3.2. GAS

GAS is the GNU Assembler, that GCC relies upon.

3.2.1. Where to find it

Find it at the same place where you've found GCC, in the binutils package. The latest version of binutils is available from http://sources.redhat.com/binutils/.

3.2.2. What is this AT&T syntax

Because GAS was invented to support a 32-bit unix compiler, it uses standard AT&T syntax, which resembles a lot the syntax for standard m68k assemblers, and is standard in the UNIX world. This syntax is neither worse, nor better than the Intel syntax. It's just different. When you get used to it, you find it much more regular than the Intel syntax, though a bit boring.

Here are the major caveats about GAS syntax:

Note: There are few programs which may help you to convert source code between AT&T and Intel assembler syntaxes; some of the are capable of performing conversion in both directions.

GAS has comprehensive documentation in TeXinfo format, which comes at least with the source distribution. Browse extracted .info pages with Emacs or whatever. There used to be a file named gas.doc or as.doc around the GAS source package, but it was merged into the TeXinfo docs. Of course, in case of doubt, the ultimate documentation is the sources themselves! A section that will particularly interest you is Machine Dependencies::i386-Dependent::

Again, the sources for Linux (the OS kernel) come in as excellent examples; see under linux/arch/i386/ the following files: kernel/*.S, boot/compressed/*.S, math-emu/*.S.

If you are writing kind of a language, a thread package, etc., you might as well see how other languages ( OCaml, Gforth, etc.), or thread packages (QuickThreads, MIT pthreads, LinuxThreads, etc), or whatever else do it.

Finally, just compiling a C program to assembly might show you the syntax for the kind of instructions you want. See section Do you need assembly? above.

3.2.3. Intel syntax

Good news are that starting from binutils 2.10 release, GAS supports Intel syntax too. It can be triggered with .intel_syntax directive. Unfortunately this mode is not documented (yet?) in the official binutils manual, so if you want to use it, try to examine http://home.snafu.de/phpr/lhpas86.html.gz, which is an extract from AMD 64bit port of binutils 2.11.

3.2.4. 16-bit mode

Binutils ( now fully support 16-bit mode (registers and addressing) on i386 PCs. Use .code16 and .code32 to switch between assembly modes.

Also, a neat trick used by several people (including the oskit authors) is to force GCC to produce code for 16-bit real mode, using an inline assembly statement asm(".code16\n"). GCC will still emit only 32-bit addressing modes, but GAS will insert proper 32-bit prefixes for them.

3.2.5. Macro support

GAS has some macro capability included, as detailed in the texinfo docs. Moreover, while GCC recognizes .s files as raw assembly to send to GAS, it also recognizes .S files as files to pipe through CPP before feeding them to GAS. Again and again, see Linux sources for examples.

GAS also has GASP (GAS Preprocessor), which adds all the usual macroassembly tricks to GAS. GASP comes together with GAS in the GNU binutils archive. It works as a filter, like CPP and M4. I have no idea on details, but it comes with its own texinfo documentation, which you would like to browse (info gasp), print, grok. GAS with GASP looks like a regular macro-assembler to me.