Shozo TAKEOKA had described to work RetroBSD on PIC32 1chip micro computer in 14th JNUG BOF.
The board is Chipkit MAX32 with SD card socket. He bought it 5,900 JPY. More detail is http://ameblo.jp/takeoka/entry-11308169137.html (Japanese).
RetroBSD is based on 2.2 BSD (!), and work without MMU. So it can work only one process in their ram, other processes are in swap space.
It was really funny presentation.