There’s a lot of speculation regarding Apple’s reasoning for using a 64-bit ARM processor in the iPhone 5s. Aside from the obvious fact that the 64-bit processor will allow Apple to include more RAM in future devices, there are all sorts of rumors ranging from easier cross-platform development all the way to an ARM-powered MacBook Air.
Everyone seems to be missing the simplest reason for the early move to 64-bit: the iPhone 5s’s “motion co-processor.” There isn’t any hard data available on how this co-processor works yet, but it sure sounds a lot like the NVidia Tegra’s power-saving “companion core.” Adding something like that to a processor is a pretty significant architectural change.
Apple isn’t likely to make a change this drastic to their ARM CPU for many years. Wouldn’t it make sense to also move to a 64-bit architecture at the same time? That way, they will be prepared to break through the 4-GB RAM barrier without any hassle.
Apple has been using the 32-bit ARMv7 instruction set since 2009. They most likely want to make sure they can use this shiny new 64-bit ARMv8 design for at least four years as well. I bet their road map has them hitting 4-GB of RAM within that time frame.