OSX is commonly seen as the graphical shell on top of BSD. Lately, it seems as though Apple is adding more and more to 'core' OS functionality (sandboxing, versioning). Is there a steady progression to an eventually all-proprietary kernel, or is the microkernel approach flexible enough to be persisted?
closed as not constructive by Daniel Beck♦, slhck, Nifle, studiohack♦ Aug 1 '11 at 1:05
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The Wikipedia article on XNU answers much of this question.
So OS X's graphical shell does not sit atop a BSD kernel. The kernel is a hybrid one and I believe it is therefore not the same as any kernel in any mainstream system from FreeBSD, OpenBSD, NetBSD or any other BSD variant. Only a relatively small (but important) part of BSD is in OSX.