YACC1 (Yet Another Custom CPU) Version 1

I have been thinking about a custom CPU project for a very very very long time, something like the past 40 years to be exact. The idea started when I first started dabbling with TTL chips. Back then I had no idea how to proceed, i knew some sort of instruction decoder, sequencer, ALU were needed but not long after first dreaming about the idea of building a CPU the RCA 1802 showed up and that stole all my attention. I will leave out the details but I have never stopped thinking about a custom CPU project and every once in a while when a new website pops up dedicated to a new custom CPU (I have a small list here) I once again would get the bug.

Time to take another run at it.


This is an MVP (Yes I am dreaming about future projects but lets see how this goes)

Open, all designs will be available for anyone to use

Simple, Simple, Simple, therefore 8 bits for data and addressing (yep only 256 x 8 memory)

Must fit on one PCB, all through hole.

Minimal magic, basic TTL chips, no FPGAs and possibly even nothing as fancy as a ALU chip (although single PCB trumps this requirement)

Does not need to to be compatible with anything past, present, or future

Function is more important than optimization

Test ideas but avoid “kitchen sink” phenomenon




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