Roll your own Unix
Also see xv6, "a simple Unix-like teaching operating system": <a href="http://pdos.csail.mit.edu/6.828/2011/xv6.html" rel="nofollow">http://pdos.csail.mit.edu/6.828/2011/xv6.html</a> and its textbook: <a href="http://pdos.csail.mit.edu/6.828/2011/xv6/book-rev6.pdf" rel="nofollow">http://pdos.csail.mit.edu/6.828/2011/xv6/book-rev6.pdf</a>