I don't think it's accurate to call it a "BSD coat of paint." Practically an entire BSD kernel is run as a single "server" in Mach. In Apple's open source rel

I don't think it's accurate to call it a "BSD coat of paint." Practically an entire BSD kernel is run as a single "server" in Mach. In Apple's open source release, a majority of the XNU kernel code lies in the "bsd" subdirectory -- Apple's heavily patched 4.3BSD / 4.4BSD / FreeBSD hybrid. Originally the idea was that having most of a BSD kernel running inside Mach would make it easy for researchers to run tools on their research kernel. One suspects NeXT had a simpler motive: They just wanted to avoid paying money to AT&T. I'm not sure they had any particular interest in Mach itself. (Although obviously Mach's strange multi-architecture binary format paid off for them big time, some years later!)