9.16.3.1 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. Mnemonic suffixes of ‘x’, ‘y’ and ‘z’ specify xmm (128-bit vector), ymm (256-bit vector) and zmm (512-bit vector) memory references, only when there’s no other way to disambiguate an instruction. Intel syntax accomplishes this by prefixing memory operands (not the instruction mnemonics) with ‘byte ptr’, ‘word ptr’, ‘dword ptr’, ‘qword ptr’, ‘xmmword ptr’, ‘ymmword ptr’ and ‘zmmword ptr’. Thus, Intel syntax ‘mov al, byte ptr foo’ is ‘movb foo, %al’ in AT&T syntax. In Intel syntax, ‘fword ptr’, ‘tbyte ptr’ and ‘oword ptr’ specify 48-bit, 80-bit and 128-bit memory references.

  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.