GNU Assembler

GNU Assembler

Developer(s)	GNU Project
Stable release	2.34 / February 1, 2020 (2020-02-01)
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 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.

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.

• Hash symbols (#) are used for the platforms: i386, x86-64, i960, 68HC11, 68HC12, VAX, V850, M32R, PowerPC, MIPS, M680x0, and RISC-V.
• 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 program

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

.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 value of (current address - address of msg start) to symbol "len" 

Intel syntax

Since version 2.10, Intel syntax can be used through use of the .intel_syntax directive.^[4][5][6]

See also

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

References

External links

• 
• Gas manual
• as(1): the portable GNU assembler – Linux User Commands Manual