F9: an L4-style microkernel for ARM Cortex-M

F9 is an L4-inspired microkernel designed for ARM Cortex-M, targeting real-time embedded systems with hard determinism requirements. It implements the fundamental microkernel principles-address spaces, threads, and IPC, while adding advanced features from industrial RTOSes.

F9 kernel GitHub page

For once, not written in Rust, and comes with both an L4-style native API and a userspace POSIX API, and there's a ton of documentation to get you started.