A nifty little Unix clone/rewrite is OMU (One Man Unix): http://discordia.org.uk/~steve/omu.html Also Xv6, Fuzix, and UZI: https://pdos.csail.mit.edu/6.828/2012/xv6.html http://www.fuzix.org/ http://www.dougbraun.com/uzi.html