The iPhone 5's A6 SoC: Not A15 or A9, a Custom Apple Core Instead
by Anand Lal Shimpi on September 15, 2012 2:15 PM EST- Posted in
- Smartphones
- Apple
- Mobile
- SoCs
- iPhone 5
When Apple announced the iPhone 5, Phil Schiller officially announced what had leaked several days earlier: the phone is powered by Apple's new A6 SoC.
As always, Apple didn't announce clock speeds, CPU microarchitecture, memory bandwidth or GPU details. It did however give us an indication of expected CPU performance:
Prior to the announcement we speculated the iPhone 5's SoC would simply be a higher clocked version of the 32nm A5r2 used in the iPad 2,4. After all, Apple seems to like saving major architecture shifts for the iPad.
However, just prior to the announcement I received some information pointing to a move away from the ARM Cortex A9 used in the A5. Given Apple's reliance on fully licensed ARM cores in the past, the expected performance gains and unpublishable information that started all of this I concluded Apple's A6 SoC likely featured two ARM Cortex A15 cores.
It turns out I was wrong. But pleasantly surprised.
The A6 is the first Apple SoC to use its own ARMv7 based processor design. The CPU core(s) aren't based on a vanilla A9 or A15 design from ARM IP, but instead are something of Apple's own creation.
Hints in Xcode 4.5
The iPhone 5 will ship with and only run iOS 6.0. To coincide with the launch of iOS 6.0, Apple has seeded developers with a newer version of its development tools. Xcode 4.5 makes two major changes: it drops support for the ARMv6 ISA (used by the ARM11 core in the iPhone 2G and iPhone 3G), keeps support for ARMv7 (used by modern ARM cores) and it adds support for a new architecture target designed to support the new A6 SoC: armv7s.
What's the main difference between the armv7 and armv7s architecture targets for the LLVM C compiler? The presence of VFPv4 support. The armv7s target supports it, the v7 target doesn't. Why does this matter?
Only the Cortex A5, A7 and A15 support the VFPv4 extensions to the ARMv7-A ISA. The Cortex A8 and A9 top out at VFPv3. If you want to get really specific, the Cortex A5 and A7 implement a 16 register VFPv4 FPU, while the A15 features a 32 register implementation. The point is, if your architecture supports VFPv4 then it isn't a Cortex A8 or A9.
It's pretty easy to dismiss the A5 and A7 as neither of those architectures is significantly faster than the Cortex A9 used in Apple's A5. The obvious conclusion then is Apple implemented a pair of A15s in its A6 SoC.
For unpublishable reasons, I knew the A6 SoC wasn't based on ARM's Cortex A9, but I immediately assumed that the only other option was the Cortex A15. I foolishly cast aside the other major possibility: an Apple developed ARMv7 processor core.
Balancing Battery Life and Performance
There are two types of ARM licensees: those who license a specific processor core (e.g. Cortex A8, A9, A15), and those who license an ARM instruction set architecture for custom implementation (e.g. ARMv7 ISA). For a long time it's been known that Apple has both types of licenses. Qualcomm is in a similar situation; it licenses individual ARM cores for use in some SoCs (e.g. the MSM8x25/Snapdragon S4 Play uses ARM Cortex A5s) as well as licenses the ARM instruction set for use by its own processors (e.g. Scorpion/Krait implement in the ARMv7 ISA).
For a while now I'd heard that Apple was working on its own ARM based CPU core, but last I heard Apple was having issues making it work. I assumed that it was too early for Apple's own design to be ready. It turns out that it's not. Based on a lot of digging over the past couple of days, and conversations with the right people, I've confirmed that Apple's A6 SoC is based on Apple's own ARM based CPU core and not the Cortex A15.
Implementing VFPv4 tells us that this isn't simply another Cortex A9 design targeted at higher clocks. If I had to guess, I would assume Apple did something similar to Qualcomm this generation: go wider without going substantially deeper. Remember Qualcomm moved from a dual-issue mostly in-order architecture to a three-wide out-of-order machine with Krait. ARM went from two-wide OoO to three-wide OoO but in the process also heavily pursued clock speed by dramatically increasing the depth of the machine.
The deeper machine plus much wider front end and execution engines drives both power and performance up. Rumor has it that the original design goal for ARM's Cortex A15 was servers, and it's only through big.LITTLE (or other clever techniques) that the A15 would be suitable for smartphones. Given Apple's intense focus on power consumption, skipping the A15 would make sense but performance still had to improve.
Why not just run the Cortex A9 cores from Apple's A5 at higher frequencies? It's tempting, after all that's what many others have done in the space, but sub-optimal from a design perspective. As we learned during the Pentium 4 days, simply relying on frequency scaling to deliver generational performance improvements results in reduced power efficiency over the long run.
To push frequency you have to push voltage, which has an exponential impact on power consumption. Running your cores as close as possible to their minimum voltage is ideal for battery life. The right approach to scaling CPU performance is a combination of increasing architectural efficiency (instructions executed per clock goes up), multithreading and conservative frequency scaling. Remember that in 2005 Intel hit 3.73GHz with the Pentium Extreme Edition. Seven years later Intel's fastest client CPU only runs at 3.5GHz (3.9GHz with turbo) but has four times the cores and up to 3x the single threaded performance. Architecture, not just frequency, must improve over time.
At its keynote, Apple promised longer battery life and 2x better CPU performance. It's clear that the A6 moved to 32nm but it's impossible to extract 2x better performance from the same CPU architecture while improving battery life over only a single process node shrink.
Despite all of this, had it not been for some external confirmation, I would've probably settled on a pair of higher clocked A9s as the likely option for the A6. In fact, higher clocked A9s was what we originally claimed would be in the iPhone 5 in our NFC post.
I should probably give Apple's CPU team more credit in the future.
The bad news is I have no details on the design of Apple's custom core. Despite Apple's willingness to spend on die area, I believe an A15/Krait class CPU core is a likely target. Slightly wider front end, more execution resources, more flexible OoO execution engine, deeper buffers, bigger windows, etc... Support for VFPv4 guarantees a bigger core size than the Cortex A9, it only makes sense that Apple would push the envelope everywhere else as well. I'm particularly interested in frequency targets and whether there's any clever dynamic clock work happening. Someone needs to run Geekbench on an iPhone 5 pronto.
I also have no indication how many cores there are. I am assuming two but Apple was careful not to report core count (as it has in the past). We'll get more details as we get our hands on devices in a week. I'm really interested to see what happens once Chipworks and UBM go to town on the A6.
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UpSpin - Sunday, September 16, 2012 - link
If you own a second device, the device will have a microUSB port most probably. If you visit a friend and forgot your cable, it's very likely that he has a microUSB cable because it's the standard, unlike the lightning cable.How do you intend to benchmark the cable? Transfer speed to your computer? Well, you have to connect it to a USB port on your computer, so you get the USB limitations. What needs more bandwith? Video link, HDMI. MHL is supported via USB which supports 1080p, maybe even Apple uses MHL in their 'lightning' connector. So where exactly should it be faster? It's a marketing name, just as retina, for people like you, who blindly believe what Apple tells them.
doobydoo - Sunday, September 16, 2012 - link
'If you own a second device, the device will have a microUSB port most probably'Um, no? There are thousands of devices which don't. And why does that matter anyway? I get a lightning cable,
'If you visit a friend and forgot your cable, it's very likely that he has a microUSB cable because it's the standard, unlike the lightning cable.'
Given how many iPhones sell, I doubt this anyway - but it's irrelevant. An iPhone has far longer battery life than say, the Samsung Galaxy S3 - so charging is less of an issue. I can't say I've EVER been caught at a friends without battery life and no-one there had an iPhone charger - it just doesn't happen.
And no - uses of the adapter aren't limited to just connecting to the PC - that's a primitive uneducated response. There could be all kinds of after market accessories which utilise a potentially faster connection - or advances in USB technology which surpass micro USB.
doobydoo - Sunday, September 16, 2012 - link
And we haven't even considered power requirements, size & space requirements, CPU utilisation etc.All in all you're just posting uneducated claims about a connector you know nothing about.
doobydoo - Monday, September 17, 2012 - link
Another factor is how robust the connection is.I believe MicroUSB can be quite brittle when attached to a long device.
Which MicroUSB variant are you pushing anyway, there's the 3.0 version which is ugly and has an extra piece on the side, or there's the 2.0 version which is as slow as USB 2.0 - can't even do 3.0 speeds.
repoman27 - Sunday, September 16, 2012 - link
USB 2.0 has one pair for signaling and one for voltage. USB 3.0 retains those pairs and adds 2 more signaling pairs for SuperSpeed USB. None of these can be repurposed within the confines of the standard.Lighting is a general purpose connector which provides 8 pairs. This allows for much greater flexibility as Apple can now accommodate digital audio in/out, USB 2.0, USB 3.0, DVI, HDMI, DisplayPort, and SDIO, or a combination thereof, over a single connector simply by using different cables. Other legacy connections such as analog audio and video are possible through the use of active adapters containing DACs.
By the end of the month there will be over 10 million Lightning cables in the wild, so I don't imagine they'll be too hard to come by in the near future. Arguably, until not that long ago, chargers with Apple's 30-pin dock connector were easier to come by than those sporting Micro USB.
The iPhones sold in the EU will be required to ship with a Lightning to Micro USB adapter anyway, so they will also be available for those who choose to buy them stateside. I currently have a Micro USB charger in my car with a 30-pin dock connector to Micro USB adapter so I can accommodate most folks who happen to be riding with. I picked up an official Apple model for $7 on Amazon, and I don't imagine the case will be much different for the Lightning version before too long.
Death666Angel - Sunday, September 16, 2012 - link
There is MHL for your digital connection needs. I have a camera that has a micro-USB port which has some added height for a special cable that gets me analog video and audio out. My old Touch Pro 2 had a modified micro-USB plug that was also used for the ear plugs. I'm pretty sure they could have gotten all the wanted from a micro-USB port with some modification. Minus the symmetry. They just don't care. Which is one of the reasons I don't like Apple.repoman27 - Sunday, September 16, 2012 - link
No, they care a lot. Lightning has the potential to be far more flexible than simply providing A/V support. MHL is connector agnostic, so technically Lightning could qualify as an MHL connector.And as you said, "I have a camera that has a micro-USB port which has some added height for a special cable..." The whole point of Lightning is to make a multi-purpose connector that needs no additional increase in dimension to support functionality beyond USB sync / charge.
Fx1 - Sunday, September 16, 2012 - link
Apple screwed the pooch on this one nicely for Samsung. Lets hope Samsung puts DDR3 and A15 Exynox in the next superphone. At this rate ill never need a HTPC i just use my phone!darkcrayon - Sunday, September 16, 2012 - link
Yes, by designing and implementing their own SoC and CPU cores with double the performance as the last, with better battery life, they really screwed up....
What?
Fx1 - Sunday, September 16, 2012 - link
Wont be as powerful as the Exynos 5 and Samsung will have a much more powerful and open Smartphone from which to do things that Apple cant do and wont let you do. Win/Win