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).
The Intel-syntax conversion instructions
cbw -- sign-extend byte in %al to word in %ax,
cwde -- sign-extend word in %ax to long in %eax,
cwd -- sign-extend word in %ax to long in %dx:%ax,
cdq -- sign-extend dword in %eax to quad in %edx:%eax,
cdqe -- sign-extend dword in %eax to quad in %rax
(x86-64 only),
cdo -- sign-extend quad in %rax to octuple in
%rdx:%rax (x86-64 only),
are called cbtw, cwtl, cwtd, cltd, cltq, and
cqto in AT&T naming. as accepts either naming for these
instructions.
Far call/jump instructions are lcall and ljmp in
AT&T syntax, but are call far and jump far in Intel
convention.