[Phrodo_00]

I thought it was a mixture of OSFMK and BSD, which is why I said it can act like a microkernel. Don't know more than what wikipedia says, though.

[rayiner]

Yes, it is, but the Mach layer and BSD layer were fused into a monolithic kernel before NeXT Step turned into OS X. In OS X, HFS (and presumably APFS) are in the BSD layer.