BUAA MOS Operating System, with Challenge Swap

Not all branches are present, for these are local files while other codes were developed on the course's platform.

A mini operating system provided by BUAA-OS and finished as the course's project.

All exercises (except for lab1-extra) have been finished, with the implementation of "supporting memory swapping" in branch "challenge-swap".

Note the kern/swap.c and kern/sdisk.c are deprecated, as they've now been merged into kern/pmap.c for requirement of the function test.