GNU Assembler

<h2 id="history">History</h2>
<p>The first version of GAS was released in 1986–1987.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> It was written by Dean Elsner and supported the <a href="/facts/VAX/N3r13IHH">VAX</a> architecture.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
</p>
<h2 id="general-syntax">General syntax</h2>
<p>GAS supports a general syntax that works for all of the supported architectures. The general syntax includes assembler directives and a method for commenting. The default syntax is <a href="/facts/AT%26T_syntax/GTUtvkMm">AT&T syntax</a>.
</p>
<h3>Directives</h3>
<p>GAS uses assembler <a href="/facts/Directive_(programming)/RsozxGG0">directives</a> (also known as pseudo ops), which are keywords beginning with a period that behave similarly to preprocessor directives in the <a href="/facts/C_(programming_language)/Ky2No763">C programming language</a>. While most of the available assembler directives are valid regardless of the target architecture, some directives are machine dependent.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a>
</p><p>Since version 2.10, Intel syntax can be used through use of the .intel_syntax directive.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a><a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a>
</p>
<h3>Comments</h3>
<p>GAS supports two comment styles.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a>
</p><p>Multi-line
</p><p>As in C, multi-line comments start and end with mirroring slash-asterisk pairs:
</p>
/* 
comment
*/

<p>Single-line
</p><p>Single line comments have a few different formats varying on which architecture is being assembled for.
</p>
<ul><li>A <a href="/facts/Hash_symbol/bHYCsWs1">hash symbol</a> (#) — <a href="/facts/I386/ag5eSgWv">i386</a>, <a href="/facts/X86-64/cyWvUFlP">x86-64</a>, <a href="/facts/I960/xnFNnjsX">i960</a>, <a href="/facts/68HC11/7p1pn24u">68HC11</a>, <a href="/facts/68HC12/ugRJu0xH">68HC12</a>, <a href="/facts/VAX/N3r13IHH">VAX</a>, <a href="/facts/V850/f9bPoRqa">V850</a>, <a href="/facts/M32R/SztNrULr">M32R</a>, <a href="/facts/PowerPC/DqmM9y50">PowerPC</a>, <a href="/facts/MIPS_architecture/efV4KgK4">MIPS</a>, <a href="/facts/Motorola_68000_series/jxd7VPlW">M680x0</a>, and <a href="/facts/RISC-V/pUSKDat1">RISC-V</a></li>
<li>A <a href="/facts/Semicolon/VY8VsSPC">semicolon</a> (;) — <a href="/facts/AMD_29k/ssWQ4bUL">AMD 29k</a> family, <a href="/facts/Advanced_RISC_Computing/E0xAU5FS">ARC</a>, <a href="/facts/H8/300/5aLYjhfP">H8/300</a> family, <a href="/facts/PA-RISC/gjDs1HEh">HPPA</a>, <a href="/facts/PDP-11/9HkGUzFn">PDP-11</a>, <a href="/facts/PicoJava/21YPafja">picoJava</a>, Motorola, and <a href="/facts/M32C/ujhb9Dcz">M32C</a></li>
<li>The <a href="/facts/At_sign/6Q03Vo2z">at sign</a> (@) — 32-bit <a href="/facts/ARM_architecture/SALsYA79">ARM</a></li>
<li>A double <a href="/facts/Slash_(punctuation)/fuaRyKzc">slash</a> (//) — <a href="/facts/AArch64/bauXRos7">AArch64</a></li>
<li>A <a href="/facts/Vertical_bar/Yq77nJ6f">vertical bar</a> (|) — <a href="/facts/Motorola_68000_series/jxd7VPlW">M680x0</a></li>
<li>An <a href="/facts/Exclamation_mark/joGCCBau">exclamation mark</a> (!) — <a href="/facts/SuperH/4GTLXc1N">Renesas SH</a></li></ul>
<h2 id="usage">Usage</h2>
<p>Being the back-end for a popular compiler suite, namely GCC, the GNU Assembler is very widely used in compiling modern <a href="/facts/Free_and_open_source_software/1cHtjLCB">free and open source software</a>. GAS is often used as the assembler on Linux operating systems in conjunction with other GNU software. A modified version of GAS can also be found in the <a href="/facts/MacOS/Y93vVpTm">macOS</a> development tools package.
</p>
<h2 id="example-program">Example program</h2>
<p>A standard "Hello, world!" program for <a href="/facts/Linux/9FvCKSTq">Linux</a> on <a href="/facts/IA-32/7yTv1aHH">IA-32</a>:
</p>
.global	_start

.text
_start:
	movl  $4, %eax   # 4 (code for "write" syscall) -> EAX register
	movl  $1, %ebx   # 1 (file descriptor for stdout) -> EBX (1st argument to syscall)
	movl  $msg, %ecx # 32-bit address of msg string -> ECX (2nd argument)
	movl  $len, %edx # length of msg string -> EDX (3rd arg)
	int   $0x80      # interrupt with location 0x80 (128), which invokes the kernel's system call procedure

movl  $1, %eax   # 1 ("exit") -> EAX
	movl  $0, %ebx   # 0 (with success) -> EBX
	int   $0x80      # see previous
.data
msg:
	.ascii  "Hello, world!\n" # inline ascii string
	len =   . - msg           # assign (current address - address of msg start) to symbol "len"

<h2 id="see-also">See also</h2>
<ul>
<li>Free and open-source software portal</li></ul>
<ul><li><a href="/facts/GNU_toolchain/8BfT9wfD">GNU toolchain</a></li>
<li><a href="/facts/Binary_File_Descriptor_library/ctHcXxcd">Binary File Descriptor library</a></li>
<li><a href="/facts/Comparison_of_assemblers/gXrTEcNP">Comparison of assemblers</a></li></ul>

<h2 id="external-links">External links</h2>

The Wikibook <i>X86 Assembly</i> has a page on the topic of: <i>GAS Syntax</i>

<ul><li><a href="https://www.gnu.org/s/binutils/">Official website</a> </li>
<li><a href="http://sourceware.org/binutils/docs/as/">Gas manual</a></li>
<li><a href="https://linux.die.net/man/1/as">as(1)</a>: the portable GNU assembler – <a href="/facts/Linux/9FvCKSTq">Linux</a> User Commands <a href="/facts/Man_page/S2yYjy0J">Manual</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"The GNU Assembler". CiteSeerX 10.1.1.32.4503. <a href="/wiki/CiteSeerX_(identifier)" target="_blank">/wiki/CiteSeerX_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"The GNU Assembler". CiteSeerX 10.1.1.32.4503. <a href="/wiki/CiteSeerX_(identifier)" target="_blank">/wiki/CiteSeerX_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"The GNU Assembler - Assembler Directives". Archived from the original on 2012-02-22. Retrieved 2008-04-13. <a href="https://web.archive.org/web/20120222005603/http://sources.redhat.com/binutils/docs-2.12/as.info/Pseudo-Ops.html#Pseudo%20Ops" target="_blank">https://web.archive.org/web/20120222005603/http://sources.redhat.com/binutils/docs-2.12/as.info/Pseudo-Ops.html#Pseudo%20Ops</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>"GNU Assembler News". A new pseudo-op .intel_syntax has been implemented to allow gas to parse i386 assembly programs with intel syntax. <a href="https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gas/NEWS;hb=HEAD" target="_blank">https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gas/NEWS;hb=HEAD</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>"AT&T Syntax versus Intel Syntax". Archived from the original on 20 June 2011. Retrieved 28 July 2014. <a href="https://web.archive.org/web/20110620054249/http://sources.redhat.com/binutils/docs-2.12/as.info/i386-Syntax.html" target="_blank">https://web.archive.org/web/20110620054249/http://sources.redhat.com/binutils/docs-2.12/as.info/i386-Syntax.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>Ram Narayan (2007-10-17). "Linux assemblers: A comparison of GAS and NASM". IBM DeveloperWorks. Archived from the original on 3 March 2009. Retrieved 28 July 2014. <a href="https://web.archive.org/web/20090303224539/http://ibm.com/developerworks/linux/library/l-gas-nasm.html" target="_blank">https://web.archive.org/web/20090303224539/http://ibm.com/developerworks/linux/library/l-gas-nasm.html</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
<li id="fn:7"><p>Red Hat Inc. "Using as". Archived from the original on June 20, 2011. Retrieved Jan 10, 2013. <a href="https://web.archive.org/web/20110620054223/http://sources.redhat.com/binutils/docs-2.12/as.info/Comments.html#Comments" target="_blank">https://web.archive.org/web/20110620054223/http://sources.redhat.com/binutils/docs-2.12/as.info/Comments.html#Comments</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li>
</ol>

GNU Assembler open-in-new

GNU Assembler