If you really wanto to "go low level", I can recommend to write progs for NES(6502)/Genesis(68000) - the hardware is simple, processor - simple, and there is a

If you really wanto to "go low level", I can recommend to write progs for NES(6502)/Genesis(68000) - the hardware is simple, processor - simple, and there is a plenty of open source emulators which are in fact a great documentation for target machine. Modern PC's unfortunately are so complicated...