Editing GNU Assembler

{{Short description|Free and open-source assembler}}
{{Infobox software
| name = GNU Assembler
| logo = Heckert GNU white.svg
| logo size = 100px
| developer = [[GNU Project]]
| latest release version = GNU Binutils {{wikidata|property|reference|edit|Q1144975|P348}}
| latest release date = {{start date and age|{{wikidata|qualifier|Q1144975|P348|P577}}}}
| programming language = [[C (programming language)|C]]
| platform = [[Cross-platform]]
| genre = [[Assembly language#Assembler|Assembler]]
| license = [[GNU General Public License]] v3
| website = {{URL|https://www.gnu.org/software/binutils/}}
}}

The '''GNU Assembler''', commonly known as '''gas''' or '''as''', is the [[Assembler (computer programming)|assembler]] developed by the [[GNU Project]]. It is the default [[Front and back ends|back-end]] of [[GNU Compiler Collection|GCC]]. It is used to assemble the [[GNU operating system]] and the [[Linux kernel]], and various other software. It is a part of the [[GNU Binutils]] package.

The GAS [[executable]] is named {{mono|[[As (Unix)|as]]}}, the standard name for a [[Unix]] assembler. GAS is [[cross-platform]], and both runs on and assembles for a number of different [[computer architecture]]s. GAS is [[free software]] released under the [[GNU General Public License]] v3.

==History==
The first version of GAS was released in 1986–1987.<ref name="gasdoc1">{{cite CiteSeerX|title=The GNU Assembler|citeseerx=10.1.1.32.4503}}</ref> It was written by Dean Elsner and supported the [[VAX]] architecture.<ref name="gasdoc1"/>

==General syntax==
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 [[AT&T syntax]].

===Directives===
GAS uses assembler [[directive (programming)|directives]] (also known as pseudo ops), which are keywords beginning with a period that behave similarly to preprocessor directives in the [[C (programming language)|C programming language]]. While most of the available assembler directives are valid regardless of the target architecture, some directives are machine dependent.<ref>{{cite web | title = The GNU Assembler - Assembler Directives | url = http://sources.redhat.com/binutils/docs-2.12/as.info/Pseudo-Ops.html#Pseudo%20Ops | access-date = 2008-04-13 | archive-date = 2012-02-22 | archive-url = https://web.archive.org/web/20120222005603/http://sources.redhat.com/binutils/docs-2.12/as.info/Pseudo-Ops.html#Pseudo%20Ops | url-status = dead }}</ref>

Since version 2.10, Intel syntax can be used through use of the <code>.intel_syntax</code> directive.<ref>{{cite web| title = GNU Assembler News| url = https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gas/NEWS;hb=HEAD |quote=A new pseudo-op .intel_syntax has been implemented to allow gas to parse i386 assembly programs with intel syntax.}}</ref><ref>{{cite web| title = AT&T Syntax versus Intel Syntax| url = http://sources.redhat.com/binutils/docs-2.12/as.info/i386-Syntax.html| access-date = 28 July 2014| archive-date = 20 June 2011| archive-url = https://web.archive.org/web/20110620054249/http://sources.redhat.com/binutils/docs-2.12/as.info/i386-Syntax.html| url-status = dead}}</ref><ref>{{cite web| title = Linux assemblers: A comparison of GAS and NASM| author = Ram Narayan| date = 2007-10-17| publisher = IBM DeveloperWorks| url = http://www.ibm.com/developerworks/linux/library/l-gas-nasm.html| archive-url = https://web.archive.org/web/20090303224539/http://ibm.com/developerworks/linux/library/l-gas-nasm.html| archive-date = 3 March 2009| access-date = 28 July 2014}}</ref>

===Comments===
GAS supports two comment styles.<ref>{{cite web|last=Red Hat Inc.|title=Using as|url=http://sources.redhat.com/binutils/docs-2.12/as.info/Comments.html#Comments|access-date=Jan 10, 2013|archive-date=June 20, 2011|archive-url=https://web.archive.org/web/20110620054223/http://sources.redhat.com/binutils/docs-2.12/as.info/Comments.html#Comments|url-status=dead}}</ref>

'''Multi-line'''

As in C, multi-line comments start and end with mirroring slash-asterisk pairs:
<syntaxhighlight lang="c">
/* 
comment
*/
</syntaxhighlight>

'''Single-line'''

Single line comments have a few different formats varying on which architecture is being assembled for.
* A [[hash symbol]] (#) — [[i386]], [[x86-64]], [[i960]], [[68HC11]], [[68HC12]], [[VAX]], [[V850]], [[M32R]], [[PowerPC]], [[MIPS architecture|MIPS]], [[Motorola 68000 series|M680x0]], and [[RISC-V]]
* A [[semicolon]] (;) — [[AMD 29k]] family, [[Advanced RISC Computing|ARC]], [[H8/300]] family, [[PA-RISC|HPPA]], [[PDP-11]], [[picoJava]], Motorola, and [[M32C]]
* The [[at sign]] (@) — 32-bit [[ARM architecture|ARM]]
* A double [[Slash (punctuation)|slash]] (//) — [[AArch64]]
* A [[vertical bar]] (|) — [[Motorola 68000 series|M680x0]]
* An [[exclamation mark]] (!) — [[SuperH|Renesas SH]]

==Usage==
Being the back-end for a popular compiler suite, namely GCC, the GNU Assembler is very widely used in compiling modern [[free and open source software]]. 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 [[macOS]] development tools package.

==Example program==
A standard "Hello, world!" program for [[Linux]] on [[IA-32]]:

<syntaxhighlight lang="asm">
.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"
</syntaxhighlight>

==See also==
{{Portal|Free and open-source software}}

* [[GNU toolchain]]
* [[Binary File Descriptor library]]
* [[Comparison of assemblers]]

==References==
{{Reflist}}

==External links==
{{Wikibooks|X86 Assembly|GAS Syntax}}
* {{official website}}
* [http://sourceware.org/binutils/docs/as/ Gas manual]
* {{man|1|as|die.net|the portable GNU assembler}}

{{GNU}}
{{X86 assembly topics}}

{{DEFAULTSORT:Gnu Assembler}}
[[Category:Assemblers]]
[[Category:Free and open source compilers]]
[[Category:GNU Project software|Assembler]]
[[Category:Linux programming tools]]
[[Category:Unix programming tools]]