chipguy, many years. This is a strategic architecture which removes segmentation from x86 in two stages, an OS stage and an application stage. AMD64 is not a tactical design for a single generation of chips but rather something which is meant to grow and distance itself from legacy x86 over time.
Maybe that is why Microsoft insisted on this as the only 64-bit version of x86. It is a design meant to have a long future, and it simplifies the software requirements over time.
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