Re:6502 “Illegal” Opcodes Demystified
by flohofwoe on Sunday June 06, 2021 @07:35AM
(#27412407)
I bet there wasn't one. The undocumented instructions are just unintended side effects of how the hardware works, not a ploy to give some programming teams an advantage via secret knowledge ;)
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| on HN ]
Yeah, it was my understanding that the opcodes directly encoded what we would nowadays call microcode, that is, bits which selected which CPU component acted on which data[1], so these illegal instructions are just unused bit patterns that select various operations just by having those bits set, not something that was ever intended to be actually used. So it would make sense that they're not documented, since they're entirely incidental.