KnightOS is an operating system I started writing about 10 years ago, for Texas Instruments line of z80 calculators — the TI-73, TI-83+, TI-84+, and similar calculators are supported. It still gets the rare improvements, but these days myself and most of the major contributors are just left with starry eyed empty promises to themselves that one day they’ll do one of those big refactorings we’ve been planning… for 4 or 5 years now. Still, it was a really interesting operating system which was working under some challenging constraints, and overcame them to offer a rather nice Unix-like environment, with a filesystem, preemptive multiprocessing and multithreading, assembly and C programming environments, and more. The entire system was written in handwritten z80 assembly, almost 50,000 lines of it, on a compiler toolchain we built from scratch. The things people can squeeze out of these limited devices astounds me every single time.