GNU Assembler
| Developer(s) | GNU Project |
|---|---|
| Stable release | 2.24
/ December 2, 2013 |
| Written in | C |
| Platform | Cross-platform |
| Type | Assembler |
| License | GNU General Public License v3 |
| Website | www |
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.[according to whom?]
However, since version 2.10, Intel syntax can be used through use of the .intel_syntax directive.[3][4][5]
See also
References
- ^ "The GNU Assembler - Assembler Directives".
- ^ Red Hat Inc. "Using as". Retrieved Jan 10, 2013.
- ^ "GNU Assembler News".
- ^ "AT&T Syntax versus Intel Syntax". Retrieved 28 July 2014.
- ^ 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.
External links
- Official website
- Gas manual
- : the portable GNU assembler – Linux User Commands Manual
