And yes, it is pretty bizarre when you think about it. Seems like it would be easier to just not support it in the BIOS.
That would be an invitation to wholesale remarking, something AMD finally shut down. A remarked AXP -> A64 processor with a BIOS patch diskette - it would be a very popular product in some places.
Secondary benefit of K8 AthlonXP: There may be dies which fail in Long Mode but pass every other test.
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