Operating system written in C++ for educational purposes when I was a student.
mainline and atomic are actually branches (imported from SVN). Mainline was initial attempt which ended up with minimalistic more or less traditional OS with functional user space. Its key feature was transparent transition from user space to kernel mode just by calling virtual methods of specially crafted classes. This was achieved by some hacking and exploiting compiler specifics for compiled C++ code (looks ugly for me now, but these days I wanted some cool way for making syscalls without traditional syscalls declaration). atomic was attepmpt for full rewrite with new idea to make orthogonally persistent OS. However it was developped later in other repository (under PhoenixOS name) and probably do not have any useful changes in this repository. It did not achieve same completion state as mainline but there are some good example, e.g. fully completed EFI loader implementation.
Both projects do not pretend to be used or even compiled nowadays, but might have historical value and have some useful code snippets.