Apple has unveiled today the iPhone 5S which features a new A7 chip which is the first 64-bit chip in a smartphone… ever. With the new chip, Apple has optimized iOS 7 and all the stock apps to 64-bit. In order for users to take full advantage of the new chip, developers also need to update their apps for 64-bit. Xcode can build an app with both 32-bit and 64-bit binaries included. This combined binary requires a minimum deployment target of iOS 6 or later. The 64-bit binary runs only on iOS 7 or later.
The only problem is that, if developers fail to update their apps, and you download an app that runs 32 bit on your iPhone 5S, you slow down the entire operating system! The below paragraph is taken form a NDA developer doc titled 64-Bit Transition Guide for Cocoa Touch.
When iOS is executing on a 64-bit device, iOS includes separate 32-bit and 64-bit versions of the system frameworks. When all apps running on the device are compiled for the 64-bit runtime, iOS never loads the 32-bit versions of those libraries, which means that the system uses less memory and launches apps more quickly. Because all of the built-in apps already support the 64-bit runtime, it is to everyone’s benefit that all apps running on 64-bit devices be compiled for the 64-bit runtime, especially apps that support background processing. Even apps that are not performance sensitive gain from this memory efficiency.
Bottom line, it may take a while until you can actually feel the power of the iPhone 5S. But hey, you’ll still feel like you have a new iPhone 5. New is exciting, right?