GNU Assembler

History

The first version of GAS was released in 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

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