GNU Info

(as.info)i386-Notes

---

Prev: i386-Bugs Up: i386-Dependent

---

Enter node , (file) or (file)node

Notes
-----

   There is some trickery concerning the `mul' and `imul' instructions
that deserves mention.  The 16-, 32-, 64- and 128-bit expanding
multiplies (base opcode `0xf6'; extension 4 for `mul' and 5 for `imul')
can be output only in the one operand form.  Thus, `imul %ebx, %eax'
does _not_ select the expanding multiply; the expanding multiply would
clobber the `%edx' register, and this would confuse `gcc' output.  Use
`imul %ebx' to get the 64-bit product in `%edx:%eax'.

   We have added a two operand form of `imul' when the first operand is
an immediate mode expression and the second operand is a register.
This is just a shorthand, so that, multiplying `%eax' by 69, for
example, can be done with `imul $69, %eax' rather than `imul $69, %eax,
%eax'.