NASM Manual

# The Netwide Assembler: NASM

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## Chapter 2: Running NASM

### 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.

#### 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

#### 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`.

#### 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 section 4.2.9) on the right. For example:

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

#### 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

#### 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, section 2.1.4.

#### 2.1.6 The `-i` Option: Include File Search Directories

When NASM sees the `%include` directive in a source file (see 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 section 2.1.13).

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

#### 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`.

#### 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`.

#### 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`.

#### 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.

#### 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.

#### 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 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 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.

#### 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.

### 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.

#### 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 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.

#### 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.

#### 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.

#### 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.

#### 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.

#### 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.

#### 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 chapter 4 and chapter 5 for further details.

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