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The GNU Assembler, commonly known as gas or simply as, its executable name, is the assembler used by the GNU Project. It is the default back-end of 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.

GNU Assembler
Developer(s) GNU Project
Stable release
2.27 / August 3, 2016; 10 months ago (2016-08-03)
Written in C
Platform Cross-platform
Type Assembler
License GNU General Public License v3

The GAS executable is named as, the standard name for a Unix assembler. GAS is cross-platform, and both runs on and assembles for a number of different computer architectures. Released under the GNU General Public License v3, GAS is free software.


General syntaxEdit

GAS supports a general syntax that works for all of the supported architectures. The general syntax includes assembler directives and a method for commenting.


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.[1]


GAS supports two comment styles:[2]

Multi-line commentsEdit

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


Single-Line commentsEdit

Single line comments have a few different formats varying on which architecture is being assembled for.


Being the back-end for a popular compiler suite, namely GCC, the GNU Assembler is very widely used in compiling modern open source software. GAS is often used as the assembler on GNU/Linux operating systems in conjunction with other GNU software. A modified version of GAS can also be found in the Macintosh operating system's development tools package since OS X.

Example ProgramsEdit

A standard “Hello, world!” program for Linux on IA-32 using the default AT&T syntax:

.global	_start

	movl  $4, %eax
	movl  $1, %ebx
	movl  $msg, %ecx
	movl  $len, %edx
	int   $0x80

	movl  $1, %eax
	movl  $0, %ebx
	int   $0x80
	.ascii  "Hello, world!\n"
	len =   . - msg


Those more accustomed to writing in Intel syntax have argued that not supporting the Intel syntax for assembly on the x86 and x86-64 platforms, as many other assemblers do, is a flaw.[according to whom?]

However, since version 2.10, Intel syntax can be used through use of the .intel_syntax directive.[3][4][5]

See alsoEdit


  1. ^ "The GNU Assembler - Assembler Directives". 
  2. ^ Red Hat Inc. "Using as". Retrieved Jan 10, 2013. 
  3. ^ "GNU Assembler News". 
  4. ^ "AT&T Syntax versus Intel Syntax". Retrieved 28 July 2014. 
  5. ^ Ram Narayan (2007-10-17). "Linux assemblers: A comparison of GAS and NASM". IBM DeveloperWorks. Archived from the original on 3 Mar 2009. Retrieved 28 July 2014. 

External linksEdit