Then you take an illegal instruction exception, and you have a choice to request a rebuild, patch it, or emulate it. (Yeah, sometimes the vendors just don't cooperate.)
Incidentally, emulating opcodes is quite often practical (unless the performance must not be affected), and is greatly helped by having the plainest, cleanest instruction encoding possible, and a well designed system register & exception architecture.
Incidentally, emulating opcodes is quite often practical (unless the performance must not be affected), and is greatly helped by having the plainest, cleanest instruction encoding possible, and a well designed system register & exception architecture.