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A generic desktop CPU maps a fairly standard set of generic abstracted peripherals to it's address space, but if you're really focused on your specific embedded system needs (as Apple has plenty of capability to address), you could use that extra space to map specific system peripherals. More than 4G worth of space? I'm not sure. In general though, the 64bit address space would make for fewer constraints as you consider mapping in peripherals. Maybe you assign different sections of address space even if you manufacture a SoC with different mixes of those peripherals actually present on the die - but you won't have rearchitecting the chip as much when you go between mixes.

The other thought I have is you might want the option of pushing iPad memory out past 4G within the lifetime of the A7 (or soon after in a follow on A8 and Apple wants to spread the technical pain of the transition a bit).

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