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Why we don't have 128-bit CPUs
(www.xda-developers.com)
This is a most excellent place for technology news and articles.
I expect the engineers are telling the marketing people "No! You can't do that. You'll scare everyone that it's incompatible."
32bits is compatible with 64bits, why wouldn't 128 bits be too?
64bit cut out 16bit compatibility. So I'm guessing the fear is that 128 would cut 32.