Nachos is instructional software for teaching undergraduate, and potentially
graduate, level operating systems courses.  The Nachos distribution
comes with: 

   simple baseline code for a working operating system
   a simulator for a generic personal computer/workstation

The assignments illustrate and explore all areas of modern operating
systems, including threads and concurrency, multiprogramming, 
system calls, virtual memory, software-loaded TLB's, file systems, 
network protocols, remote procedure call, and distributed systems.

To get started, you should:
  1. lpr nachos.ps  -- print out the paper describing nachos
  2. cd code; make print -- print out the nachos source code
  3. cd code/c++example; lpr *.ps *.h *.cc -- print out C++ primer
  4. cd code; gmake all -- compile nachos source code

The basic Nachos system was written for a MIPS workstation.  It has
been ported to other platforms, but as of now, there are a few gotchas.
The Nachos kernel and machine simulator run directly on the host machine,
but user-level programs running on top of Nachos are simulated instruction-by-
instruction.  The simulator assumes MIPS object code, in little endian format.
It would take much more work to complete the port and change the CPU 
simulator to simulate other instruction sets (although this is under
investigation).  Keeping the MIPS CPU causes a one problem:

The Nachos kernel runs runs native mode while the user programs
runs on the simulated CPU.  This is a little weird on the non-MIPS workstations
because the user programs are using little endian (typically) and the kernel is
using big endian.  Some information (such as the argv[] array) that
is passed between the kernel and the user though user memory must be
byte swapped.  (Unfortunately, this isn't as easy to fix as simply
cross-compiling to the SGI MIPS, which is big endian; in a few places,
the simulation assumes little endian format.  We're working on fixing this.)