Thanks to author and programmer Mike Ash for this developer-focused explanation of Apple’s move to the 64-bit A7 chip in the iPhone 5s. In short, although details will vary by app, 64-bit code that takes advantage of new programming techniques can be faster and use less memory (though 64-bit apps use more memory in general). Plus, old 32-bit code can run on the A7 with no changes or performance penalty.follow link
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What 64-bit Support in iPhone 5s Really Means