Prescott's 64-bit performance is sometimes worse than its 32-bit when AMD's isn't in the same benchmark so this is not remotely a minor issue about degree here. AMD64 was designed with the K8 architecture in mind so nothing is a given as you are hopefully assuming although you would expect it to be ok given the time they have had to start a clean sheet x86-64 P-M and given the similiarity of P-M to PIII which isn't a bus ride away from the K7/8s like P4 is.
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