The GNU Assembler, commonly known as gas or as, is the assembler developed 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 The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...

, and various other software. It is a part of the GNU Binutils package. The GAS

executable In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instruction (computer science), instructi ...

is named , the standard name for a Unix assembler. GAS is cross-platform, and both runs on and assembles for a number of different computer architectures. GAS is free software released under the GNU General Public License v3.

History

The first version of GAS was released in 1986-1987. It was written by Dean Elsner, and supported the VAX architecture.

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. The default syntax is AT&T syntax.

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 ''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...

. While most of the available assembler directives are valid regardless of the target architecture, some directives are machine dependent. Since version 2.10, Intel syntax can be used through use of the .intel_syntax directive.

Comments

GAS supports two comment styles. Multi-line As in C, multi-line comments start and end with mirroring slash-asterisk pairs: /* comment */ Single-line Single line comments have a few different formats varying on which architecture is being assembled for. * A hash symbol (#) — i386, x86-64, i960, 68HC11, 68HC12, VAX, V850, M32R,

PowerPC PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple Inc., App ...

, MIPS, M680x0, and RISC-V * A semicolon (;) — AMD 29k family,

ARC ARC may refer to: Business * Aircraft Radio Corporation, a major avionics manufacturer from the 1920s to the '50s * Airlines Reporting Corporation, an airline-owned company that provides ticket distribution, reporting, and settlement services * ...

, H8/300 family, HPPA,

PDP-11 The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were sold, ...

, picoJava, Motorola, and

M32C is a Japanese semiconductor manufacturer headquartered in Tokyo, Japan, initially incorporated in 2002 as Renesas Technology, the consolidated entity of the semiconductor units of Hitachi and Mitsubishi excluding their dynamic random-access ...

* The at sign (@) — ARM * A double slash (//) — AArch64 * A

vertical bar The vertical bar, , is a glyph with various uses in mathematics, computing, and typography. It has many names, often related to particular meanings: Sheffer stroke (in logic), pipe, bar, or (literally the word "or"), vbar, and others. Usage ...

(, ) — 680x0 * An exclamation mark (!) — Renesas SH

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 macOS development tools package since

OS X macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...

Example program

A standard "Hello, world!" program for Linux on IA-32: .global _start .text _start: movl $4, %eax # 4 (code for "write" syscall) -> EAX register movl $1, %ebx # 1 (file descriptor for stdout) -> EBX (1st argument to syscall) movl $msg, %ecx # address of msg string -> ECX (2nd argument) movl $len, %edx # len (32 bit address) -> EDX (3rd arg) int $0x80 # interrupt with location 0x80 (128), which invokes the kernel's system call procedure movl $1, %eax # 1 ("exit") -> EAX movl $0, %ebx # 0 (with success) -> EBX int $0x80 # see previous .data msg: .ascii "Hello, world!\n" # inline ascii string len = . - msg # assign (current address - address of msg start) to symbol "len"

References

External links

*
Gas manual
* {{DEFAULTSORT:Gnu Assembler Assemblers Free compilers and interpreters GNU Project software Linux programming tools Unix programming tools