Re: Difference between 6502 & 65C02

[sheldon@netcom.com (Sheldon Simms)]

In article <96082.140914JCA31@MAINE.MAINE.EDU> <JCA31@MAINE.MAINE.EDU> writes:
>I'd like to know what it is that's different between these two cpu's,
>apart from one being a CMOS chip. I'm also curious as to why I my II+
>'sort-of' works with a 'C02 in it. I'd think that the machine would
>either run, or crash miserably, but instead, it just seems to have
>trouble keeping track of numbers ..?something to do with operating
>modes, maybe?

The 65C02 has some extra instructions not found on the 6502, which could
possibly be a problem for programs that used undocumented 'instructions'
on the 6502. But I think the trouble you are taking about is mainly
caused by slight differences in timing between the two chips. The
Apple II+, probably cut some of the timing pretty close, seeing as how
it was built almost entirely out of TTL chips, and the 65C02 is slightly
different here or there so some signal gets lost. As an example, if
instruction reads were done ok, but data reads were flaky because the
timing was off, the symptoms could be similar to what you describe.

-Sheldon