I had a great time following this a couple years ago. It inspired me to write a tiny micro-kernel with pre-emtive multitasking. It was probably a bug-ridden mess, but it was incredibly satisfying having a handful of threads printing 'Hello from thread #1', 'Hello from thread #2' etc.
I had to code a small OS in C in third year of college and the main piece of it was the scheduler. I remember my semaphore routines quite fondly actually.
