Another great free OS resource is MIT's 6.828: Operating System Engineering.
"This course studies fundamental design and implementation ideas in the engineering of operating systems. Lectures are based on a study of UNIX and research papers. Topics include virtual memory, threads, context switches, kernels, interrupts, system calls, interprocess communication, coordination, and the interaction between software and hardware. Individual laboratory assignments involve implementation of a small operating system in C, with some x86 assembly."
Note that this book is half-finished and work on it has been discontinued (as of 2 years ago). If you want a good resource on OSdev, start here: http://wiki.osdev.org/Main_Page
Maintainer here! We actually have more developed than the tutorial lets on; at Rust Belt Rust next week, we're running a six-hour class, so focus has been on material for that, rather than on writing more book chapters. I hope to get them out afterwards, though.
There's also some open PRs with more functionality too! Basically, check out the kernel repo if you finish the book and want more :)
"This course studies fundamental design and implementation ideas in the engineering of operating systems. Lectures are based on a study of UNIX and research papers. Topics include virtual memory, threads, context switches, kernels, interrupts, system calls, interprocess communication, coordination, and the interaction between software and hardware. Individual laboratory assignments involve implementation of a small operating system in C, with some x86 assembly."
Lecture notes from 2012: https://ocw.mit.edu/courses/electrical-engineering-and-compu...
Video lectures from 2014: https://www.youtube.com/watch?v=kDRHsNauoxk&list=PLfciLKR3Sg...