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.

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 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.

Directives

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]

Comments

GAS supports two comment styles:^[2]

Multi-line comments:

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

/* 
comment
*/

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, and M880x0.
• Semicolons (;) are used on: AMD 29k family, ARC, H8/300 family, HPPA,PDP-11, picoJava, Motorola, and PowerPC.
• The at sign (@) is used on the ARM platform.
• A vertical bar (|) is used to signify comments when assembling on 680x0.
• An exclamation mark (!) on the Renesas SH platform.

Usage

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 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 Programs

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

.globl	_start
 
.text
_start:
	movl	$len, %edx
	movl	$msg, %ecx
	movl	$1, %ebx
	movl	$4, %eax
	int	$0x80
 
	movl	$0, %ebx
	movl	$1, %eax
	int	$0x80
.data
msg:
	.ascii	"Hello, world!\n"
	len =	. - msg

Criticism

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.

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

See also

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

References

External links

The Wikibook X86 Assembly has a page on the topic of: GAS Syntax

• Official website
• Gas manual
• A comparison of GAS and NASM from IBM via archive.org
• as(1): the portable GNU assembler – Linux User Commands Manual

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