There's been some limited research in this space; see for instance xoreaxeaxeax's sandsifter tool which has found millions of undocumented processor instruction

There's been some limited research in this space; see for instance xoreaxeaxeax's sandsifter tool which has found millions of undocumented processor instructions [0]. [0] https://www.youtube.com/watch?v=ajccZ7LdvoQ https://www.youtube.com/watch?v=ajccZ7LdvoQ