GasSyntax – The Yasm Modular Assembler Project

GAS Syntax

GAS or GNU as syntax is a different form of syntax for assembly language files, known also as AT&T syntax after the original style. It is commonly used by other versions of GAS for other architectures (i.e. non-x86). This guide is not a complete reference, but merely an overview.

Differences

GAS syntax can appear foreign to someone who is familiar with Intel syntax.

  1. Registers are prefixed by a % sign, thus "eax" become "%eax"
  2. Operands are reversed, so "mov eax, ecx" (move ecx into eax) becomes "movl %ecx, %eax". Note the 'l'. This is discussed in item 4.
  3. Constants are prefixed with a "$", so "mov eax, 50" becomes "movl $50, %eax". Constants are decimal by default; hexadecimal constants are additionally prefixed with 0x, e.g. "$0x50".
  4. Opcodes do not have implied sizes nor does it specify the size as a separate word. For example, a 32 bit move with Intel syntax requires the "dword" specifier when it is ambigious, while GAS syntax uses suffixes. See below.
  5. Memory references through register are in the form of "displacement(base register, offset register, scalar)". Thus the memory reference "[eax + 4 + edx*2]" is written as "4(%eax, %edx, 2)". Note that parentheses are used, NOT square brackets.
  6. Symbol names require a "$" to load the address, and no prefix to access contents. Thus the Intel/NASM syntax memory reference "mov dword eax, [symbol]" is the same "movl symbol, %eax". To load the address of "symbol", then Intel/NASM syntax uses "mov eax, symbol", while GAS uses "movl $symbol, %eax".

Integer Suffixes

  • b -- 8 bit byte. Ex: "movb $0x40, %al", move constant 0x40 into register al.
  • w -- 16 bit word. Ex: "movw %ax, %bx", move register ax into bx.
  • l -- 32 bit long. Ex: "movl %ecx, %eax", move register ecx into eax
  • q -- 64 bit quadword. Ex: (64 bit programs only) "movq %rax, %rdx", move register rax into rdx

Floating Point (x87) Suffixes

  • s -- Short (single precision, 32 bits). Ex: "flds (%eax)", load 32 "float" from memory.
  • l -- Long (64 bits). Ex: "fldl (%eax)", load 64 bit "double" from memory.
  • t -- Ten-byte (80 bits). Ex: "fldt (%eax)", load 80 bit "long double" from memory.