The L4/Darwin Project
"The L4/Darwin project is an experimental port of Darwin to the L4 microkernel to study the characteristics of a large-scale microkernel-based system. It includes a port of IOKit to L4, a modified libc to communicate with the Darbat Server, and of course XNU with many of the machine-dependent parts heavily modified (pmap, thread/task creation, etc.) but much left unchanged (most of BSD, and large parts of OSFMK work without modification)."