Reading John Gruber’s review of the iPhone 5S and 5C, I found this very interesting part that I hadn’t realised before:

ARMv8, the architecture upon which Apple’s new A7 is designed, is a clean break. ARM’s previous instruction set dated back 20 years. ARM has always been designed for low power consumption, but 20 years ago is forever in this industry. Rather than simply add 64-bit instructions to the old ISA, ARMv8 is a clean break designed for today’s — and the future — world.

[…]

[…]the biggest reason for the performance and power consumption improvements going from the A6 to A7 is the switch from the ARMv7 to ARMv8 architectures, not 32- to 64-bit. ARMv8’s improved instruction set alone has resulted in 15-20 percent performance gains while simultaneously using less power, from what I’ve been told by informed sources. And though Apple could have gone to ARMv8 while remaining 32-bit only, it made no sense not to go 64-bit.

I was wondering where was the need for 64-bit on smartphones, but that sounds really cool.