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Quick question: why do you think AMD decided canonical addresses had to be sign-extended rather than just lead with zeroes? Wouldn't an access to 0xffff80000000

Quick question: why do you think AMD decided canonical addresses had to be sign-extended rather than just lead with zeroes? Wouldn't an access to 0xffff800000000000 also break if the address space was extended?

dmitrygr 11y ago
easier testing for whether an address belongs to userspace or kernel
- ajross 11y ago
  More "compatible" than easier really (I mean, two subtractions is hardly a heavyweight process). The trick was already in long use in shipping OSes and they didn't want to break anything.