i386-Variations - Using as
Description: Using as

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##### 9.15.3.1 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.

  In 64-bit code, ‘`movabs`’ can be used to encode the ‘`mov`’ instruction with the 64-bit displacement or immediate operand.

- 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.