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Reviving the project

A project log for JACA 1 & 2 Homebrew Computer

JACA - Just Another CPU Again Homebrew CPU, starting by a simple POC 4-bit CPU on circuit simulator soft. (done), then 8-bit (in progress)

Andre BaptistaAndre Baptista 01/19/2020 at 18:261 Comment

One year ago I made a commitment to finish this project in 4 months, one module per month (link). The first 3 months everything went alright, but the last module was very complex and blow up entirely the schedule.

Now I'm slowly coming back and putting this module on a PCB, this time without time contraints, just for fun.

Discussions

zpekic wrote 05/14/2022 at 15:29 point

Fascinating project! I just stumbled on it now and I can only realize how much time it must take to go over all the combinations and iterations, and you documented it all in blogs so we can see the evolution. I always wanted to build something in pure hardware too but now I am a bit scared... Maybe when I retire? :-)

I think the key problem is that for each doubling of word-size (e.g. going from 4 to 8 bit) the complexity increases by square or so. One way around this is to keep 4-bit internally for processing (lots of 74xx devices are 4 bit) but use time multiplexing. So 2 7489 RAMS + 1 74181 could become a core of a 16-bit CPU with 4 16 bit registers, but the lower two bits or register RAM are actually time sequence. Yes, the CPU is 4 times slower but it could still reasonably run at few MHz, quite sufficient for hobby purposes. Did you maybe explore that option? 

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