GNU Info

(as.info)i386-Arch

---

Next: i386-Bugs Prev: i386-16bit Up: i386-Dependent

---

Enter node , (file) or (file)node

Specifying CPU Architecture
---------------------------

   `as' may be told to assemble for a particular CPU architecture with
the `.arch CPU_TYPE' directive.  This directive enables a warning when
gas detects an instruction that is not supported on the CPU specified.
The choices for CPU_TYPE are:

`i8086'        `i186'         `i286'         `i386'
`i486'         `i586'         `i686'         `pentium'
`pentiumpro'   `pentium4'     `k6'           `athlon'
`sledgehammer'                               

   Apart from the warning, there are only two other effects on `as'
operation;  Firstly, if you specify a CPU other than `i486', then shift
by one instructions such as `sarl $1, %eax' will automatically use a
two byte opcode sequence.  The larger three byte opcode sequence is
used on the 486 (and when no architecture is specified) because it
executes faster on the 486.  Note that you can explicitly request the
two byte opcode by writing `sarl %eax'.  Secondly, if you specify
`i8086', `i186', or `i286', _and_ `.code16' or `.code16gcc' then byte
offset conditional jumps will be promoted when necessary to a two
instruction sequence consisting of a conditional jump of the opposite
sense around an unconditional jump to the target.

   Following the CPU architecture, you may specify `jumps' or `nojumps'
to control automatic promotion of conditional jumps.  `jumps' is the
default, and enables jump promotion;  All external jumps will be of the
long variety, and file-local jumps will be promoted as necessary.
(Note: i386-Jumps)  `nojumps' leaves external conditional jumps as
byte offset jumps, and warns about file-local conditional jumps that
`as' promotes.  Unconditional jumps are treated as for `jumps'.

   For example

      .arch i8086,nojumps