DARBES, I'd like to emphasize that the slightly different versions concept is a bit of a myth. See my earlier answer to Semi on this topic. AMD64 is compatible from a customer standpoint (applications and performance) and an application writer's standpoint, but not from the compiler writer's standpoint. 64-bit mode is a whole new design running alongside the old compatibility mode, requiring compilers to be fully re-written for that mode. It does share certain CISC family resemblences to x86-32, but there the similarities end. It contains new opcodes, new registers and new instructions.
64-bit mode is a much cleaner break with the past than generally recognized.
Real world evidence for this can be seen by the amount of steady improvement we are seeing with each new update of compilers released for AMD64. If it were only 'slightly different' then we would have had optimized compilers from the start.
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