Might want to look at MicroPython. The ports have various init functions for various chips /system. It might not be a full OS, but it is pretty low level code y

Might want to look at MicroPython. The ports have various init functions for various chips /system. It might not be a full OS, but it is pretty low level code you can compare to various other systems. https://github.com/micropython/micropython/tree/master/ports https://github.com/micropython/micropython/tree/master/ports