Instruction mnemonics are suffixed with one character modifiers which
specify the size of operands. The letters `b', `w', `l'
and `q' specify byte, word, long and quadruple word operands. If
no suffix is specified by an instruction then
as tries to
fill in the missing suffix based on the destination register operand
(the last one by convention). Thus, `mov %ax, %bx' is equivalent
to `movw %ax, %bx'; also, `mov $1, %bx' is equivalent to
`movw $1, bx'. Note that this is incompatible with the AT&T Unix
assembler which assumes that a missing mnemonic suffix implies long
operand size. (This incompatibility does not affect compiler output
since compilers always explicitly specify the mnemonic suffix.)
Almost all instructions have the same names in AT&T and Intel format. There are a few exceptions. The sign extend and zero extend instructions need two sizes to specify them. They need a size to sign/zero extend from and a size to zero extend to. This is accomplished by using two instruction mnemonic suffixes in AT&T syntax. Base names for sign extend and zero extend are `movs...' and `movz...' in AT&T syntax (`movsx' and `movzx' in Intel syntax). The instruction mnemonic suffixes are tacked on to this base name, the from suffix before the to suffix. Thus, `movsbl %al, %edx' is AT&T syntax for “move sign extend from %al to %edx.” Possible suffixes, thus, are `bl' (from byte to long), `bw' (from byte to word), `wl' (from word to long), `bq' (from byte to quadruple word), `wq' (from word to quadruple word), and `lq' (from long to quadruple word).
Different encoding options can be specified via optional mnemonic suffix. `.s' suffix swaps 2 register operands in encoding when moving from one register to another. `.d32' suffix forces 32bit displacement in encoding.
The Intel-syntax conversion instructions
are called `cbtw', `cwtl', `cwtd', `cltd', `cltq', and
`cqto' in AT&T naming.
as accepts either naming for these
Far call/jump instructions are `lcall' and `ljmp' in AT&T syntax, but are `call far' and `jump far' in Intel convention.
as supports assembly using Intel mnemonic.
.intel_mnemonic selects Intel mnemonic with Intel syntax, and
.att_mnemonic switches back to the usual AT&T mnemonic with AT&T
syntax for compatibility with the output of
Several x87 instructions, `fadd', `fdiv', `fdivp',
`fdivr', `fdivrp', `fmul', `fsub', `fsubp',
`fsubr' and `fsubrp', are implemented in AT&T System V/386
assembler with different mnemonics from those in Intel IA32 specification.
gcc generates those instructions with AT&T mnemonic.