From what I understand the 65C22 is superior to the 6821 PIA used in the Apple 1 (and many others).

So, is it conceivable that replacing the 6821 for a 65C22 in an Apple 1 replica/clone should be somewhat trivial?

I know the pinouts aren't the same...ignoring that for the moment.

How much work do you think it would be? Especially to enable legacy software to work?

The WozMon shouldn't be too hard, I would imagine, to use the 65C22.

Thoughts?

Thanks!

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Cat; the other white meat.

It should be doable. The hard part will be getting the addresses for the Woz Mon character I/O entry points the same. Here's the source: https://github.com/jefftranter/6502/blo ... n/wozmon.s

There's little space there for padding to keep the entry points at the same absolute address. If you alter those addresses any legacy software that calls the WozMon will fail. Also, any software which does its own PIA I/O will break.

If you can't get it to fit then consider using a previously unused area of the high addresses space. With some judicious code shuffling and a few JMP's it would work.

What I'm trying to do is wire-wrap a SBC that is as compatible as possible with the Apple 1. But, I want to use all new parts and the PIA is the one I'm concerned with. I have a PIA, I just don't want to use it if I don't have to.



That's what has me worried. How many different entry points do you think legacy software would use? The ECHO comes to mind. Others?



Shoot. I didn't think about that one. That might be a bummer too.

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Cat; the other white meat.

ECHO ends in an RTS, so anything that calls it could also be used as an entry point. The obvious candidates are GETLINE and NEXTCHAR.

Wait!

Duh...I just realized there is a 65C21.

http://www.mouser.com/search/ProductDet ... 21N6TPG-14

I haven't read the datasheet yet, but do you think it would work as a drop-in replacement for the PIA?

If so, that would work for me because it satisfies my requirements of new parts only.

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Cat; the other white meat.

The WDC 65C21 is supposed to be an exact replacement for the 6521/6821 of olden days. Assuming it is, it would be less work to integrate it than the 65C22.

Incidentally, you want the W65C21S, not the W65C21N.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Oh yeah, it sure would be. I've been burning too many cycles wondering what all I would need to do to replace with a 65C22 and it never occurred to me to see if there was a modern 6821 replacement.

I've been called senile but I always thought they were joking...maybe they were right as rain.

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Cat; the other white meat.

Far be it from me at my advanced age to pass any judgment on senility.

Incidentally, note my above remark about the W65C21S versus the W65C21N.

Speaking of the 65C21, my preliminary design for POC V3 includes a Centronics-compatible port that will be driven by a W65C21S. As the Centronics port requires certain timing criteria be met in order to get the connected peripheral (usually a printer) to respond, one of those handy timers in an NXP QUART or OCTART would give me a way to sequence the port at a rate that is independent of the Ø2 clock rate.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Ah, yes. Good call on the difference.

Thanks!

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Cat; the other white meat.