All chips now out are based on 32 bit architecture, no surprises there,.
64 bit is an extension of that architecture, no surprises there either.
You enhance the memory architecture to get it to do that and the chip is backwards compatible with 32 bit programs.
To put it another way, you can run 32 bit programs on a cpu that can handle 64 bit processing.
So XP runs just fine.
You cannot, however, run 64 bit programs on a 32 bit processor.
So, even if you wanted to, you cannot run Vista 64 on that XP Athlon.
Quote:
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64-bit computer architecture provides higher performance than 32-bit architecture by handling twice as many bits of information in the same clock cycle.
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Think of it as DDR for processors.
edit : The point is that it's a hardware change.