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

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).