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#### 9.13.3 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 bound, invlpga, and instructions with 2 immediate operands, 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.