NMOS 6510 Unintended Opcodes

by groepaz

'Back in the days' so called 'illegal' opcodes were researched independently by different parties, and detail knowledge about them was considered 'black magic' for many conventional programmers. They first appeared in the context of copy protection schemes, so keeping the knowledge secret was crucial.

When some time later some of these opcodes were documented by various book authors and magazines, a lot of misinformation was spread and a number of weird myths were born. It took another few years until some brave souls started to systematically investigate each and every opcode, and until the mid 90s that Wolfgang Lorenz came up with his test suite that finally contained elaborated test programs for them.

Still, a few opcodes were considered witchcraft for a while (the so called 'unstable' ones), until other people finally de-capped an actual CPU and solved the remaining riddles.

61 pages, ebook

Published December 24, 2017

Reviews

[David · Rating 5 out of 5]

Instead of microcode, the 6502 family uses a PLA to define its instructions. PLAs use "don't care" bits, and these birthed some very interesting unintended instructions.
This text is an amazing deep dive into all the MOS 6510 unintended opcodes, and practical use-cases for each. Impressive effort.