GNU Assembler

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GNU Assembler
Heckert GNU white.svg
Developer(s)GNU Project
Stable release
2.34 / February 1, 2020; 16 months ago (2020-02-01)
Written inC
LicenseGNU General Public License v3

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.

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 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.


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 comments

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


Single-Line comments

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

  • Hash symbols (#) are used for the platforms: i386, x86-64, i960, 68HC11, 68HC12, VAX, V850, M32R, PowerPC, MIPS, M680x0, and RISC-V.
  • Semicolons (;) are used on: AMD 29k family, ARC, H8/300 family, HPPA, PDP-11, picoJava, Motorola, and M32C.
  • The at sign (@) is used on the ARM platform.
  • A double slash (//) is used on the AArch64 platform.
  • A vertical bar (|) is used to signify comments when assembling on 680x0.
  • An exclamation mark (!) on the Renesas SH platform.


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 program

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 syntax

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

See also


  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 links

Edited: 2021-06-18 19:34:04