After a bit of googling I found this:<p><i>The caller ensures that the stack is 16-byte aligned at the point of the function call.<p>“At the point of the function call” means just before the CALL instruction is executed, the top of the stack. No further explanation is given, but an inference can be made: the Darwin kernel is using SIMD instructions to quickly move data on the stack, possibly during context switches.</i><p><a href="http://pages.cs.wisc.edu/~weinrich/papers/method_dispatch.pdf" rel="nofollow">http://pages.cs.wisc.edu/~weinrich/papers/method_dispatch.pd...</a>