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

The GAS executable 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.

Since version 2.10, Intel syntax can be used through use of the <code>.intel_syntax</code> directive.

Comments

GAS supports two comment styles.

Multi-line

As in C, multi-line comments start and end with mirroring slash-asterisk pairs:

<syntaxhighlight lang="c">

/*

comment

  • /

</syntaxhighlight>

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, MIPS, M680x0, and RISC-V
  • A semicolon (;) — AMD 29k family, ARC, H8/300 family, HPPA, PDP-11, picoJava, Motorola, and M32C
  • The at sign (@) — 32-bit ARM
  • A double slash (//) — AArch64
  • A vertical bar (|) — M680x0
  • 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 free and 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.

Example program

A standard "Hello, world!" program for Linux on IA-32:

<syntaxhighlight lang="asm">

.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 # 32-bit address of msg string -> ECX (2nd argument)

movl $len, %edx # length of msg string -> 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"

</syntaxhighlight>

See also

  • GNU toolchain
  • Binary File Descriptor library
  • Comparison of assemblers
  • List of assembly software and tools

References

  • Gas manual