#### 9.15.17 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`'.