Apple’s iPhone 5s, the A7 Chip, and That 64-Bit Question

When Apple unveiled the iPhone 5s on Tuesday, the company touted as one of its tentpole features the 64-bit desktop-class processing power of its new custom-made A7 chip. “The A7 is up to twice as fast as the previous-generation system at CPU tasks,” Apple marketing chief Phil Schiller said. “This is the first-ever 64-bit processor in a phone of any kind. I don’t think the other guys are even talking about it yet.”

According to Schiller, the A7 is “up to twice as fast” in raw processing power and graphics performance as its predecessor, the A6. And when paired with Apple’s forthcoming iOS 7 operating system — which has been designed with native 64-bit kernel, libraries and drivers — it provides unparalleled performance. “The benefits are huge,” Schiller said. “This is a huge leap forward.”

And there’s no question that the benefits of 64-bit are huge. What’s less clear is how evident they’ll be in the iPhone 5s. Because, as innumerable observers have pointed out this week, in order to tap into the biggest performance gains offered by a 64-bit chip, you need a smartphone with more than four gigabytes of RAM. And, while Apple hasn’t said how much RAM it has built into the 5s, it’s highly unlikely that it’s enough to meet that requirement.

That has led some to wonder whether this move to 64-bit is a long-term play whose only real short-term benefit is marketing — the ability to tout the 5s as the first 64-bit smartphone ever, even though it doesn’t have the internals to fully take advantage of that 64-bit A7 chip.

But chip experts say that there are some gains to be had even from a 64-bit chip without 4GB of addressable memory.

For one thing, 64-bit integer math will allow the A7 to execute operations much faster than the 32-bit A6. “The fact that the A7 has twice as many processor registers means that more operations can occur without the processor using main memory, which is slower to access,” Carl Howe, VP of research and data sciences at the Yankee Group told AllThingsD. “This means that for some codes, the A7 will be twice as fast (or faster depending on how many memory accesses the original code had) to run code because the processor doesn’t have to use main memory as much.”

But for most, the gains found here will be marginal, said Moor Insights analyst Paul Teich. “Double the register file adds a few percent to performance,” he said. “It’s a deep compiler and runtime VM issue. … So marginal improvements for most apps, at best. Depending on how code is written it can run a little longer without hitting main memory, but it really depends on whether you are writing a computationally intensive app or not.”

Another benefit: ARMv8, the architecture on which the A7 is likely based, has a very efficient instruction set that’s great for resource-intensive applications. As Kevin Krewell, senior analyst at the Linley Group and a senior editor of Microprocessor Report, told AllThingsD, “The ARMv8 instruction set is a clean-slate approach with many improvements. Even without 4GB of RAM, the A7 should make it easier to build larger applications like PC-class games and programs. Apps can now become real desktop-class programs and games.”

The caveat here is that Apple must maintain backward compatibility with legacy 32-bit apps until the 64-bit ecosystem really kicks in. Said Krewell, “The vast majority of programs will still use the 32-bit mode, so the benefits may be slow to come.”

What about immediate improvements to battery life? Teich thinks that’s unlikely. “The only way battery life gets better with greater than 32-bit address spaces is to prevent swapping apps from physical memory to either SSD or HDD and then back again,” he said. “If you move from 4GB of physical system memory to 6GB, that extra 2GB is always on when the system is on. All things being equal, you burn 50 percent more power for memory. You only see a return on that if you use apps that push the boundaries of your physical memory space.”

Finally, with the 64-bit A7, Apple has made it possible for developers to take the 64-bit apps they’ve written for the Mac and bring them to iOS 7 with relative ease. And that is a huge benefit, indeed.

“Because Apple makes the development environment and has updated those tools for 64-bit architectures, a developer only really needs to recompile their application to make it 64-bit compatible — assuming they haven’t done anything non-standard with their code,” said Howe. “This will not be true with Android, by the way. The Android Java app and native app environment will need support from Oracle, who owns the Java environment, as well as 64-bit support from the Android kernel. Android has a lot more moving pieces to coordinate, and will take longer to go to 64-bit.”

So, while this early transition to 64-bit is most certainly a long-term play for Apple, it’s not without some immediate and important short-term benefits — even without more than 4GB of memory.

This post was updated to include new information from Moor Insights analyst Paul Teich.