<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML
><HEAD
><TITLE
>GAS</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
REL="HOME"
TITLE="Linux Assembly HOWTO"
HREF="index.html"><LINK
REL="UP"
TITLE="Assemblers"
HREF="assemblers.html"><LINK
REL="PREVIOUS"
TITLE="GCC Inline Assembly"
HREF="gcc.html"><LINK
REL="NEXT"
TITLE="NASM"
HREF="nasm.html"></HEAD
><BODY
CLASS="SECTION"
BGCOLOR="#FFFFFF"
TEXT="#000000"
LINK="#0000FF"
VLINK="#840084"
ALINK="#0000FF"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>Linux Assembly HOWTO</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="gcc.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
>Chapter 3. Assemblers</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="nasm.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECTION"
><H1
CLASS="SECTION"
><A
NAME="P-GAS"
></A
>3.2. GAS</H1
><P
>GAS is the GNU Assembler, that GCC relies upon.</P
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN339"
></A
>3.2.1. Where to find it</H2
><P
>Find it at the same place where you've found GCC,
in the binutils package.
The latest version of binutils is available from
<A
HREF="http://sources.redhat.com/binutils/"
TARGET="_top"
>http://sources.redhat.com/binutils/</A
>.</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN343"
></A
>3.2.2. What is this AT&T syntax</H2
><P
>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.</P
><P
>Here are the major caveats about GAS syntax:
<P
></P
><UL
><LI
><P
>Register names are prefixed with <TT
CLASS="LITERAL"
>%</TT
>, so that registers
are <TT
CLASS="LITERAL"
>%eax</TT
>, <TT
CLASS="LITERAL"
>%dl</TT
> and so on,
instead of just <TT
CLASS="LITERAL"
>eax</TT
>, <TT
CLASS="LITERAL"
>dl</TT
>, etc.
This makes it possible to include external C symbols directly
in assembly source, without any risk of confusion, or any need
for ugly underscore prefixes.</P
></LI
><LI
><P
>The order of operands is source(s) first, and destination last,
as opposed to the Intel convention of destination first and sources last.
Hence, what in Intel syntax is
<TT
CLASS="FUNCTION"
>mov eax,edx</TT
>
(move contents of register <TT
CLASS="LITERAL"
>edx</TT
> into register <TT
CLASS="LITERAL"
>eax</TT
>)
will be in GAS syntax
<TT
CLASS="FUNCTION"
>mov %edx,%eax</TT
>.</P
></LI
><LI
><P
>The operand size is specified as a suffix to the instruction name.
The suffix is
<TT
CLASS="LITERAL"
>b</TT
> for (8-bit) byte,
<TT
CLASS="LITERAL"
>w</TT
> for (16-bit) word, and
<TT
CLASS="LITERAL"
>l</TT
> for (32-bit) long.
For instance, the correct syntax for the above instruction
would have been <TT
CLASS="FUNCTION"
>movl %edx,%eax</TT
>.
However, gas does not require strict AT&T syntax,
so the suffix is optional when size can be guessed from register operands,
and else defaults to 32-bit (with a warning).</P
></LI
><LI
><P
>Immediate operands are marked with a <TT
CLASS="LITERAL"
>$</TT
> prefix,
as in <TT
CLASS="FUNCTION"
>addl $5,%eax</TT
>
(add immediate long value 5 to register <TT
CLASS="LITERAL"
>%eax</TT
>).</P
></LI
><LI
><P
>Missing operand prefix indicates that it is memory-contents;
hence <TT
CLASS="FUNCTION"
>movl $foo,%eax</TT
>
puts the <EM
>address</EM
> of variable <TT
CLASS="LITERAL"
>foo</TT
>
into register <TT
CLASS="LITERAL"
>%eax</TT
>,
but <TT
CLASS="FUNCTION"
>movl foo,%eax</TT
>
puts the <EM
>contents</EM
> of variable <TT
CLASS="LITERAL"
>foo</TT
>
into register <TT
CLASS="LITERAL"
>%eax</TT
>.</P
></LI
><LI
><P
>Indexing or indirection is done by enclosing the index register
or indirection memory cell address in parentheses,
as in <TT
CLASS="FUNCTION"
>testb $0x80,17(%ebp)</TT
>
(test the high bit of the byte value at offset 17
from the cell pointed to by <TT
CLASS="LITERAL"
>%ebp</TT
>).</P
></LI
></UL
> </P
><P
><A
NAME="P-CONVERT"
></A
>
Note: There are <A
HREF="resources.html"
>few programs</A
> 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.</P
><P
>GAS has comprehensive documentation in TeXinfo format,
which comes at least with the source distribution.
Browse extracted <TT
CLASS="FILENAME"
>.info</TT
> 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
<TT
CLASS="LITERAL"
>Machine Dependencies::i386-Dependent::</TT
></P
><P
>Again, the sources for Linux (the OS kernel) come in as excellent examples;
see under <TT
CLASS="FILENAME"
>linux/arch/i386/</TT
> the following files:
<TT
CLASS="FILENAME"
>kernel/*.S</TT
>,
<TT
CLASS="FILENAME"
>boot/compressed/*.S</TT
>,
<TT
CLASS="FILENAME"
>math-emu/*.S</TT
>.</P
><P
>If you are writing kind of a language, a thread package, etc.,
you might as well see how other languages (
<A
HREF="http://para.inria.fr/"
TARGET="_top"
>OCaml</A
>,
<A
HREF="http://www.jwdt.com/~paysan/gforth.html"
TARGET="_top"
>Gforth</A
>,
etc.),
or thread packages (QuickThreads, MIT pthreads, LinuxThreads, etc),
or whatever else do it.</P
><P
>Finally, just compiling a C program to assembly
might show you the syntax for the kind of instructions you want.
See section <A
HREF="doyouneed.html"
>Do you need assembly?</A
> above.</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN402"
></A
>3.2.3. Intel syntax</H2
><P
>Good news are that starting from binutils 2.10 release,
GAS supports Intel syntax too.
It can be triggered with <TT
CLASS="LITERAL"
>.intel_syntax</TT
> directive.
Unfortunately this mode is not documented (yet?) in the official
binutils manual, so if you want to use it, try to examine
<A
HREF="http://www.lxhp.in-berlin.de/lhpas86.html"
TARGET="_top"
>http://www.lxhp.in-berlin.de/lhpas86.html</A
>,
which is an extract from AMD 64bit port of binutils 2.11.</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN407"
></A
>3.2.4. 16-bit mode</H2
><P
>Binutils (2.9.1.0.25+) now fully support 16-bit mode
(registers <EM
>and</EM
> addressing) on i386 PCs.
Use <TT
CLASS="LITERAL"
>.code16</TT
> and <TT
CLASS="LITERAL"
>.code32</TT
>
to switch between assembly modes.</P
><P
>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
<TT
CLASS="LITERAL"
>asm(".code16\n")</TT
>.
GCC will still emit only 32-bit addressing modes,
but GAS will insert proper 32-bit prefixes for them.</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN415"
></A
>3.2.5. Macro support</H2
><P
>GAS has some macro capability included, as detailed in the texinfo docs.
Moreover, while GCC recognizes <TT
CLASS="FILENAME"
>.s</TT
> files as raw assembly
to send to GAS, it also recognizes <TT
CLASS="FILENAME"
>.S</TT
> files as files
to pipe through CPP before feeding them to GAS.
Again and again, see Linux sources for examples.</P
><P
>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 <A
HREF="external.html#P-CPP"
>CPP</A
> and <A
HREF="external.html#P-M4"
>M4</A
>.
I have no idea on details, but it comes with its own texinfo documentation,
which you would like to browse (<B
CLASS="COMMAND"
>info gasp</B
>), print, grok.
GAS with GASP looks like a regular macro-assembler to me.</P
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="gcc.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="nasm.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>GCC Inline Assembly</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="assemblers.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>NASM</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
