If Darwin uses a Microkernel then so does Microsoft Windows. They are very similar designs. In case you are wondering: I don't think that Microsoft Windows use

If Darwin uses a Microkernel then so does Microsoft Windows. They are very similar designs. In case you are wondering: I don't think that Microsoft Windows uses a microkernel. Some versions of Mach are micro, other versions are not. OS X doesn't use a microkernel, neither does Darwin. They use XNU kernel. XNU has code from Mach and it also uses code from FreeBSD. People call it a 'hybrid kernel', but I think that is a misnomer and only really exists through the effectiveness of Apple's marketing. XNU is a monolithic kernel with message passing capabilities inspired from microkernels. Makes it so they can isolate certain features and drivers better then, say, Linux can. Unlike Apple, Microsoft actually did release a version of NT that was a true microkernel, but they quickly realized that it is not a practical approach to creating a OS that was competitive with Unix. Regardless... This seems pretty much Darwin grafted with FreeBSD. For a commercial network appliance device it's probably a good approach. They get all the benefits from using source code from Apple, FreeBSD, and ZFS without having to actually provide much source code in the way of real improvements that could be used to enhance a competitor's product.