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, PowerPC, MIPS and M880x0.
• 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.

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 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 Programs

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

.global	_start

.text
_start:
	movl  $4, %eax
	movl  $1, %ebx
	movl  $msg, %ecx
	movl  $len, %edx
	int   $0x80

	movl  $1, %eax
	movl  $0, %ebx
	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
• as(1): the portable GNU assembler – Linux User Commands Manual

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