xv6 is an amazing teaching OS - it is very simple to dive into and play around with. A while ago I wanted to explore some filesystem&#x2F;permissions stuff and use xv6 to play with some concepts (see <a href="http:&#x2F;&#x2F;sarahjamielewis.com&#x2F;posts&#x2F;file-system-permissions-and-xv6.html" rel="nofollow">http:&#x2F;&#x2F;sarahjamielewis.com&#x2F;posts&#x2F;file-system-permissions-and...</a> - still lots I want to play around with there when I find some time) - If you are at all interested in OS dev it is a great gateway.