Re: [stella] "Illegal" opcodes

Subject: Re: [stella] "Illegal" opcodes
From: Erik Mooney <emooney@xxxxxxxxxxxxxxxx>
Date: Mon, 19 Jun 2000 00:19:21 -0400
>As others have pointed out, they're not really illegal, but merely
>undocumented.  But IIRC the reason they used a stronger-sounding term
>was because they're not guaranteed to be there in new revisions of the
>chip, or in the next generation with the same architecture.

In practice, most undocumented functions of both hardware and software
usually do stay in for the lifetime of the product, because some weirdo
is always going to find out about them, find a good use for them, and
release something popular enough that uses them that the designers of
the next generation won't want to break it.  I recall reading that
something like a third of all the functions available in the collective
Windows/DOS environment started off undocumented.

>>That they work at all is a side effect of the way the 65xx's microcode
>>is lashed together.
>Great, careless coding even on that level...
>I've always wondered why they didn't just tie off any used opcodes as
>NOPs to actually MAKE them unuseable.

As I said, because tying them off as NOPs would increase complexity and
cost for an actual _reduction_ in chip functionality, which is rather a
bad idea on the engineering level.  I'd still maintain that the chip
designers knew exactly that those operations would result from those
opcodes and intentionally left it that way.  They're accidents, but
supervised accidents.

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