MacOS XNU is based on OSFMK which was also used by Tru64 and few others, and sometimes called "Mach 3.5" IIRC. One of the major differences was that it embedded

MacOS XNU is based on OSFMK which was also used by Tru64 and few others, and sometimes called "Mach 3.5" IIRC. One of the major differences was that it embedded the BSD server into kernel space.