os161 kernel course from harvard.
assignments and course information: https://www.ops-class.org/
System/161 is a machine simulator that provides a simplified but still realistic environment for OS hacking. It is a 32-bit MIPS system supporting up to 32 processors, with up to 31 hardware slots each holding a single simple device (disk, console, network, etc.) It was designed to support OS/161, with a balance of simplicity and realism chosen to make it maximally useful for teaching. However, it also has proven useful as a platform for rapid development of research kernel projects.
more info on os161 can be found at: http://os161.eecs.harvard.edu/