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
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'.