Initially I planned to just fork xv6 (MIT's teaching OS) to demonstrate the C compiler but after the basics were working I kept going.
The kernel itself (not including tools) is now >50% new code, including features like priority-based scheduling and a reworked filesystem with support for multiple drives.
Some new tools have been added to the repository including a new mkfs implementation, while things like libc have been factored out into their own repositories. Other simple system tools like the shell & test programs remain from xv6 with small modifications.