GNU Assembler

GNU Assembler

Developer(s)	GNU Project
Stable release	2.21 / December 8, 2010 (2010-12-08)
Written in	C
Platform	Cross-platform
Type	Assembler
License	GNU General Public License v3
Website	www.gnu.org/software/binutils

The GNU Assembler, commonly known as GAS (even though the program itself is as), 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.

GAS' executable is named after as, 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.

Comments

GAS comments start with a number sign and span up to a newline character. Example:

# comment
popl %edx # comment

Criticism

One source of criticism was that on the x86 and x86-64 architecture it uses the AT&T assembler syntax, rather than the Intel syntax used in many other assemblers; however, in versions from 2.10 support for the Intel syntax via the .intel_syntax directive has been added.

Here is the example as C-inline assembler in Intel syntax:

__asm__ __volatile__("\t.intel_syntax noprefix\n"
                     "\tpop edx\n"
                     "\tmov eax,edx\n"
                     "\t.att_syntax prefix\n"
                     : /* no outputs */
                     : "d" (save_var), "a" (temp_var) /* inputs */
                     : "eax", "edx" /* clobber list */);

See also

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

External links

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