Pintos is an educational operating system for the x86. Pintos was developed for Stanford's CS 140 operating systems course as a successor to Nachos, a less realistic educational operating system.
A paper about Pintos was presented at SIGCSE 2009. Slides from the talk are available in PDF and PowerPoint 2007 formats.
Pintos source code, documentation, and suggested assignments are available in the public Git repository. Sample solutions and suggestions for grading are available in a private repository. For access to the private repository, if you are teaching with Pintos, is available by sending an SSH public key and requested username to Ben Pfaff.