I am looking for a decent source on how AT&T syntax assembler instructions are formed (the syntax used by gcc inline asm).
There are some semi decent ones covering gcc such as the DJGPP guide or the GCC inline asm howto, but none of these describe the syntax detailed enough so one could deduct all instructions' formats using an opcode guide written for the Intel syntax.
I see there are already questions like this one on the subject, but without suitable answers.
Most particularly currently I am stumped about how a three operand imul should be formatted. The only pointer I could find on this is this forum post, with apparently just as stumped people as me on this matter.
(OK, it is true I could try and see, but really there should be some proper docs out in the wild for these types of things which I just can't find)