Here is a link (http://retrobsd.org/wiki/doku.php) to a true port of the Unix (BSD) OS to the PIC32 (MIPS)!
This code will work on a humble MIPS M4K core that you can find inside the PIC32MX series devices. It will work with 128K of RAM which means we are talking PIC32MX7 at least.
Lots of common hardware development boards (Explorer 16, Chipkit…) supported too..
