You may know that the MINIX operating system, the inspiration for Linux, was created for teaching purposes (namely, Andrew Tanenbaum’s “Operating Systems” textbook). In the same vein, Xv6 is a newer teaching OS from MIT <https://pdos.csail.mit.edu/6.828/2022/xv6.html>, for use in an undergraduate operating systems course. Originally it was implemented on x86, but now it has moved to RISC-V. The inspiration for Xv6 was the old 6th Edition Unix from Bell Labs from 1975, back when it only ran on DEC PDP-11 processors. This OS version was the subject of the famous “Lions book” <https://en.wikipedia.org/wiki/A_Commentary_on_the_UNIX_Operating_System>, circulation of which was supposed to be restricted to those with licences to the Unix source code. And then with the 7th Edition, AT&T even forbade classroom use of this source code. Nevertheless, the book still spread via, shall we say, samizdat ... The Xv6 GitHub area is here <https://github.com/mit-pdos>.