StateOS
Free real-time operating system designed for microcontrollers
Free, extremely simple, amazingly tiny and very fast real-time operating system (RTOS) designed for deeply embedded applications.
Target: ARM Cortex-M, STM8.
It was inspired by the concept of a state machine. Procedure executed by the task (task state) doesn't have to be noreturn-type. It will be executed into an infinite loop. There's a dedicated function for immediate change the task state.