This is the MIT xv6 teaching operating system, with features added by Nick Girardo and Myself.
The features we've added in different branches are support for things like user level signal handling, copy-on-write virtual memory management, semaphores, and user and kernel level preemptive threading using linux ucontexts.
The features are all sitting in these separate branches. I will merge these at some point... maybe.
There are many other cool features and things laying around, (like curse words) in the source code, so feel free to check it out!