Aren't all of them microcoded? Some years back root was achieved on a line of intel processors and new instructions implemented as proof of concept. There's an academic paper, citation not immediately to hand.
https://github.com/google/security-research/blob/master/pocs...
Tavis spells it out there pretty quickly:
“ The simplest instructions (add, sub, mov, etc) are all implemented in hardware. The more complicated instructions like rdrand, fpatan and cmpxchg are microcoded. You can think of them as a bit like calling into a library of functions written in that RISC-like code.”