Ah OK I see. Yes as others have pointed out Minix. But for something you can really understand have a look at Xinu - small and very minimal, there's a great boo

Ah OK I see. Yes as others have pointed out Minix. But for something you can really understand have a look at Xinu - small and very minimal, there's a great book on it too. You probably aren't going to run Xinu as your main desktop though! ;) Others worth mentioning are SkyOS. I don't think Robert has worked on it for years, but I believe the source code is still available and there used to be quite a lot of very readable documentation outlining the architecture. ChrysaLisp is certainly one to watch. He's the guy behind the now defunct Tao OS. The Tao tech was way too far ahead of its time. Maybe now is the time...