OS/161 is a simplified operating system we use for teaching the undergraduate operating systems class. It includes a standalone kernel and a simple userland, all written in C. It runs on a machine simulator, System/161, that offers simplified but still realistic hardware devices. (Neither OS/161 nor System/161 is in any way affiliated with IBM.)

OS/161 Features

Architectural features:

• BSD-like source tree and kernel build environment
• Portable kernel split into machine-dependent and machine-independent portions
• Extensible device framework akin to that found in NetBSD
• VFS layer to allow multiple file system types

Code supplied:

• MIPS port (other ports under development)
• Device driver set for System/161
• Pass-through file system device to access host system’s files
• Absolutely minimal virtual memory system
• Simple skeleton file system
• One sample system call implementation
• Simple round-robin scheduler
• In-kernel threads package
• Semaphore implementation

Code left for students:

• Lock and condition-variable implementation
• Process support and system call layer
• More interesting schedulers
• Virtual memory system
• Full-featured file system
• Network stack and any/all network-related code
• Other more advanced features (see our Assignment 5 for some examples/suggestions).

The source code is in the SVN repository, we implemented three assignments in about 251 revisions. I am still writing the system calls in my free time. Eventually I am going to move on to the File system part of the code, implement a better file system. From this I’m hoping to be able to contribute to the linux kernel in the future.