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8.11.13 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'.