I really liked ThreadX when I was working with it a while ago. Calling it an "operating system" is charitable in the sense that it's an interrupt service routin

I really liked ThreadX when I was working with it a while ago. Calling it an "operating system" is charitable in the sense that it's an interrupt service routine and a bunch of functions to handle 'creating threads' and managing memory allocations. It was super light weight and quite nice to get stuff done. You call tx_thread_create or some such and then tx_malloc and so on and then they have tx queues and semaphores and mutexes and so on. Neat to see it's still living on. In our case we were doing real-time power/fan/thermal control loops so we needed control over when certain actions were happening and needed to be able to guarantee that we could sense a problem and then (within X ticks) issue commands to respond, or make the system safe to prevent damage.