> An undocumented opcode in a processor is generally a side-effect that was
> never intended to be there in the first place. The original chip
> manufacturer (or a licensed second source) is under no obligation to keep
> that side-effect present in future versions of the chip.
>
> The Z80 has quite a few of these and, while I may be able to save a byte
or
> two occasionally, the risk of having a future die revision break my code
> just isn't worth it to me. YMMV

6800 from long ago had a  nice one -
We called it   HCF
aka "Halt & Catch Fire" :-)

When implemented the cpu turned into a binary hardware counter with
addresses appearing on address bus at clockspeed.
An excellent instruction for testing address decoders etc and a good trap if
the cpu ran away ever. When it did it would often as not strike a HCF and
the system would lock in this interesting mode.

Presumably it was intended as  a hardware test.


RM

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