(This is version 1 of LILOS, which nobody ever used for anything. I later reused the name for my embedded async real-time operating system in Rust. See: https://github.com/cbiffle/lilos/)
LILOS is a little operating system I built over Spring Break (edit: ...of 2011). It provides multitasking and basic hardware abstraction for the Atmel AVR (currently, the ATmega328p).
There's a working example in main.cc. It assumes an Arduino-style
configuration where an LED is connected to Port B 5. It starts four user tasks
and demonstrates inter-task messaging, GPIO, and use of the USART.
It currently uses 2804 bytes of Flash and 384 bytes of RAM. I hope to reduce both numbers now that everything works.