Not Another Completely Heuristic Op

Not Another Completely Heuristic Operating System, or Nachos, is instructional software for teaching undergraduate, and potentially graduate level operating systems courses. It was developed at the University of California, Berkeley, designed by Thomas Anderson, and is used by numerous schools including the University of Southern California, Indian Institute of Technology Kanpur, University of California, Irvine and Aristotle University of Thessaloniki.

Originally written in C++ for MIPS, Nachos runs as a user-process on a host operating system. A MIPS simulator executes the code for any user programs running on top of the Nachos operating system. Ports of the Nachos code exist for a variety of architectures.

In addition to the Nachos code, a number of assignments are provided with the Nachos system. The goal of Nachos is to introduce students to concepts in operating system design and implementation by requiring them to implement significant pieces of functionality within the Nachos system.

In Nachos' case Operating System simulator simply means, that you can run an OS (a guest OS) on top of another one (the host OS). If this sounds a little like Bochs/VMWare, then you're right, because it works similar to them. It actually features emulation for

a CPU (a MIPS CPU)
a hard drive
misc. stuff like an interrupt controller, timer,...
which are there to run the Nachos[1] user space applications. That means that you can actually write programs for Nachos, compile them with a real compiler (an old gcc compiler[2] that produces code for MIPS) and run them. The Nachos kernel instead is compiled to the platform of the Host OS and thus runs natively on the Host OS' CPU.

Nachos version 3.4 has been the stable, commonly used version of Nachos for many years. Nachos version 4.0 has existed as a beta since approximately 1996.