[gchadwick]

> Can you give a reference for that? As far as I know, Apple has an architectural license that allows them to ship anything that passes a test suite.

I can't find one actually, so perhaps I am wrong. If it strictly based upon the test suite then maybe it's sufficiently thorough that interesting alterations are effectively impossible (e.g. it could test you get invalid instruction exceptions when you feed in unused bits of instruction encoding space, preventing you from adding any).