The original NMOS version of the MOS 6502, used in computers like the Commodore 64, the Apple II and the Nintendo Entertainment System (NES), is well-known for its illegal opcodes: Out of 256 possible opcodes, 151 are defined by the architecture, but many of the remaining 105 undefined opcodes do useful things. Many articles have been written to test and document these, but I am not aware of any article that tries to explain where exactly they come from. I’ll do this here. The Block Diagram Ever...