Posted By
Csabo
on 2002-10-23
|  Opcodes
While I can't give you a proper explanations on WHY they exist (I have my ideas, but they are probably not 100% right), I do know that these work on the C16/Plus4 as well. Some demos used them, possibly even some games (but that's rare). One area where they were used extensively was "protectors" or "coders" (take a look at GOTU, Terrorists and Methabolix demos/games). They used these coders to "protect" their work. Since these instructions cannot be read by the built in monitor, the average user wouldn't understand the actual code (but you could use one of the several more advanced monitors like Kalimon).
"Illegal instructions" is a very unfortunate and inappropriate name. "Undocumented instructions" would be much better. Most of them have very well defined behavior, but some of them are extremely obscure (and therefore useless). The Op-code police won't mind if you use them at all 
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