eXtreme Minimal Kernel is a preemptive multithreaded real-time operating system for microcontrollers. XMK's primary design goal is to be small, extremely small, without sacrificing performance or functionality. XMK's minimal footprint makes it ideal for running on 8bit microcontrollers, while its feature content makes it a excellent choice for 16bit and 32bit microcontrollers.
Platform: unknown; License: Shift-Right's Open Repository licensing
Reference: https://www.shift-right.com/xmk/