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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.  Note: 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.