MacOS X is BSD with the low-level part of the kernel replaced by the Mach microkernel. Another way to say this is that the high-level parts of the BSD kernel go

MacOS X is BSD with the low-level part of the kernel replaced by the Mach microkernel. Another way to say this is that the high-level parts of the BSD kernel got stuffed into the Mach microkernel. This is a nasty horrid way to design an OS. Microkernels add lots of pointless overhead while making the code hard to clean up.