Fuzix is a very simple UNIX clone (and a for of UZI) started by Alan Cox, an ex-Linux kernel developer) as his retirement project.

It aims to run on old CPUs like Z80 and on microcontrollers. I found it when searching what OS can I run on the $4 Raspberry Pi Pico.

https://codeberg.org/EtchedPixels/FUZIX

https://codeberg.org/EtchedPixels/FUZIX last updated 4 days ago

Alan's currently putting most of his energy into the compiler. It's a C compiler in C which can compile itself, and compile FUZIX, for 8080 and Z80 targets. The goal is to make it compile itself on all the platforms it can run on eventually. :)

https://codeberg.org/EtchedPixels/Fuzix-Compiler-Kit