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Re: 6502 illegal opcodes questions
heuser.marcus@freenet.de wrote:
It's kind of a self-fulfilling prophecy. If enough people write applications
that depend on undocumented behavior, then that behavior becomes
a "feature" that cannot be changed in future versions--usually
preventing useful improvements.
IMHO Atari didn't even seriously consider replacing the 6502. The 8-bit
platform was virtually unchanged for ten years. The machines were cash
cows and the Tramiel-aera Atari concentrated in making them cheaper and
cheaper (to compete with Commodore and reach the Eastern Europe market)
- so there was small room to get innovative.
So it seems the message is, "If you have good reason to believe that
you are programming for an 'end of the line' platform, then do anything
that works--it won't matter anyway".
Of course, now *all* 8-bit platforms are unchanging, but some of them,
in particular the Apple II, went through several implementations of
the processor.
So, even though I don't feel any need to provide for *future* changes,
I still am motivated to cover all the *past* changes, by coding for the
widest range of systems that makes sense--particularly since there is a
negligible "cost" of doing so.
The case with the 2600 is even clearer IMHO: It used the cheaper 6507
and the video game market crash in 1983 did the rest.
In the case of a video game machine, I wouldn't expect any compatible
upgrade path, and the entire hardware arrangement was very ideosyncratic
anyway--so there would be no reason not to do "anything that worked".
-michael
Music synthesis for 8-bit Apple II's!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
tool--and it is seriously underused."