This is a wee operating system written to support the async style of programming in Rust on microcontrollers. It fits in about 2 kiB of Flash and uses about 20 bytes of RAM (before your tasks are added). In that space, you get a full async runtime with multiple tasks, support for complex concurrency via join and select, and a lot of convenient but simple APIs. (If you want to see what a lilos program looks like, look in the examples directory, or read the intro guide.)

Source: GitHub - cbiffle/lilos: A wee async RTOS for Cortex-M

Interesting! Very, very tiny.