GNU Assembler

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
Heckert GNU white.svg
Developer(s)GNU Project
Stable release
2.34 / February 1, 2020; 14 months ago (2020-02-01)
Written inC
LicenseGNU 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.


The first version of GAS was released in 1986-1987.[1] It was written by Dean Elsner, and supported the VAX architecture.[1]

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.[2]


GAS supports two comment styles:[3]

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 programEdit

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

.global	_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 # address of msg string -> ECX (2nd argument)
	movl  $len, %edx # len (32 bit address) -> 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
	.ascii  "Hello, world!\n" # inline ascii string
	len =   . - msg           # assign value of (current address - address of msg start) to symbol "len"

Intel syntaxEdit

Since version 2.10, Intel syntax can be used through use of the .intel_syntax directive.[4][5][6]

See alsoEdit


  1. ^ a b "The GNU Assembler". CiteSeerX Cite journal requires |journal= (help)
  2. ^ "The GNU Assembler - Assembler Directives".
  3. ^ Red Hat Inc. "Using as". Retrieved Jan 10, 2013.
  4. ^ "GNU Assembler News".
  5. ^ "AT&T Syntax versus Intel Syntax". Retrieved 28 July 2014.
  6. ^ 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.

External linksEdit