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
Developer(s)GNU Project
Stable release
2.32 / February 2, 2019 (2019-02-02)
Written inC
PlatformCross-platform
TypeAssembler
LicenseGNU General Public License v3
Websitewww.gnu.org/software/binutils/

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.

History

The first version of GAS was published c. 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.

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.[2]

Comments

GAS supports two comment styles:[3]

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.

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 macOS.

Example program

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

Intel syntax

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.[4][5][6]

See also

References

  1. "The GNU Assembler".
  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 Mar 2009. Retrieved 28 July 2014.
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