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Comment by zahlman

4 hours ago

> illegal instructions... were often used by programmers

Intentionally, with an expected effect? I'd need a citation for that.

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zahlman

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chuckadams 3 hours ago

Some desultory googling turned up:

* https://www.nesdev.org/wiki/CPU_unofficial_opcodes#Games_usi...

* https://hitmen.c02.at/files/docs/c64/NoMoreSecrets-NMOS6510U... (doesn't name any software, but some copy protection schemes were already known to use them)