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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.
Why are we not using them in end-user devices
We are.
Addressing-wise, no we don't have consumer level 128bit CPUs and probably won't ever need them.
Instructions though, SSE had some 128bit ops (OR/XOR, MOVE) and AVX is 128bit vector math. AVX2 is 256bit vector math, AVX512 is- you guessed it 512bit vector math. AltiVec on PPC had 128bit vectors 20 years ago.
There's no benefit.