[Wonder Warthog]

"MacOS X is built on FreeBSD."

I thought MacOS was built around the Mach kernel???

[AnotherUnixGeek]

I thought MacOS was built around the Mach kernel???

From the XNU Wiki entry, the MacOS X XNU kernel is built around both Mach and FreeBSD, with BSD providing the system calls, IPC, the network stack, NFS, and the Unix process model (layered on top of Mach tasks). An applications programmer accustomed to working with 4.3 BSD or SunOS 4.1 would probably feel at home writing application code for XNU.