If it must not be linux, I would recommend going for something embedded, like FreeRTOS or similar. The advantages: - Linux + Real Time has become a buzz word, and you will find lots of bad information - If you want to learn HW, nothing better as embedded, where you interact with the hardware. - The FreeRTOS is so small you can really understand it pretty fast. You can see how it is made, much easier than Linux. - The essentials you learn with that, will work with Linux later.