[monocasa]

Oh, sure, my bad.

OK, in the context of 'why can't you cleanly have the kernel in a different address space from user processes on x86', the same reasons apply. It's a chicken/egg thing, as a syscall instruction executes and touches the kernel stack before you have a chance to change mmu mappings.

There are versions of Darwin for x86 (but no released versions of full OSX AFAIK) that separate the address spaces, but they reserve a (albeit much smaller) piece of virtual address space at the top for the kernel in all address spaces in order to facilitate the transition to the full kernel address space.

[bogomipz]

Right that would be a pretty awful place to segfault :)

Thanks for the clarification.