[dfox]

Mac OS/XNU is in fact derivate of DEC's OSF/1 (later called Tru64 Unix). It has very weird hybrid design where essentially anything that would be in monolitic kernel runs as one big Mach process.

Edit: it is somewhat ironic that Alpha's memory protection model is designed such way that the natural way to implement any OS would be to write your own microkernel as OS-specific PALcode (something between firmware and microcode, written in extended Alpha ISA and the only thing that the CPU hardware sees as privileged code), but none of the Alpha OSes is implemented this way. In OSF/1 you thus get limited microkerne-ish thing that runs two process-ish things, one of which is Mach kernel and the other currently running Mach task, which in turn is either the essentially monolithic Unix kernel or Unix userspace process.

[TickleSteve]

Sorry, but MacOS/OSX is a deriviative of NeXTStep, not Tru64.

There is no relationship to Tru64 except that HP did also support OpenSTEP at one point.

[lukeh]

From memory, they did swap out Mach from the version 2.5 used in NS/OS for version 3 from MkLinux (which was from OSF). This was at the time of the Rhapsody to OS X transition. My memory is hazy on the BSD kernel mode component history.