GNU Assembler

The first version of GAS was released in 1986–1987.{{cite CiteSeerX|title=The GNU Assembler|citeseerx=10.1.1.32.4503}} It was written by Dean Elsner and supported the VAX architecture.

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.

GAS uses assembler directives (also known as pseudo ops), which are keywords beginning with a period that behave similarly to preprocessor directives in the C programming language. While most of the available assembler directives are valid regardless of the target architecture, some directives are machine dependent.{{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 }}

Since version 2.10, Intel syntax can be used through use of the .intel_syntax directive.{{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.}}{{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}}{{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}}

GAS supports two comment styles.{{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}}

Multi-line

As in C, multi-line comments start and end with mirroring slash-asterisk pairs:

/*

comment

• /

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, M680x0, and RISC-V
• A semicolon (;) — AMD 29k family, ARC, H8/300 family, HPPA, PDP-11, picoJava, Motorola, and M32C
• The at sign (@) — 32-bit ARM
• A double slash (//) — AArch64
• A vertical bar (|) — M680x0
• An exclamation mark (!) — Renesas SH

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.

A standard "Hello, world!" program for Linux on IA-32:

.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"

{{Portal|Free and open-source software}}

• GNU toolchain
• Binary File Descriptor library
• Comparison of assemblers

{{Reflist}}

{{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

Assembler

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