https://github.com/hoglet67/PiTubeDirect
(The BBC Micro itself was quite remarkable for supporting multiple processors, which did not have to be the same architecture as the host 6502. Amazing for 1981.)
Two I can think of off the top of my head:
- FPGA 6581 to replace a Commodore 64 SID chip.
- There's a drop-in sorta Z-80 so you can run CP/M on the TRS-80 Model 100.
Dangerous Prototypes wrote up a pretty good post about it: http://dangerousprototypes.com/blog/2012/02/22/prop-6502-pro...
Do you happen to still have this information somewhere?
http://dennisferron.blogspot.com/2008/12/prop6502-laptop-pro...
Looks like Parallax still has a link to their old site, but I couldn't find the contest pages there, either.
I'm pretty sure the code I wrote for it ended up on the Propeller object exchange, but that site also is gone... It probably lives on in the migration to a github repo, but it isn't so easy to search by author AFAICT.
My picture (along with the other design contest winners and honorable mentions) was on the back of the May 2009 Nuts and Volts magazine, but I just checked my copy and there's no article inside, just the blurb on the back cover.
So start with a Pi running around 1/30th normal speed, then get the memory card working and remove that from the pi, then get the video and BIOS support working and remove that from the pi, eventually remove the last thing from the pi and crank the speed up about 30-times faster and you've got a gradually bootstrapped fully operational XT-clone.
Its an interesting project plan, usually people bring up a system by having just CPU and memory at least partially working, then add peripherals like disk, display, rs232, GPIO, etc. But this way you can bring it all up, at least enough to run CP/M (err, ms-dos I guess?), admittedly glacially slowly and mostly emulated, and then upgrade the parts to hardware as see fit.
Could probably do something with a FPGA that would be much more difficult but could at least run full speed (or maybe faster?). Of course in early 2022 what's more unobtanium, RasPis or FPGAs? If you can't buy either I guess it doesn't matter.
