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.
GAS syntax can appear foreign to someone who is familiar with Intel syntax.
- Registers are prefixed by a % sign, thus
- Operands are reversed, so
mov eax, ecx(move ecx into eax) becomes
movl %ecx, %eax. Note the "l". This is discussed in item 4.
- Constants are prefixed with a "$", so
mov eax, 50becomes
movl $50, %eax. Constants are decimal by default; hexadecimal constants are additionally prefixed with 0x, e.g. "$0x50".
- Opcodes do not have implied sizes nor does it specify the size as a separate word. For example, a 32 bit move with Intel syntax requires the "dword" specifier when it is ambigious, while GAS syntax uses suffixes. See below.
- Memory references through register are in the form of "displacement(base register, offset register, scalar)". Thus the memory reference
[eax + 4 + edx*2]is written as
4(%eax, %edx, 2). Note that parentheses are used, NOT square brackets.
- Symbol names require a "$" 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.
- 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 bit "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.