I just scanned the Xv6 book [0] and it seems the book's exercises aren't really to build an OS from scratch, but modify an existing bare-bones OS.

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...