[artlogic]

I own volumes 1, 2 and the PC version. I can vouch for the quality and understandability of the code. I've read a number of operating systems books, and I've found volume 1 to be one of the most straightforward - if you can get past the fact it's written with the PDP-11 in mind. I bought the PC version on a whim after reading volume 1, and found it to be lacking in a number of ways, using BIOS for many system services and glossing over memory protection (not surprising as it targeted the 8086).

One of the more interesting things about Xinu is that its security model is ring based, and if I remember right, it takes advantage of rings 0-3, unlike many modern operating systems. I can't actually remember if the PDP-11's hardware supported this, or if it was emulated by the kernel.

One of my rainy day projects has always been to get Xinu up and running on a PDP-11 emulator - too many operating systems, too little time.

[Erwin]

OS/2 was another one using multiple rings.

Interestingly VirtualBox takes advantage of x86's multiple rings: http://www.virtualbox.org/manual/ch10.html#idp12629776 (which contains other interessting details about VirtualBox, like the disassembles and patches code running in the VM in certain cases)