Posted By
Hypex
on 2017-08-08
10:11:30
|  Re: Possible 8501 replacement kit?
Well I don't mean to steal one from a C128. Even if it is stuck with an inferior 16 colour palette. 
There are sources of 8502 CPUs and they tend to be cheaper than 8501s. I don't know if they are any more reliable. It would have been easier if C= used the same pin layout since the 8502 would be a logical follow up to the 8501. From what I can tell one pin side has the same lines but the other side differs, mostly being offset down one set of pins, from an extra pin on top section.
Given the port lines are read internally from location $01 I don't know if they can be set externally. Perhaps a bit of logic involved can detect a read /write on addresses $00/$01 and fill in the missing bits. But I don't know if the memory can be overridden.
It isn't only our machines that have this problem but the C64 as well. Which AFAIK was fairly popular. The CPU lasts longer but 6510 aren't around any more either. Be good if a run could be made of ASIC CPU's to satisfy both markets and any others. I am surprised an FPGA 6510 design isn't out by now that slots in place even if it would be seen as a "fake" CPU.
Given WDC used the 6502 design and bolted things onto it I'm surprised they didn't base it on the 6510 or 8502. It is the closest to being an ASIC 65xx CPU still available. Like above, I don't know if it could be bolted onto a PCB in a DIP 40 pin layout with logic to bring the extra port lines in. Perhaps easier to do if $00/$01 isn't mapped internally but can be set externally.
Another idea I have would be to have some kind of CPU on a cartridge. But I have read the 264 port isn't like the C64 one where it can have an external CPU. Still having a RAM cart would be good. It has been before. And there are cart PCBs available as seen on eBay. Bare and with EPROM.
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