I also used Xinu in my O/S course. I'll mention that one of Doug Comer's (http://www.wikipedia.org/wiki/Douglas_Comer) intentions with Xinu was to purposefully make a different design decision than Unix when pedagogically relevant. For example, instead of treating everything like a file (Unix/Linux) everything is treated like a device.

Its various iterations over the year do not necessarily remain true to that principle, but it is something different about the O/S.