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 is used for teaching; the OS/161 instructional OS runs on it.
However, it has also proven useful as a rapid development platform for
bringing up research or experimental kernels.

System/161 supports fully transparent debugging, via remote gdb into
the simulator. It also provides transparent kernel profiling,
statistical monitoring, event tracing (down to the level of individual
machine instructions) and one can connect multiple running System/161
instances together into a network using a "hub" program.

Homepage:
http://os161.eecs.harvard.edu/#sys161
