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.