There are tons of patents out there for the circuits which efficiently implement some instructions. Most of those patents have expired by now, but new ones pop

There are tons of patents out there for the circuits which efficiently implement some instructions. Most of those patents have expired by now, but new ones pop up every once in a while. One recent example would be the circuit which computes steering bits for implementing pext and pdep on a butterfly network. There are also plenty of patents for particular descriptions of instructions. They may be fairly narrow in scope, depending on how close they are to prior art. But they don't have to be particularly wide in scope in order to prevent cloning. It would not surprise me at all if the x86's encoding choice for `reg + reg * scale + displacement` was patented. Emulators can absolutely require licenses. The ARM emulator shipped with Android for x86 is licensed by ARM, for example. While there aren't very many (uh, any at all?) x86 phones, x86 Chromebooks with Android support is a thing.