pagetable.com » Blog Archive » How MOS 6502 Illegal Opcodes really work
http://www.pagetable.com/?p=39
"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."