Russ Cox is one of the persons behind Xv6 (https://pdos.csail.mit.edu/6.828/2011/xv6.html https://pdos.csail.mit.edu/6.828/2011/xv6.html), an x86 port of Versio

Russ Cox is one of the persons behind Xv6 (https://pdos.csail.mit.edu/6.828/2011/xv6.html https://pdos.csail.mit.edu/6.828/2011/xv6.html), an x86 port of Version 6 Unix, which is the version of Unix described in the famous Lion’s book (https://en.wikipedia.org/wiki/A_Commentary_on_the_UNIX_Operating_System https://en.wikipedia.org/wiki/A_Commentary_on_the_UNIX_Opera...). Xv6 was prepared for MIT’s 6.828, an undergraduate operating systems course. In this syllabus from 2018 (https://pdos.csail.mit.edu/6.828/2018/overview.html https://pdos.csail.mit.edu/6.828/2018/overview.html), students are expected to write significant subsystems in Xv6. I think many good computer science programs offer a similar course where students make modifications to an open source Unix-like OS like Xv6, Minix, Linux, or one of the BSDs. I took such a course at Cal Poly San Luis Obispo in 2008. However, I don’t know of any undergraduate programs where students are required to code an entire OS from the boot loader all the way up to basic drivers. Building an OS is part of the final chapter of From NAND to Tetris, so perhaps there might be some universities out there that do require their students to write an OS from scratch as part of the larger curriculum.