MacOSX's kernel? you mean darwin? described by apple as follows
"This foundation is a core operating system commonly known as Darwin. Darwin integrates a number of technologies, most importantly Mach 3.0, operating-system services based on 4.4BSD (Berkeley Software Distribution)"
http://developer.apple.com/darwin/
"This foundation is a core operating system commonly known as Darwin. Darwin integrates a number of technologies, most importantly Mach 3.0, operating-system services based on 4.4BSD (Berkeley Software Distribution)"
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The operative words there are *based on*. Mach is driven by a microkernel whereas BSD's core is monolithic.
They are very different in that respect.