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(as.info)i386-Mnemonics

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Instruction Naming
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   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.