NASM Manual

# The Netwide Assembler: NASM

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## Chapter 1: Introduction

### 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.

#### 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.

#### 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.

### 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 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`.

### 1.3 Installation

#### 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.

#### 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.

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