Comment by onetom

I'm surprised that no one has mentioned https://flashforth.com/

There is no intriguing backstory for it, like for CollapseOS, but it's a ~6 kiloword, practical 4th environment for Microchip PIC microcontrollers, which are a lot simpler than Z80, btw... The source code is trivial to understand too. My father is still using it daily to replace/substitute Caterpillar machine electronics or build custom instruments for biological research projects.

We started with Mary(Forth) back then, when the first, very constrained PIC models came out, with 8 deep stack and ~200 bytes of RAM. Later we used the https://rfc1149.net/devel/picforth.html compiler for those, which doesn't provide an interactive environment.

I made a MIDI "flute" with that for example, which was fabricated from sawing out a row of keys from a keyboard and used a pen house as a blow pipe and a bent razor with a photo-gate as the blow-pressure detector...

There are more minimal Forth OSes, which might be more accessible than a Z80-based one.

I would think those are more convenient for learning, how can you have video, keyboard and disk IO, an interactive REPL and compiler in less than 10KB

I remember, I played a lot with https://wiki.c2.com/?EnthForth

But if you really want to see something mind-bending, then you should study Moore's ColorForth! I found it completely unusable, BUT I've learnt immense amount of stuff from it: https://colorforth.github.io/

There are more usable variants of it, btw. Also worth looking into Low Fat computing: http://www.ultratechnology.com/lowfat.htm I think it's still relevant today.