critical expression                      (2): Section 3.2.4.
* critical expression                      (3): Section 3.7.
* critical expression                      (4): Section 4.1.3.
* critical expression                      (5): Section 5.3.
* `CWD'                                    (1): Section A.14.
* `CWDE'                                   (1): Section A.14.
* `-D' option                              (1): Section 2.1.8.
* `-d' option                              (1): Section 2.1.8.
* `DAA'                                    (1): Section A.23.
* `DAS'                                    (1): Section A.23.
* `.data'                                  (1): Section 6.1.
* `.data'                                  (2): Section 6.5.1.
* `.data'                                  (3): Section 6.6.
* `.data'                                  (4): Section 6.7.
* `.data'                                  (5): Section 6.8.
* `.data'                                  (6): Section 6.9.
* `_DATA'                                  (1): Section 7.4.2.
* `data'                                   (1): Section 6.5.3.
* data structure                           (1): Section 7.4.4.
* data structure                           (2): Section 8.1.3.
* `DB'                                     (1): Section 3.2.
* `DB'                                     (2): Section 3.2.1.
* `DB'                                     (3): Section 3.4.3.
* `dbg'                                    (1): Section 6.10.
* `DD'                                     (1): Section 3.2.
* `DD'                                     (2): Section 3.2.1.
* `DD'                                     (3): Section 3.4.3.
* `DD'                                     (4): Section 3.4.4.
* debug registers                          (1): Section A.2.1.
* `DEC'                                    (1): Section A.24.
* declaring structures                     (1): Section 4.7.3.
* default macro parameters                 (1): Section 4.2.4.
* default name                             (1): Chapter 6.
* default-`WRT' mechanism                  (1): Section 6.2.7.
* `%define'                                (1): Section 2.1.8.
* `%define'                                (2): Section 4.1.1.
* defining sections                        (1): Section 5.2.
* design goals                             (1): Section 2.2.2.
* DevPac                                   (1): Section 3.2.3.
* DevPac                                   (2): Section 3.8.
* disabling listing expansion              (1): Section 4.2.9.
* `DIV'                                    (1): Section A.25.
* division                                 (1): Section 3.5.6.
* DJGPP                                    (1): Section 6.4.
* DJGPP                                    (2): Chapter 8.
* `djlink'                                 (1): Section 7.1.1.
* DLL symbols, exporting                   (1): Section 6.2.5.
* DLL symbols, importing                   (1): Section 6.2.4.
* DOS                                      (1): Section 1.3.1.
* DOS                                      (2): Section 2.1.4.
* DOS                                      (3): Section 2.1.5.
* DOS archive                              (1): Section 1.3.1.
* DOS source archive                       (1): Section 1.3.1.
* `DQ'                                     (1): Section 3.2.
* `DQ'                                     (2): Section 3.2.1.
* `DQ'                                     (3): Section 3.4.3.
* `DQ'                                     (4): Section 3.4.4.
* `.drectve'                               (1): Section 6.3.1.
* `DT'                                     (1): Section 3.2.
* `DT'                                     (2): Section 3.2.1.
* `DT'                                     (3): Section 3.4.3.
* `DT'                                     (4): Section 3.4.4.
* `DUP'                                    (1): Section 2.2.7.
* `DUP'                                    (2): Section 3.2.5.
* `DW'                                     (1): Section 3.2.
* `DW'                                     (2): Section 3.2.1.
* `DW'                                     (3): Section 3.4.3.
* `DWORD'                                  (1): Section 3.1.
* `-E' option                              (1): Section 2.1.4.
* `-e' option                              (1): Section 2.1.10.
* effective addresses                      (1): Section 3.1.
* effective addresses                      (2): Section 3.3.
* effective addresses                      (3): Section 3.7.
* effective addresses                      (4): Section A.2.3.
* element size, in common variables        (1): Section 6.2.8.
* `elf'                                    (1): Section 2.1.1.
* `elf'                                    (2): Section 6.5.
* `elf' shared libraries                   (1): Section 6.5.2.
* `%elif'                                  (1): Section 4.3.
* `%elif'                                  (2): Section 4.3.3.
* `%elifctx'                               (1): Section 4.3.2.
* `%elifdef'                               (1): Section 4.3.1.
* `%elifid'                                (1): Section 4.3.5.
* `%elifidn'                               (1): Section 4.3.4.
* `%elifidni'                              (1): Section 4.3.4.
* `%elifnctx'                              (1): Section 4.3.2.
* `%elifndef'                              (1): Section 4.3.1.
* `%elifnid'                               (1): Section 4.3.5.
* `%elifnidn'                              (1): Section 4.3.4.
* `%elifnidni'                             (1): Section 4.3.4.
* `%elifnnum'                              (1): Section 4.3.5.
* `%elifnstr'                              (1): Section 4.3.5.
* `%elifnum'                               (1): Section 4.3.5.
* `%elifstr'                               (1): Section 4.3.5.
* `%else'                                  (1): Section 4.3.
* e-mail                                   (1): Section 1.2.
* `EMMS'                                   (1): Section A.26.
* `endproc'                                (1): Section 7.4.5.
* `endproc'                                (2): Section 8.1.4.
* `%endrep'                                (1): Section 4.4.
* `ENDSTRUC'                               (1): Section 4.7.3.
* `ENDSTRUC'                               (2): Section 5.3.
* `ENTER'                                  (1): Section A.27.
* environment                              (1): Section 2.1.13.
* `EQU'                                    (1): Section 3.2.
* `EQU'                                    (2): Section 3.2.4.
* `EQU'                                    (3): Section 3.7.
* `%error'                                 (1): Section 4.3.6.
* error messages                           (1): Section 2.1.4.
* error messages                           (2): Section 2.1.5.
* `EVEN'                                   (1): Section 4.7.5.
* `.EXE'                                   (1): Section 6.2.
* `.EXE'                                   (2): Section 7.1.
* `EXE_begin'                              (1): Section 7.1.2.
* `EXE2BIN'                                (1): Section 7.2.2.
* `exebin.mac'                             (1): Section 7.1.2.
* `exec'                                   (1): Section 6.5.1.
* executable and linkable format           (1): Section 6.5.
* `EXE_end'                                (1): Section 7.1.2.
* `EXE_stack'                              (1): Section 7.1.2.
* `%exitrep'                               (1): Section 4.4.
* `EXPORT'                                 (1): Section 6.2.5.
* exporting symbols                        (1): Section 5.5.
* expressions                              (1): Section 2.1.10.
* expressions                              (2): Section 3.5.
* extension                                (1): Section 2.1.1.
* extension                                (2): Chapter 6.
* `EXTERN'                                 (1): Section 5.4.
* extern, obj extensions to                (1): Section 6.2.7.
* `-f' option                              (1): Section 2.1.2.
* `-f' option                              (2): Chapter 6.
* `FABS'                                   (1): Section A.29.
* `FADD'                                   (1): Section A.30.
* `FADDP'                                  (1): Section A.30.
* far call                                 (1): Section 2.2.5.
* far call                                 (2): Section A.13.
* far common variables                     (1): Section 6.2.8.
* far jump                                 (1): Section A.88.
* far pointer                              (1): Section 3.6.
* `FARCODE'                                (1): Section 7.4.5.
* `FARCODE'                                (2): Section 7.5.3.
* `FBLD'                                   (1): Section A.31.
* `FBSTP'                                  (1): Section A.31.
* `FCHS'                                   (1): Section A.32.
* `FCLEX'                                  (1): Section A.33.
* `FCMOVcc'                                (1): Section A.34.
* `FCOM'                                   (1): Section A.35.
* `FCOMI'                                  (1): Section A.35.
* `FCOMIP'                                 (1): Section A.35.
* `FCOMP'                                  (1): Section A.35.
* `FCOMPP'                                 (1): Section A.35.
* `FCOS'                                   (1): Section A.36.
* `FDECSTP'                                (1): Section A.37.
* `FDIV'                                   (1): Section A.39.
* `FDIVP'                                  (1): Section A.39.
* `FDIVR'                                  (1): Section A.39.
* `FDIVRP'                                 (1): Section A.39.
* `FFREE'                                  (1): Section A.40.
* `FIADD'                                  (1): Section A.41.
* `FICOM'                                  (1): Section A.42.
* `FICOMP'                                 (1): Section A.42.
* `FIDIV'                                  (1): Section A.43.
* `FIDIVR'                                 (1): Section A.43.
* `FILD'                                   (1): Section A.44.
* `__FILE__'                               (1): Section 4.7.2.
* `FIMUL'                                  (1): Section A.45.
* `FINCSTP'                                (1): Section A.46.
* `FINIT'                                  (1): Section A.47.
* `FIST'                                   (1): Section A.44.
* `FISTP'                                  (1): Section A.44.
* `FISUB'                                  (1): Section A.48.
* `FLAT'                                   (1): Section 6.2.1.
* flat memory model                        (1): Chapter 8.
* flat-form binary                         (1): Section 6.1.
* `FLD'                                    (1): Section A.49.
* `FLDCW'                                  (1): Section A.51.
* `FLDENV'                                 (1): Section A.52.
* `FLDxx'                                  (1): Section A.50.
* floating-point                           (1): Section 2.2.6.
* floating-point                           (2): Section 3.1.
* floating-point                           (3): Section 3.2.1.
* floating-point                           (4): Section 3.4.4.
* floating-point, constants                (1): Section 3.4.4.
* floating-point, registers                (1): Section A.2.1.
* `FMUL'                                   (1): Section A.53.
* `FMULP'                                  (1): Section A.53.
* `FNINIT'                                 (1): Section A.47.
* `FNOP'                                   (1): Section A.54.
* format-specific directives               (1): Chapter 5.
* forward references                       (1): Section 3.7.
* `FPATAN'                                 (1): Section A.55.
* `FPREM'                                  (1): Section A.56.
* `FPREM1'                                 (1): Section A.56.
* `FPTAN'                                  (1): Section A.55.
* frame pointer                            (1): Section 7.4.3.
* frame pointer                            (2): Section 7.5.1.
* frame pointer                            (3): Section 8.1.2.
* FreeBSD                                  (1): Section 6.7.
* FreeBSD                                  (2): Section 8.2.
* FreeLink                                 (1): Section 7.1.1.
* `FRNDINT'                                (1): Section A.57.
* `FRSTOR'                                 (1): Section A.58.
* `FSAVE'                                  (1): Section A.58.
* `FSCALE'                                 (1): Section A.59.
* `FSETPM'                                 (1): Section A.60.
* `FSIN'                                   (1): Section A.61.
* `FSINCOS'                                (1): Section A.61.
* `FSQRT'                                  (1): Section A.62.
* `FST'                                    (1): Section A.63.
* `FSTCW'                                  (1): Section A.64.
* `FSTENV'                                 (1): Section A.65.
* `FSTP'                                   (1): Section A.63.
* `FSTSW'                                  (1): Section A.66.
* `FSUB'                                   (1): Section A.67.
* `FSUBP'                                  (1): Section A.67.
* `FSUBR'                                  (1): Section A.67.
* `FSUBRP'                                 (1): Section A.67.
* `ftp.coast.net'                          (1): Section 1.2.
* `ftp.kernel.org'                         (1): Section 1.2.
* `ftp.simtel.net'                         (1): Section 1.2.
* `ftp.simtel.net'                         (2): Section 7.1.1.
* `FTST'                                   (1): Section A.68.
* `FUCOMxx'                                (1): Section A.69.
* `function'                               (1): Section 6.5.3.
* functions, C calling convention          (1): Section 7.4.3.
* functions, C calling convention          (2): Section 8.1.2.
* functions, Pascal calling convention     (1): Section 7.5.1.
* `FXAM'                                   (1): Section A.70.
* `FXCH'                                   (1): Section A.71.
* `FxDISI'                                 (1): Section A.38.
* `FxENI'                                  (1): Section A.38.
* `F2XM1'                                  (1): Section A.28.
* `FXTRACT'                                (1): Section A.72.
* `FYL2X'                                  (1): Section A.73.
* `FYL2XP1'                                (1): Section A.73.
* `gas'                                    (1): Section 1.1.1.
* `gcc'                                    (1): Section 1.1.1.
* general purpose register                 (1): Section A.1.
* `GLOBAL'                                 (1): Section 5.5.
* `GLOBAL', `aoutb' extensions to          (1): Section 6.5.3.
* `GLOBAL', `elf' extensions to            (1): Section 6.5.3.
* global offset table                      (1): Section 8.2.
* `_GLOBAL_OFFSET_TABLE_'                  (1): Section 6.5.2.
* `..got'                                  (1): Section 6.5.2.
* `GOT' relocations                        (1): Section 8.2.3.
* GOT                                      (1): Section 6.5.2.
* GOT                                      (2): Section 8.2.
* `..gotoff'                               (1): Section 6.5.2.
* `GOTOFF' relocations                     (1): Section 8.2.2.
* `..gotpc'                                (1): Section 6.5.2.
* `GOTPC' relocations                      (1): Section 8.2.1.
* graphics                                 (1): Section 3.2.3.
* greedy macro parameters                  (1): Section 4.2.3.
* `GROUP'                                  (1): Section 6.2.2.
* groups                                   (1): Section 3.6.
* hex                                      (1): Section 3.4.1.
* `HLT'                                    (1): Section A.74.
* hybrid syntaxes                          (1): Section 2.2.2.
* `-I' option                              (1): Section 2.1.6.
* `-i' option                              (1): Section 2.1.6.
* `%iassign'                               (1): Section 4.1.3.
* `IBTS'                                   (1): Section A.75.
* `ICEBP'                                  (1): Section A.82.
* `%idefine'                               (1): Section 4.1.1.
* `IDIV'                                   (1): Section A.76.
* `IEND'                                   (1): Section 4.7.4.
* `%if'                                    (1): Section 4.3.
* `%if'                                    (2): Section 4.3.3.
* `%ifctx'                                 (1): Section 4.3.2.
* `%ifctx'                                 (2): Section 4.6.5.
* `%ifdef'                                 (1): Section 4.3.1.
* `%ifid'                                  (1): Section 4.3.5.
* `%ifidn'                                 (1): Section 4.3.4.
* `%ifidni'                                (1): Section 4.3.4.
* `%ifnctx'                                (1): Section 4.3.2.
* `%ifndef'                                (1): Section 4.3.1.
* `%ifnid'                                 (1): Section 4.3.5.
* `%ifnidn'                                (1): Section 4.3.4.
* `%ifnidni'                               (1): Section 4.3.4.
* `%ifnnum'                                (1): Section 4.3.5.
* `%ifnstr'                                (1): Section 4.3.5.
* `%ifnum'                                 (1): Section 4.3.5.
* `%ifstr'                                 (1): Section 4.3.5.
* `%imacro'                                (1): Section 4.2.
* immediate operand                        (1): Section A.1.
* `IMPORT'                                 (1): Section 6.2.4.
* import library                           (1): Section 6.2.4.
* importing symbols                        (1): Section 5.4.
* `IMUL'                                   (1): Section A.77.
* `IN'                                     (1): Section A.78.
* `INC'                                    (1): Section A.79.
* `INCBIN'                                 (1): Section 3.2.
* `INCBIN'                                 (2): Section 3.2.3.
* `INCBIN'                                 (3): Section 3.4.3.
* `%include'                               (1): Section 2.1.6.
* `%include'                               (2): Section 2.1.7.
* `%include'                               (3): Section 4.5.
* include search path                      (1): Section 2.1.6.
* including other files                    (1): Section 4.5.
* inefficient code                         (1): Section 10.1.1.
* infinite loop                            (1): Section 3.5.
* informational section                    (1): Section 6.3.1.
* `INSB'                                   (1): Section A.80.
* `INSD'                                   (1): Section A.80.
* `INSTALL'                                (1): Section 1.3.2.
* installing                               (1): Section 1.3.1.
* instances of structures                  (1): Section 4.7.4.
* `INSW'                                   (1): Section A.80.
* `INT'                                    (1): Section A.81.
* `INT01'                                  (1): Section A.82.
* `INT1'                                   (1): Section A.82.
* `INT3'                                   (1): Section A.82.
* integer overflow                         (1): Section 3.5.
* intel number formats                     (1): Section 3.4.4.
* `INTO'                                   (1): Section A.83.
* `INVD'                                   (1): Section A.84.
* `INVLPG'                                 (1): Section A.85.
* `IRET'                                   (1): Section A.86.
* `IRETD'                                  (1): Section A.86.
* `IRETW'                                  (1): Section A.86.
* `ISTRUC'                                 (1): Section 4.7.4.
* iterating over macro parameters          (1): Section 4.2.6.
* `Jcc'                                    (1): Section A.89.
* `Jcc NEAR'                               (1): Section 10.1.2.
* `JCXZ'                                   (1): Section A.87.
* `JECXZ'                                  (1): Section A.87.
* `JMP'                                    (1): Section A.88.
* `JMP DWORD'                              (1): Section 9.1.
* jumps, mixed-size                        (1): Section 9.1.
* `-l' option                              (1): Section 2.1.3.
* label prefix                             (1): Section 3.8.
* `LAHF'                                   (1): Section A.90.
* `LAR'                                    (1): Section A.91.
* `ld86'                                   (1): Section 6.8.
* `LDS'                                    (1): Section A.92.
* `LEA'                                    (1): Section A.93.
* `LEAVE'                                  (1): Section A.94.
* `LES'                                    (1): Section A.92.
* `LFS'                                    (1): Section A.92.
* `LGDT'                                   (1): Section A.95.
* `LGS'                                    (1): Section A.92.
* `LIBRARY'                                (1): Section 6.9.1.
* licence                                  (1): Section 1.1.2.
* `LIDT'                                   (1): Section A.95.
* `__LINE__'                               (1): Section 4.7.2.
* linker, free                             (1): Section 7.1.1.
* Linux ELF                                (1): Section 6.5.
* listing file                             (1): Section 2.1.3.
* little-endian                            (1): Section 3.4.2.
* `LLDT'                                   (1): Section A.95.
* `LMSW'                                   (1): Section A.96.
* `LOADALL'                                (1): Section A.97.
* `LOADALL286'                             (1): Section A.97.
* local labels                             (1): Section 3.8.
* `LODSB'                                  (1): Section A.98.
* `LODSD'                                  (1): Section A.98.
* `LODSW'                                  (1): Section A.98.
* logical AND                              (1): Section 4.3.3.
* logical OR                               (1): Section 4.3.3.
* logical XOR                              (1): Section 4.3.3.
* `LOOP'                                   (1): Section A.99.
* `LOOPE'                                  (1): Section A.99.
* `LOOPNE'                                 (1): Section A.99.
* `LOOPNZ'                                 (1): Section A.99.
* `LOOPZ'                                  (1): Section A.99.
* `LSL'                                    (1): Section A.100.
* `LSS'                                    (1): Section A.92.
* `LTR'                                    (1): Section A.101.
* `%macro'                                 (1): Section 4.2.
* macro library                            (1): Section 2.1.6.
* macro processor                          (1): Chapter 4.
* macro-local labels                       (1): Section 4.2.2.
* `macro-params'                           (1): Section 2.1.12.
* macros                                   (1): Section 3.2.5.
* `make'                                   (1): Section 1.3.2.
* makefiles                                (1): Section 1.3.1.
* makefiles                                (2): Section 1.3.2.
* `Makefile.unx'                           (1): Section 1.3.2.
* man pages                                (1): Section 1.3.2.
* MASM                                     (1): Section 1.1.1.
* MASM                                     (2): Section 2.2.
* MASM                                     (3): Section 3.2.5.
* MASM                                     (4): Section 6.2.
* memory models                            (1): Section 2.2.5.
* memory models                            (2): Section 7.4.2.
* memory operand                           (1): Section 3.1.
* memory references                        (1): Section 2.2.2.
* memory references                        (2): Section 3.3.
* memory references                        (3): Section A.1.
* Microsoft OMF                            (1): Section 6.2.
* `misc' subdirectory                      (1): Section 7.1.2.
* `misc' subdirectory                      (2): Section 7.4.5.
* `misc' subdirectory                      (3): Section 8.1.4.
* mixed-language program                   (1): Section 7.4.
* mixed-size addressing                    (1): Section 9.2.
* mixed-size instruction                   (1): Section 9.1.
* MMX registers                            (1): Section A.2.1.
* ModR/M byte                              (1): Section A.2.
* ModR/M byte                              (2): Section A.2.3.
* modulo operators                         (1): Section 3.5.6.
* `MOV'                                    (1): Section A.102.
* `MOVD'                                   (1): Section A.103.
* `MOVQ'                                   (1): Section A.104.
* `MOVSB'                                  (1): Section A.105.
* `MOVSD'                                  (1): Section A.105.
* `MOVSW'                                  (1): Section A.105.
* `MOVSX'                                  (1): Section A.106.
* `MOVZX'                                  (1): Section A.106.
* MS-DOS                                   (1): Section 6.1.
* MS-DOS device drivers                    (1): Section 7.3.
* `MUL'                                    (1): Section A.107.
* multi-line macros                        (1): Section 2.1.12.
* multi-line macros                        (2): Section 4.2.
* multiplication                           (1): Section 3.5.6.
* `multipush' macro                        (1): Section 4.2.6.
* `nasm.1'                                 (1): Section 1.3.2.
* NASM version                             (1): Section 4.7.1.
* `__NASMDEFSEG'                           (1): Section 6.2.
* `nasm.exe'                               (1): Section 1.3.1.
* `nasm -h'                                (1): Section 2.1.2.
* `__NASM_MAJOR__'                         (1): Section 4.7.1.
* `__NASM_MINOR__'                         (1): Section 4.7.1.
* `nasm.out'                               (1): Section 2.1.1.
* `nasmw.exe'                              (1): Section 1.3.1.
* `nasmXXXs.zip'                           (1): Section 1.3.1.
* `nasm-X.XX.tar.gz'                       (1): Section 1.3.2.
* `nasmXXX.zip'                            (1): Section 1.3.1.
* `ndisasm.1'                              (1): Section 1.3.2.
* `ndisasm.exe'                            (1): Section 1.3.1.
* `ndisasmw.exe'                           (1): Section 1.3.1.
* near call                                (1): Section 2.2.5.
* near call                                (2): Section A.13.
* near common variables                    (1): Section 6.2.8.
* near jump                                (1): Section A.88.
* `NEG'                                    (1): Section A.108.
* NetBSD                                   (1): Section 6.7.
* NetBSD                                   (2): Section 8.2.
* new releases                             (1): Section 1.2.
* `noalloc'                                (1): Section 6.5.1.
* `nobits'                                 (1): Section 6.5.1.
* `noexec'                                 (1): Section 6.5.1.
* `.nolist'                                (1): Section 4.2.9.
* `NOP'                                    (1): Section A.109.
* `NOT'                                    (1): Section A.108.
* `nowait'                                 (1): Section 2.2.6.
* `nowrite'                                (1): Section 6.5.1.
* `number-overflow'                        (1): Section 2.1.12.
* numeric constants                        (1): Section 3.2.1.
* numeric constants                        (2): Section 3.4.1.
* `-o' option                              (1): Section 2.1.1.
* `o16'                                    (1): Section 9.3.
* `o16'                                    (2): Section A.126.
* `o16'                                    (3): Section A.134.
* `o32'                                    (1): Section 9.3.
* `o32'                                    (2): Section A.126.
* `o32'                                    (3): Section A.134.
* `.OBJ'                                   (1): Section 7.1.
* `obj'                                    (1): Section 2.1.1.
* `obj'                                    (2): Section 6.2.
* `object'                                 (1): Section 6.5.3.
* octal                                    (1): Section 3.4.1.
* `OF_DBG'                                 (1): Section 6.10.
* `OF_DEFAULT'                             (1): Section 2.1.2.
* `OFFSET'                                 (1): Section 2.2.2.
* OMF                                      (1): Section 6.2.
* omitted parameters                       (1): Section 4.2.4.
* one's complement                         (1): Section 3.5.7.
* OpenBSD                                  (1): Section 6.7.
* OpenBSD                                  (2): Section 8.2.
* operands                                 (1): Section 3.1.
* operand-size prefixes                    (1): Section 3.1.
* operating system, writing                (1): Section 9.1.
* operating system                         (1): Section 6.1.
* operators                                (1): Section 3.5.
* `OR'                                     (1): Section A.110.
* `ORG'                                    (1): Section 6.1.1.
* `ORG'                                    (2): Section 7.2.1.
* `ORG'                                    (3): Section 7.2.2.
* `ORG'                                    (4): Section 10.1.3.
* `orphan-labels'                          (1): Section 2.1.12.
* `orphan-labels'                          (2): Section 3.1.
* OS/2                                     (1): Section 6.2.
* OS/2                                     (2): Section 6.2.1.
* `OUT'                                    (1): Section A.111.
* out of range, jumps                      (1): Section 10.1.2.
* output file format                       (1): Section 2.1.2.
* output formats                           (1): Chapter 6.
* `OUTSB'                                  (1): Section A.112.
* `OUTSD'                                  (1): Section A.112.
* `OUTSW'                                  (1): Section A.112.
* overlapping segments                     (1): Section 3.6.
* `OVERLAY'                                (1): Section 6.2.1.
* overloading multi-line macros            (1): Section 4.2.1.
* overloading, single-line macros          (1): Section 4.1.1.
* `-P' option                              (1): Section 2.1.7.
* `-p' option                              (1): Section 2.1.7.
* `-p' option                              (2): Section 4.5.
* `PACKSSDW'                               (1): Section A.113.
* `PACKSSWB'                               (1): Section A.113.
* `PACKUSWB'                               (1): Section A.113.
* `PADDSIW'                                (1): Section A.115.
* `PADDxx'                                 (1): Section A.114.
* `PAND'                                   (1): Section A.116.
* `PANDN'                                  (1): Section A.116.
* paradox                                  (1): Section 3.7.
* `PASCAL'                                 (1): Section 7.5.3.
* Pascal calling convention                (1): Section 7.5.1.
* passes, assembly                         (1): Section 3.7.
* `PATH'                                   (1): Section 1.3.1.
* `PAVEB'                                  (1): Section A.117.
* `PCMPxx'                                 (1): Section A.118.
* `PDISTIB'                                (1): Section A.119.
* period                                   (1): Section 3.8.
* Perl                                     (1): Section 1.3.1.
* perverse                                 (1): Section 2.1.6.
* PharLap                                  (1): Section 6.2.1.
* PIC                                      (1): Section 6.5.2.
* PIC                                      (2): Section 6.7.
* PIC                                      (3): Section 8.2.
* `..plt'                                  (1): Section 6.5.2.
* `PLT' relocations                        (1): Section 6.5.2.
* `PLT' relocations                        (2): Section 8.2.4.
* `PLT' relocations                        (3): Section 8.2.5.
* plt relocations                          (1): Section 8.2.5.
* `PMACHRIW'                               (1): Section A.120.
* `PMADDWD'                                (1): Section A.121.
* `PMAGW'                                  (1): Section A.122.
* `PMULHRIW'                               (1): Section A.123.
* `PMULHRW'                                (1): Section A.123.
* `PMULHW'                                 (1): Section A.124.
* `PMULLW'                                 (1): Section A.124.
* `PMVccZB'                                (1): Section A.125.
* `%pop'                                   (1): Section 4.6.
* `%pop'                                   (2): Section 4.6.1.
* `POP'                                    (1): Section A.126.
* `POPAx'                                  (1): Section A.127.
* `POPFx'                                  (1): Section A.128.
* `POR'                                    (1): Section A.129.
* position-independent code                (1): Section 6.5.2.
* position-independent code                (2): Section 6.7.
* position-independent code                (3): Section 8.2.
* precedence                               (1): Section 3.5.
* pre-defining macros                      (1): Section 2.1.8.
* pre-defining macros                      (2): Section 4.1.1.
* preferred                                (1): Section 3.6.
* $prefix                                  (1): Section 3.1.
* $prefix                                  (2): Section 3.4.1.
* pre-including files                      (1): Section 2.1.7.
* preprocess-only mode                     (1): Section 2.1.10.
* preprocessor                             (1): Section 2.1.10.
* preprocessor                             (2): Section 2.1.11.
* preprocessor                             (3): Section 3.2.4.
* preprocessor                             (4): Section 3.5.6.
* preprocessor                             (5): Chapter 4.
* preprocessor expressions                 (1): Section 2.1.10.
* preprocessor loops                       (1): Section 4.4.
* preprocessor variables                   (1): Section 4.1.3.
* primitive directives                     (1): Chapter 5.
* `PRIVATE'                                (1): Section 6.2.1.
* `proc'                                   (1): Section 7.4.5.
* `proc'                                   (2): Section 8.1.4.
* procedure linkage table                  (1): Section 6.5.2.
* procedure linkage table                  (2): Section 8.2.4.
* procedure linkage table                  (3): Section 8.2.5.
* processor mode                           (1): Section 5.1.
* `progbits'                               (1): Section 6.5.1.
* program entry point                      (1): Section 6.2.6.
* program entry point                      (2): Section 7.1.1.
* program origin                           (1): Section 6.1.1.
* pseudo-instructions                      (1): Section 3.2.
* `PSLLx'                                  (1): Section A.130.
* `PSRAx'                                  (1): Section A.130.
* `PSRLx'                                  (1): Section A.130.
* `PSUBSIW'                                (1): Section A.132.
* `PSUBxx'                                 (1): Section A.131.
* `PUBLIC'                                 (1): Section 5.5.
* `PUBLIC'                                 (2): Section 6.2.1.
* `PUNPCKxxx'                              (1): Section A.133.
* pure binary                              (1): Section 6.1.
* `%push'                                  (1): Section 4.6.
* `%push'                                  (2): Section 4.6.1.
* `PUSH'                                   (1): Section A.134.
* `PUSHAx'                                 (1): Section A.135.
* `PUSHFx'                                 (1): Section A.136.
* `PXOR'                                   (1): Section A.137.
* quick start                              (1): Section 2.2.
* `QWORD'                                  (1): Section 3.1.
* `RCL'                                    (1): Section A.138.
* `RCR'                                    (1): Section A.138.
* `rdf'                                    (1): Section 2.1.1.
* `rdf'                                    (2): Section 6.9.
* `RDMSR'                                  (1): Section A.139.
* `rdoff' subdirectory                     (1): Section 1.3.2.
* `rdoff' subdirectory                     (2): Section 6.9.
* `RDPMC'                                  (1): Section A.140.
* `RDTSC'                                  (1): Section A.141.
* redirecting errors                       (1): Section 2.1.4.
* register push                            (1): Section A.134.
* relational operators                     (1): Section 4.3.3.
* Relocatable Dynamic Object File Format   (1): Section 6.9.
* relocations, PIC-specific                (1): Section 6.5.2.
* removing contexts                        (1): Section 4.6.1.
* renaming contexts                        (1): Section 4.6.4.
* `%rep'                                   (1): Section 3.2.5.
* `%rep'                                   (2): Section 4.4.
* repeating                                (1): Section 3.2.5.
* repeating                                (2): Section 4.4.
* `%repl'                                  (1): Section 4.6.4.
* reporting bugs                           (1): Section 10.2.
* `RESB'                                   (1): Section 2.2.7.
* `RESB'                                   (2): Section 3.2.
* `RESB'                                   (3): Section 3.2.2.
* `RESB'                                   (4): Section 3.7.
* `RESD'                                   (1): Section 3.2.
* `RESD'                                   (2): Section 3.2.2.
* `RESQ'                                   (1): Section 3.2.
* `RESQ'                                   (2): Section 3.2.2.
* `REST'                                   (1): Section 3.2.
* `REST'                                   (2): Section 3.2.2.
* restricted memory references             (1): Section A.1.
* `RESW'                                   (1): Section 3.2.
* `RESW'                                   (2): Section 3.2.2.
* `RET'                                    (1): Section A.142.
* `RETF'                                   (1): Section A.142.
* `RETN'                                   (1): Section A.142.
* `ROL'                                    (1): Section A.143.
* `ROR'                                    (1): Section A.143.
* `%rotate'                                (1): Section 4.2.6.
* rotating macro parameters                (1): Section 4.2.6.
* `RSM'                                    (1): Section A.144.
* `-s' option                              (1): Section 2.1.5.
* `SAHF'                                   (1): Section A.145.
* `SAL'                                    (1): Section A.146.
* `SALC'                                   (1): Section A.147.
* `SAR'                                    (1): Section A.146.
* `SBB'                                    (1): Section A.148.
* `SCASB'                                  (1): Section A.149.
* `SCASD'                                  (1): Section A.149.
* `SCASW'                                  (1): Section A.149.
* searching for include files              (1): Section 4.5.
* `__SECT__'                               (1): Section 5.2.1.
* `__SECT__'                               (2): Section 5.3.
* `SECTION'                                (1): Section 5.2.
* `SECTION', `elf' extensions to           (1): Section 6.5.1.
* `SECTION', `win32' extensions to         (1): Section 6.3.1.
* section alignment, in `bin'              (1): Section 6.1.2.
* section alignment, in `elf'              (1): Section 6.5.1.
* section alignment, in `obj'              (1): Section 6.2.1.
* section alignment, in `win32'            (1): Section 6.3.1.
* section, bin extensions to               (1): Section 6.1.2.
* `SEG'                                    (1): Section 3.5.7.
* `SEG'                                    (2): Section 3.6.
* `SEG'                                    (3): Section 6.2.
* `SEGMENT'                                (1): Section 5.2.
* `SEGMENT', `elf' extensions to           (1): Section 6.2.1.
* segment address                          (1): Section 3.5.7.
* segment address                          (2): Section 3.6.
* segment alignment, in `bin'              (1): Section 6.1.2.
* segment alignment, in `obj'              (1): Section 6.2.1.
* segment names, Borland Pascal            (1): Section 7.5.2.
* segment override                         (1): Section 2.2.4.
* segment override                         (2): Section 3.1.
* segment registers                        (1): Section A.2.1.
* segments                                 (1): Section 3.6.
* segments, groups of                      (1): Section 6.2.2.
* separator character                      (1): Section 2.1.13.
* `SETcc'                                  (1): Section A.150.
* `SGDT'                                   (1): Section A.151.
* shared libraries                         (1): Section 6.7.
* shared libraries                         (2): Section 8.2.
* shared library                           (1): Section 6.5.3.
* shift command                            (1): Section 4.2.6.
* `SHL'                                    (1): Section A.152.
* `SHLD'                                   (1): Section A.153.
* `SHR'                                    (1): Section A.152.
* `SHRD'                                   (1): Section A.153.
* SIB byte                                 (1): Section A.2.
* SIB byte                                 (2): Section A.2.3.
* `SIDT'                                   (1): Section A.151.
* signed division                          (1): Section 3.5.6.
* signed modulo                            (1): Section 3.5.6.
* single-line macros                       (1): Section 4.1.
* size, of symbols                         (1): Section 6.5.3.
* `SLDT'                                   (1): Section A.151.
* `SMI'                                    (1): Section A.154.
* `SMSW'                                   (1): Section A.155.
* `-soname'                                (1): Section 8.2.6.
* sound                                    (1): Section 3.2.3.
* source code                              (1): Section 1.3.1.
* source-listing file                      (1): Section 2.1.3.
* square brackets                          (1): Section 2.2.2.
* square brackets                          (2): Section 3.3.
* `STACK'                                  (1): Section 6.2.1.
* standard macros                          (1): Section 4.7.
* standardised section names               (1): Section 5.2.
* standardised section names               (2): Section 6.1.
* standardised section names               (3): Section 6.3.1.
* standardised section names               (4): Section 6.5.1.
* standardised section names               (5): Section 6.6.
* standardised section names               (6): Section 6.7.
* standardised section names               (7): Section 6.8.
* standardised section names               (8): Section 6.9.
* `..start'                                (1): Section 6.2.6.
* `..start'                                (2): Section 7.1.1.
* `STC'                                    (1): Section A.156.
* `STD'                                    (1): Section A.156.
* `stderr'                                 (1): Section 2.1.4.
* `stdout'                                 (1): Section 2.1.5.
* `STI'                                    (1): Section A.156.
* `STOSB'                                  (1): Section A.157.
* `STOSD'                                  (1): Section A.157.
* `STOSW'                                  (1): Section A.157.
* `STR'                                    (1): Section A.158.
* string constant                          (1): Section 3.2.1.
* `STRUC'                                  (1): Section 4.7.3.
* `STRUC'                                  (2): Section 5.3.
* `STRUC'                                  (3): Section 7.4.4.
* `STRUC'                                  (4): Section 8.1.3.
* stub preprocessor                        (1): Section 2.1.11.
* `SUB'                                    (1): Section A.159.
* subtraction                              (1): Section 3.5.5.
* `sunsite.unc.edu'                        (1): Section 1.2.
* suppressible warning                     (1): Section 2.1.12.
* suppressing preprocessing                (1): Section 2.1.11.
* switching between sections               (1): Section 5.2.
* `..sym'                                  (1): Section 6.5.2.
* symbol sizes, specifying                 (1): Section 6.5.3.
* symbol types, specifying                 (1): Section 6.5.3.
* symbols, exporting from DLLs             (1): Section 6.2.5.
* symbols, importing from DLLs             (1): Section 6.2.4.
* `.SYS'                                   (1): Section 6.1.
* `.SYS'                                   (2): Section 7.3.
* TASM                                     (1): Section 1.1.1.
* TASM                                     (2): Section 2.2.
* TASM                                     (3): Section 6.2.
* `TBYTE'                                  (1): Section 2.2.7.
* `TEST'                                   (1): Section A.160.
* `test' subdirectory                      (1): Section 7.1.1.
* test registers                           (1): Section A.2.1.
* testing arbitrary numeric expressions    (1): Section 4.3.3.
* testing exact text identity              (1): Section 4.3.4.
* testing single-line macro existence      (1): Section 4.3.1.
* testing the context stack                (1): Section 4.3.2.
* testing token types                      (1): Section 4.3.5.
* `.text'                                  (1): Section 6.1.
* `.text'                                  (2): Section 6.5.1.
* `.text'                                  (3): Section 6.6.
* `.text'                                  (4): Section 6.7.
* `.text'                                  (5): Section 6.8.
* `.text'                                  (6): Section 6.9.
* `_TEXT'                                  (1): Section 7.4.2.
* `TIMES'                                  (1): Section 3.2.
* `TIMES'                                  (2): Section 3.2.5.
* `TIMES'                                  (3): Section 3.7.
* `TIMES'                                  (4): Section 10.1.3.
* `TIMES'                                  (5): Section 10.1.4.
* TLINK                                    (1): Section 7.2.2.
* trailing colon                           (1): Section 3.1.
* two-pass assembler                       (1): Section 3.7.
* `TWORD'                                  (1): Section 2.2.7.
* `TWORD'                                  (2): Section 3.1.
* type, of symbols                         (1): Section 6.5.3.
* `-U' option                              (1): Section 2.1.9.
* `-u' option                              (1): Section 2.1.9.
* `UMOV'                                   (1): Section A.161.
* unary operators                          (1): Section 3.5.7.
* `%undef'                                 (1): Section 2.1.9.
* `%undef'                                 (2): Section 4.1.2.
* undefining macros                        (1): Section 2.1.9.
* underscore, in C symbols                 (1): Section 7.4.1.
* uninitialised                            (1): Section 3.2.
* uninitialised                            (2): Section 3.2.2.
* uninitialised storage                    (1): Section 2.2.7.
* Unix                                     (1): Section 1.3.2.
* Unix source archive                      (1): Section 1.3.2.
* unrolled loops                           (1): Section 3.2.5.
* unsigned division                        (1): Section 3.5.6.
* unsigned modulo                          (1): Section 3.5.6.
* `UPPERCASE'                              (1): Section 2.2.1.
* `UPPERCASE'                              (2): Section 6.2.3.
* `USE16'                                  (1): Section 6.2.1.
* `USE32'                                  (1): Section 6.2.1.
* user-defined errors                      (1): Section 4.3.6.
* user-level assembler directives          (1): Section 4.7.
* user-level directives                    (1): Chapter 5.
* VAL                                      (1): Section 7.1.1.
* valid characters                         (1): Section 3.1.
* variable types                           (1): Section 2.2.3.
* `VERR'                                   (1): Section A.162.
* version number of NASM                   (1): Section 4.7.1.
* `VERW'                                   (1): Section A.162.
* Visual C++                               (1): Section 6.3.
* `-w' option                              (1): Section 2.1.12.
* `WAIT'                                   (1): Section A.163.
* warnings                                 (1): Section 2.1.12.
* `WBINVD'                                 (1): Section A.164.
* Win32                                    (1): Section 1.3.1.
* Win32                                    (2): Section 2.1.1.
* Win32                                    (3): Section 6.2.
* Win32                                    (4): Section 6.3.
* Win32                                    (5): Chapter 8.
* Windows                                  (1): Section 7.1.
* Windows 95                               (1): Section 1.3.1.
* Windows NT                               (1): Section 1.3.1.
* `write'                                  (1): Section 6.5.1.
* writing operating systems                (1): Section 9.1.
* `WRMSR'                                  (1): Section A.165.
* `WRT'                                    (1): Section 3.6.
* `WRT'                                    (2): Section 6.2.
* `WRT'                                    (3): Section 6.5.2.
* `WRT'                                    (4): Section 6.7.
* `WRT ..got'                              (1): Section 8.2.3.
* `WRT ..gotoff'                           (1): Section 8.2.2.
* `WRT ..gotpc'                            (1): Section 8.2.1.
* `WRT ..plt'                              (1): Section 8.2.5.
* `WRT ..sym'                              (1): Section 8.2.4.
* WWW page                                 (1): Section 1.2.
* `www.cpan.org'                           (1): Section 1.3.1.
* `www.delorie.com'                        (1): Section 7.1.1.
* `www.pcorner.com'                        (1): Section 7.1.1.
* `XADD'                                   (1): Section A.166.
* `XBTS'                                   (1): Section A.167.
* `XCHG'                                   (1): Section A.168.
* `x2ftp.oulu.fi'                          (1): Section 7.1.1.
* `XLATB'                                  (1): Section A.169.
* `XOR'                                    (1): Section A.170.



Tag Table:
Node: Top508
Node: Chapter 11154
Node: Section 1.11387
Node: Section 1.1.12045
Node: Section 1.1.23897
Node: Section 1.24194
Node: Section 1.35417
Node: Section 1.3.15664
Node: Section 1.3.27907
Node: Chapter 29501
Node: Section 2.19733
Node: Section 2.1.112138
Node: Section 2.1.213508
Node: Section 2.1.314264
Node: Section 2.1.414916
Node: Section 2.1.515663
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Node: Section 2.1.717703
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Node: Section 2.224256
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Node: Section 2.2.225661
Node: Section 2.2.328016
Node: Section 2.2.428882
Node: Section 2.2.529289
Node: Section 2.2.630052
Node: Section 2.2.730683
Node: Chapter 331538
Node: Section 3.131965
Node: Section 3.235990
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Node: Section 3.2.238044
Node: Section 3.2.338978
Node: Section 3.2.439720
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Node: Section 3.5.351929
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Node: Section 3.5.652937
Node: Section 3.5.753690
Node: Section 3.654232
Node: Section 3.756296
Node: Section 3.859709
Node: Chapter 462170
Node: Section 4.162863
Node: Section 4.1.163159
Node: Section 4.1.265977
Node: Section 4.1.366581
Node: Section 4.267867
Node: Section 4.2.169870
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Node: Section 4.2.372392
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Node: Section 4.2.882145
Node: Section 4.2.983534
Node: Section 4.384378
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Node: Section 4.3.286571
Node: Section 4.3.387186
Node: Section 4.3.488643
Node: Section 4.3.589639
Node: Section 4.3.691818
Node: Section 4.492692
Node: Section 4.594383
Node: Section 4.695750
Node: Section 4.6.196921
Node: Section 4.6.297652
Node: Section 4.6.398673
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Node: Section 4.7102383
Node: Section 4.7.1103366
Node: Section 4.7.2103849
Node: Section 4.7.3105202
Node: Section 4.7.4107552
Node: Section 4.7.5109092
Node: Chapter 5111580
Node: Section 5.1113064
Node: Section 5.2115012
Node: Section 5.2.1116055
Node: Section 5.3117870
Node: Section 5.4119763
Node: Section 5.5121287
Node: Section 5.6122483
Node: Chapter 6123724
Node: Section 6.1125168
Node: Section 6.1.1126768
Node: Section 6.1.2127791
Node: Section 6.2128541
Node: Section 6.2.1131024
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Node: Section 6.2.5137430
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Node: Section 6.3143424
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Node: Section 6.4146753
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Node: Section 6.5.1147876
Node: Section 6.5.2149793
Node: Section 6.5.3152794
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Node: Section 6.6154996
Node: Section 6.7155842
Node: Section 6.8157006
Node: Section 6.9157874
Node: Section 6.9.1159022
Node: Section 6.10159497
Node: Chapter 7161773
Node: Section 7.1162526
Node: Section 7.1.1163621
Node: Section 7.1.2167124
Node: Section 7.2169505
Node: Section 7.2.1170014
Node: Section 7.2.2171546
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Node: Section 7.4.5188721
Node: Section 7.5191235
Node: Section 7.5.1192975
Node: Section 7.5.2197248
Node: Section 7.5.3198277
Node: Chapter 8199618
Node: Section 8.1201079
Node: Section 8.1.1201677
Node: Section 8.1.2202499
Node: Section 8.1.3207205
Node: Section 8.1.4209461
Node: Section 8.2211056
Node: Section 8.2.1213334
Node: Section 8.2.2216075
Node: Section 8.2.3217360
Node: Section 8.2.4218476
Node: Section 8.2.5221025
Node: Section 8.2.6221935
Node: Chapter 9222797
Node: Section 9.1223498
Node: Section 9.2225457
Node: Section 9.3228025
Node: Chapter 10230473
Node: Section 10.1230911
Node: Section 10.1.1231239
Node: Section 10.1.2231912
Node: Section 10.1.3233044
Node: Section 10.1.4234229
Node: Section 10.2235594
Node: Appendix A240107
Node: Section A.1250678
Node: Section A.2252742
Node: Section A.2.1256914
Node: Section A.2.2258633
Node: Section A.2.3260094
Node: Section A.3264666
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Node: Section A.167422507
Node: Section A.168423200
Node: Section A.169424290
Node: Section A.170425108
Node: Index426819

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