Xinu

XINU Is Not UNIX. A simple operating system. Often used to teach students about OSes, since its source code is freely available, and is fairly simple to understand. It features:

• Concurrent Processing
• Message Passing
• Ports
• Semaphores
• Memory Management
• Buffer Pools
• Uniform Device I/O
• Shell
• Tcl
• TCP/IP

Xinu was designed by Professor Douglas Comer, who wrote 'The Xinu Approach' book in 1979. It's been ported to the PC, Mac, DECSation3100, VAX, and Sun/Sparc stations. Xinu has been used as an embedded system in various products and has a full TCP/IP stack. The first version (for the PDP-11) supported I/O and arbitrary processes.