The book isn’t free, but I’ve found it to be fun to work through. The projects are centered around building and running on small boards like beaglebone, Galileo, and more recently raspi (although this isn’t covered in the book.)
When looking at the Xinu book index, it's not clear to me if the Xinu book is different. Does the Xinu book show how to write an OS from scratch or is the book also about making small changes to an existing OS?
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[0]: https://pdos.csail.mit.edu/6.828/2021/xv6/book-riscv-rev2.pd...
I've taken the class at Purdue. The way the class works it was divided into 4 "projects" where you had to add features to Xinu. In my semester those were: a more advanced scheduler, locks, process permissions, and a file-system.