It's very small, which is great. You can print out the entire source code (and there's a makefile target that will produce a printable PDF) and read through it and actually understand the whole system.
At the same time, it has most of the features of an OS that you might want to cover in an intro course:
1. A basic round robin scheduler that you can replace with something more interesting like priorities.
2. A simple FS that still has some advanced features like transactions / crash recovery.
3. Support for SMP, so you can teach issues like locking / deadlocks.
4. UNIX-like system call API, that's again easy to extend to show how a new system call might be implemented.
Off the top of my head, some things it lacks compared to something like Linux:
1. Driver support. Basically only the vga and old-school IDE disks are supported. I've assigned a mouse driver as a final project before and it went pretty well: https://panda.moyix.net/~moyix/cs3224/fall16/bonus_hw/bonus_...
2. Related to (1), this means you aren't going to get anything related to networking.
3. No dynamic linking; every program is static.
4. Anything graphical (although I've seen people do this as a final project)
After this course I able to translate the skills to real world results in include a small contribution to the Linux kernel, as well as a significant non-public Linux kernel module. I can recommend studying the Xv6 source code and accompanying book to anyone, without reservation. It was a great privilege to have been able to take that class.
* The system assumes a fixed amount of RAM and a fixed memory layout, so there is no discovery process and no adaptive code to go with it
* The process table is just a small array--no dynamic allocation necessary, and the system can just do a linear search to find an unused entry
* The userspace is simplified. There is a single stack (no threads) of fixed size, mapped memory starts at address 0, and memory is layed out so that only a single number is required to track the size of the entire address space: the size is the top of mapped memory.
Simple data structures are used everywhere, and the emphasis is always on clarity, not efficiency or flexibility.
1. I remember the first assignment if the class was adding a new syscall. It was quite trivial task for Xv6 because Xv6 syscall layers is thin and it implements only < 30 simple syscalls. Imagine you are going to add a Linux syscall out of > 300 syscalls.
2. Production grade kernels like Linux is very hard to read for students because they support additional configurations which, for example, are wrapped by a lot of #ifdef macros. Also there are overwhelming number of Arch/hardware support that are not needed for teaching.
It's essientially a stripped down rewrite of what existed in Lion's Commentary on Unix. When you reduce subsystems to around 1KLOC each, it's a lot easier to see the core of what they're trying accomplish.
As for what's missing... a lot. No threads, no mmap (or any way to share memory between processes), only vga, serial, lbs, and ide drivers (in the x86 port), no block devices in the fs, 20 something syscalls total.
Also isn't stuff about UNIX popular?
