Phōs is a very simple embedded OS that is used for teaching the first year course on Digital Systems. Unlike most other embedded operating systems, it is based on the single synchronisation primitive of synchronous message passing, with all interrupts converted to messages.
At present, Phōs is specific to the BBC micro:bit. But plenty of other inexpensive ARM-based development boards exist, such as the Freescale FRDM-KL25Z and FRDM-K64F boards, and many other boards that are part of the MBED ecosystem.
The aim of this project is to port Phōs to one or more of these other boards, identifying and isolating the parts that are board-specific and providing multiple implementations of device driver processes so that Phōs applications are portable between boards.
Other boards don't have the switches and lights of the micro:bit but some, including the FRDM boards, support Arduino-compatible add-on 'shield' boards, and we could easily make a shield that replicates the features of the micro:bit.
Work to extend and port Phōs is ongoing, and another page has notes and details of progress.