I printed out a copy of the source into a spiral-bound notebook to read/annotate. It can boot on real hardware which is pretty neat. It can even do multiprocessing on multiple cores (processes, no threads).

It uses "ATA PIO Mode" for the disk driver which is very simple and available on all ATA drives, but horribly slow. The console output is similarly naive but easy to understand. It even has a stripped down user-space, shell, C library etc.

It's a really good top-to-bottom look at a simple OS.