Undocumented CPU behavior: analyzing undocumented opcodes on Intel x86-64 (2018) [pdf]
Associated GitHub repository with more information: https://github.com/cattius/opcodetesterI know next to nothing about processors at this level, but I wonder if it would be possible for a skilled engineer to try to find these instructions by scrutinizing the actual physical instruction decoder on the chip and/or inspect the processor's microcode. Are these things possible to do? If they are, is it feasible to reverse engineer them?