Syntax
x86 assembly language has two main syntax branches: Intel syntax, originally used for documentation of the x86 platform, and AT&T syntax. Intel syntax is dominant in the MS-DOS and Windows world, and AT&T syntax is dominant in the Unix world, since Unix was created at AT&T Bell Labs. Here is a summary of the main differences between Intel syntax and AT&T syntax:
| AT&T | Intel | |
|---|---|---|
| Parameter order | Source before the destination
eax := 5 is mov $5, %eax |
Destination before source
eax := 5 is mov eax, 5 |
| Parameter Size | Mnemonics are suffixed with a letter indicating the size of the operands (e.g., "q" for qword, "l" for long (dword), "w" for word, and "b" for byte)
addl $4, %esp |
Derived from the name of the register that is used (e.g., rax, eax, ax, al imply q,l,w,b in that order)
add esp, 4 |
| Immediate value sigils | Prefixed with a "$", and registers must be prefixed with a "%" | The assembler automatically detects the type of symbols; i.e., if they are registers, constants or something else. |
| Effective addresses | General syntax DISP(BASE,INDEX,SCALE)
Example: movl mem_location(%ebx,%ecx,4), %eax |
Use variables, and need to be in square brackets; additionally, size keywords like byte, word, or dword have to be used.
Example: mov eax, dword |
Many x86 assemblers use Intel syntax including MASM, TASM, NASM, FASM and YASM. GAS has supported both syntaxes since version 2.10 via the .intel_syntax directive.
Read more about this topic: X86 Assembly Language