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 and x86-64 assembler designed for portability and modularity. It supports a range of object file formats, including Linux and `*BSD` `a.out`, `ELF`, `COFF`, `Mach-O`, Microsoft 16-bit `OBJ`, `Win32` and `Win64`. 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 all currently known x86 architectural extensions, and has strong support for macros.

#### 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 to 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 specific to Minix and Linux, and (my version at least) doesn't seem to have much (or any) documentation.
- `MASM` isn't very good, and it's (was) 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 License Conditions

Please see the file `LICENSE`, supplied as part of any NASM distribution archive, for the license conditions under which you may use NASM. NASM is now under the so-called 2-clause BSD license, also known as the simplified BSD license.

Copyright 1996-2011 the NASM Authors - All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

- Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

### 1.2 Contact Information

The current version of NASM (since about 0.98.08) is maintained by a team of developers, accessible through the `nasm-devel` mailing list (see below for the link). If you want to report a bug, please read section 12.2 first.

NASM has a website at `http://www.nasm.us/`. If it's not there, google for us!

New releases, release candidates, and daily development snapshots of NASM are available from the official web site.

Announcements are posted to `comp.lang.asm.x86`, and to the web site `http://www.freshmeat.net/`.

If you want information about the current development status, please subscribe to the `nasm-devel` email list; see link from the website.

### 1.3 Installation

#### 1.3.1 Installing NASM under MS-DOS or Windows

Once you've obtained the appropriate archive for NASM, `nasm-XXX-dos.zip` or `nasm-XXX-win32.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 a set of executable files: the NASM executable file `nasm.exe`, the NDISASM executable file `ndisasm.exe`, and possibly additional utilities to handle the RDOFF file format.

The only file NASM needs to run is its own executable, so copy `nasm.exe` to a directory on your PATH, or alternatively edit `autoexec.bat` to add the `nasm` directory to your `PATH` (to do that under Windows XP, go to Start > Control Panel > System > Advanced > Environment Variables; these instructions may work under other versions of Windows as well.)

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, `nasm-XXX.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. See the file `INSTALL` in the source archive.

Note that a number of files are generated from other files by Perl scripts. Although the NASM source 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-XXX.tar.gz` (where `XXX` 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-XXX`.

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.

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