Simple ports to small systems are actually very easy. To run Linux all that you strictly need is a timer driver and an interrupt controller driver, assuming the

Simple ports to small systems are actually very easy. To run Linux all that you strictly need is a timer driver and an interrupt controller driver, assuming the CPU architecture is already supported and there is a compatible MMU. On small systems, those drivers can be very simple. I was bored one evening and ported Linux to run on the Wii's IO/security processor (not the main CPU, that's Wii Linux). It only took a few hours to get the kernel booting to userspace.