To be clear, sure, that's surely the case. The REX encoding added an extra bit to the register field used by the movcr instructions, giving you 16. So... migh

To be clear, sure, that's surely the case. The REX encoding added an extra bit to the register field used by the movcr instructions, giving you 16. So... might as well. Nonetheless the "official" means for adding state to an x86 is an MSR. And in particular there's no good reason for putting this in legacy spaces like CRs: CR8 is an interrupt control register, unrelated to the MMU control in CR0/2/3. (In fact the proof of this is that within the decade, x86 interrupt handling needed to evolve in the direction of MSI's and x2apic, something that even the newly-expanded CR registers were completely inadequate to do. So we do that stuff in MSRs too.)