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### AT&T Syntax versus Intel Syntax

`as` now supports assembly using Intel assembler syntax. `.intel_syntax` selects Intel mode, and `.att_syntax` switches back to the usual AT&T mode for compatibility with the output of `gcc`. Either of these directives may have an optional argument, `prefix`, or `noprefix` specifying whether registers require a `%` prefix. AT&T System V/386 assembler syntax is quite different from Intel syntax. We mention these differences because almost all 80386 documents use Intel syntax. Notable differences between the two syntaxes are:

- AT&T immediate operands are preceded by `$`; Intel immediate operands are undelimited (Intel `push 4` is AT&T `pushl $4`). AT&T register operands are preceded by `%`; Intel register operands are undelimited. AT&T absolute (as opposed to PC relative) jump/call operands are prefixed by `*`; they are undelimited in Intel syntax.
- AT&T and Intel syntax use the opposite order for source and destination operands. Intel `add eax, 4` is `addl $4, %eax`. The `source, dest` convention is maintained for compatibility with previous Unix assemblers. Note that instructions with more than one source operand, such as the `enter` instruction, do not have reversed order. i386-Bugs.
- In AT&T syntax the size of memory operands is determined from the last character of the instruction mnemonic. Mnemonic suffixes of `b`, `w`, `l` and `q` specify byte (8-bit), word (16-bit), long (32-bit) and quadruple word (64-bit) memory references. Intel syntax accomplishes this by prefixing memory operands (not the instruction mnemonics) with `byte ptr`, `word ptr`, `dword ptr` and `qword ptr`. Thus, Intel `mov al, byte ptr` foo`` is `movb` foo`, %al` in AT&T syntax.
- Immediate form long jumps and calls are `lcall/ljmp $`section`, $`offset`` in AT&T syntax; the Intel syntax is `call/jmp far` section`:`offset``. Also, the far return instruction is `lret $`stack-adjust`` in AT&T syntax; Intel syntax is `ret far` stack-adjust``.
- The AT&T assembler does not provide support for multiple section programs. Unix style systems expect all programs to be single sections.