[anta40]

xv6 is still my favourite:

- The codebase is very small. Study the PDF references a few days, and eventually you'll start to understand how things work.

- No convoluted build system. Just "make xv6.img". Done.

- No need to build a custom compiler from source. On Linux systems: GCC supplied by your favourite package manager is sufficient. On Mac: install i386-elf-toolchain via Homebrew.

- Plenty of folks customizing it, like adding their own syscalls or a custom scheduler or a file system integrity check (Github is your friend).