8.12.2 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. 8.12.11 AT&T Syntax 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.
This document was generated
by Stephen Zander on April, 18 2003
using texi2html