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8.8.2 AT&T Syntax versus Intel Syntax

In order to maintain compatibility with the output of gcc, as supports AT&T System V/386 assembler syntax. This is quite different from Intel syntax. We mention these differences because almost all 80386 documents used only 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.

• In AT&T syntax the size of memory operands is determined from the last character of the opcode name. Opcode suffixes of `b', `w', and `l' specify byte (8-bit), word (16-bit), and long (32-bit) memory references. Intel syntax accomplishes this by prefixes memory operands (not the opcodes themselves) with `byte ptr', `word ptr', and `dword 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.