!pr1
Making a 65C02 Work in my Apple II Plus........William O'Ryan

I am writing this on my Apple II Plus running a 2 MHz 65C02 (GTE G65SC02PI-2).  All is well now, but it took some doing.

A few days after pluggin in the chip I started noticing problems.  Applesoft found itself unable to process numeric literals, and the version of FORTH I have been developing began acting weird.

Following the tip in AAL that the timing of the fetch-process- save instructions might be responsible, I ran some tests on them.  The 65C02 worked flawlessly.  Apparently the problem is elsewhere.

After further checking, especially in my FORTH, I found that a certain BNE instruction sitting in the first byte of a page and branching backward into the prior page frequently branched back one byte less than it should.

I'm not a hardware person, but I figured debugging is debugging and I really wanted that chip to work, so I began staring at the circuit diagram in the Apple Reference manual.  After several hours I concluded that I stood for input, O for output, D for data, and A for address.

The easiest hypothesis to check seemed to be that data was not getting back from the RAMs to the microprocessor in time.  So I wrote down some chip numbers and went downtown to see if I could buy some faster varients.  Well, the first two chips I replaced solved the problem.

They were 74LS257 chips at B6 and B7.  These chips multiplex the output of RAM with the output of the keyboard and send the result to the 65C02.  I replaced them with 74F257 chips.  I understand these consume less power, respond faster, and are more susceptible to electrostatic damage.

Anyway, my 65C02 is happy now.  I would like to hear whether this modification works in other Apples, and with other 65C02s.  Drop a line to Bob and Bill at S-C if you have any word on this.
