SoC Analysis: Apple A9X

Diving into the heart of the iPad Pro, we have Apple’s latest generation tablet SoC, A9X. Like the other Apple X series SoCs before it, A9X is by and large an enhanced and physically larger version of Apple’s latest phone SoC, taking advantage of the greater space and heat dissipation afforded by a tablet to produce a more powerful SoC.


Apple's A9X (Image Courtesy iFixit)

That Apple has developed a new SoC to launch alongside the iPad Pro is in no way surprising, but just as how the iPad Pro has ramifications for the overall iPad lineup as Apple gets into the productivity tablet market, iPad Pro’s genesis is reflected in its component selection. Apple already needed a powerful SoC for the iPad Air 2 in order to keep performance up with the tablet’s high resolution of the screen, and iPad Pro in turn pushes Apple’s performance needs even harder. Not only is there an even higher resolution screen to drive – the 2732x2048 display has about 66% of the pixels of a 4K display – but now Apple needs to deliver suitable performance for content creation and meaningful multitasking. I don’t want to imply that the A9X was somehow specifically designed from scratch for the iPad Pro, as there are a number of more important engineering considerations, but I do want to highlight how the iPad Pro is not just another iPad, and that as Apple expands the capabilities of the iPad they need to expand the performance as well if they wish to extend their reputation for smooth UX performance.

Looking at the specifications of the A9X, it seems like Apple always throws us a curveball on the X series SoCs, and for their latest SoC this is no different. With A8X Apple delivered more RAM on a wider memory bus, a larger GPU, and surprisingly, three Typhoon CPU cores. To date it’s still not clear just why Apple went with three CPU cores on A8X – was it for multitasking, or as an alternative means to boost performance – and A9X’s configuration only serves to highlight this enigma.

Apple SoC Comparison
  A9X A9 A8X A6X
CPU 2x Twister 2x Twister 3x Typhoon 2x Swift
CPU Clockspeed 2.26GHz 1.85GHz 1.5GHz 1.3GHz
GPU PVR 12 Cluster Series7XT PVR 6 Cluster Series7
(PVR GT7600)
PVR 8 Cluster Series6XT
(APL GXA6850)
PVR SGX554 MP4
RAM 4GB LPDDR4 2GB LPDDR4 2GB LPDDR3 1GB LPDDR2
Memory Bus Width 128-bit 64-bit 128-bit 128-bit
Memory Bandwidth 51.2GB/sec 25.6GB/sec 25.6GB/sec 17.1GB/sec
L2 Cache 3MB 3MB 2MB 1MB
L3 Cache None 4MB 4MB N/A
Manufacturing Process TSMC 16nm FinFET TSMC 16nm &
Samsung 14nm
TSMC 20nm Samsung 32nm

Instead of continuing with a triple-core CPU design for A9X, for their latest X series SoC Apple has dropped back down to just a pair of Twister CPU cores. The catch here – and why two cores is in many ways better than three – is that relative to A8X and A9, Apple has cranked up their CPU clockspeeds. Way, way up. Whereas the iPad Air 2 (A8X) shipped at 1.5GHz and the iPhone 6s (A9) at 1.85GHz, the A9X sees Apple push their clockspeed to 2.26GHz. Not counting the architectural changes, this is 22% higher clocked than the A9 and 51% higher than the A8X.

The fact that Apple dropped back down to 2 CPU cores is unexpected given that we don’t expect Apple to ever go backwards in such a fashion, and while we’ll never know the official reason for everything Apple does, in retrospect I’m starting to think that A8X was an anomaly and Apple didn’t really want a tri-core CPU in the first place. A8X came at a time where Apple was bound by TSMC’s 20nm process and couldn’t drive up their clockspeeds without vastly increasing power consumption, so a third core was a far more power effective option.


A9X Die Shot w/AT Annotations (Die Shot Courtesy Chipworks)

Overall this means that iPad Pro and A9X will set a very high bar for tablet CPU performance. As we’ve already seen in the iPhone 6s review, the Twister CPU core is very potent and in most cases faster than any other ARM CPU core by leaps and bounds. Cranking up the clockspeed a further 22% only serves to open up that gap even further, as Twister is now reaching clockspeeds similar to the likes of Cortex-A57 and A72, but with its much wider execution pipeline and greater IPC. This is also the reason that an Intel Core CPU comparison is so interesting, as Intel’s tablet-class Core processors in many ways are the target to beat on overall CPU performance, and we’ll be touching upon this subject in greater detail a bit later.

GPU: Imagination PowerVR 12 Cluster Series 7XT

Meanwhile on the GPU side, as expected Apple has further increased the number of clusters on their SoC to drive the higher resolution display of a tablet. Whereas A9 used a 6 cluster design (PVR GT7600), A9X doubles this, giving us a relatively massive 12 cluster design.

In Imagination’s PowerVR Series7XT roadmap, the company doesn’t have an official name for a 12 cluster configuration, as this falls between the 8 cluster GT7800 and 16 cluster GT7900. So for the moment I’m simply calling it a “PowerVR 12 cluster Series7XT design,” and with any luck Imagination will use a more fine-grained naming scheme for future generations of PowerVR graphics.

In any case, the use of a 12 cluster design is a bit surprising from an engineering standpoint since it means that Apple was willing to take the die space hit to implement additional GPU clusters, despite the impact this would have on chip yields and costs. If anything, with the larger thermal capacity and battery of the iPad Pro, I had expected Apple to use higher GPU clockspeeds (and eat the power cost) in order to save on chip costs. Instead what we’re seeing is a GPU that essentially offers twice the GPU power of A9’s GPU.

However to put all of this in context, keep in mind that iPad Pro’s display is 5.95Mpixels, versus the 2.07Mpixel screen on the iPhone 6s Plus. So although Apple has doubled the number of GPU clusters for A9X – and I suspect clocked it fairly similarly – that increased performance will be very quickly consumed by the iPad Pro’s high resolution screen. Consequently even a 12 cluster GPU design is something of a compromise; if Apple wanted to maintain the same level of GPU performance per pixel as in the iPhone 6s family, they would have needed an even more powerful GPU. Which just goes to show how demanding tablets can be.

Memory Subsystem: 128-bit LPDDR4-3200, No L3 Cache

Responsibility for feeding the beast that is A9X’s GPU falls to A9X’s 128-bit LPDDR4 memory controller configuration. With twice as many GPU clusters, Apple needs twice as much memory bandwidth to maintain the same bandwidth-to-core ratio, so like the past X-series tablet SoCs, A9X implements a 128-bit bus. For Apple this means they now have a sizable 51.2GB/sec of memory bandwidth to play with. For an SoC this is a huge amount of bandwidth, but at the same time it’s quickly going to be consumed by those 12 GPU clusters.

Geekbench 3 Memory Bandwidth Comparison (1 thread)
  Stream Copy Stream Scale Stream Add Stream Triad
Apple A9X 2.26GHz 20.8 GB/s 15.0 GB/s 15.3 GB/s 15.1 GB/s
Apple A8X 1.5GHz 14.2 GB/s 7.44 GB/s 7.54 GB/s 7.49 GB/s
A9X Advantage 46.4% 101% 103% 102%

It’s also while looking at A9X’s memory subsystem however that we find our second and final curveball for A9X: the L3 cache. Or rather, the lack thereof. For multiple generations now Apple has used an L3 cache on both their phone and tablet SoCs to help feed both the CPU and GPU, as even a fast memory bus can’t keep up with a low latency local cache. Even as recent as A9, Apple included a 4MB victim cache. However for A9X there is no L3 cache; the only caches on the chip are the individual L1 and L2 caches for the CPU and GPU, along with some even smaller amounts for cache for various other functional blocks..

The big question right now is why Apple would do this. Our traditional wisdom here is that the L3 cache was put in place to service both the CPU and GPU, but especially the GPU. Graphics rendering is a memory bandwidth-intensive operation, and as Apple has consistently been well ahead of many of the other ARM SoC designers in GPU performance, they have been running headlong into the performance limitations imposed by narrow mobile memory interfaces. An L3 cache, in turn, would alleviate some of that memory pressure and keep both CPU and GPU performance up.

One explanation may be that Apple deemed the L3 cache no longer necessary with the A9X’s 128-bit LPDDR4 memory bus; that 51.2GB/sec of bandwidth meant that they no longer needed the cache to avoid GPU stalls. However while the use of LPDDR4 may be a factor, Apple’s ratio of bandwidth-to-GPU cores of roughly 4.26GB/sec-to-1 core is identical to A9’s, which does have an L3 cache. With A9X being a larger A9 in so many ways, this alone isn’t the whole story.

What’s especially curious is that the L3 cache on the A9 wasn’t costing Apple much in the way of space. Chipworks puts the size of A9’s 4MB L3 cache block at a puny ~4.5 mm2, which is just 3% the size of A9X. So although there is a cost to adding L3 cache, unless there are issues we can’t see even with a die shot (e.g. routing), Apple didn’t save much by getting rid of the L3 cache.

Our own Andrei Frumusanu suspects that it may be a power matter, and that Apple was using the L3 cache to save on power-expensive memory operations on the A9. With A9X however, it’s a tablet SoC that doesn’t face the same power restrictions, and as a result doesn’t need a power-saving cache. This would be coupled with the fact that with double the GPU cores, there would be a lot more pressure on just a 4MB cache versus the pressure created by A9, which in turn may drive the need for a larger cache and ultimately an even larger die size.

As it stands there’s no one obvious reason, and it’s likely that all 3 factors – die size, LPDDR4, and power needs – all played a part here, with only those within the halls of One Infinite Loop knowing for sure. However I will add that since Apple has removed the L3 cache, the GPU L2 cache must be sizable. Imagination’s tile based deferred rendering technology needs an on-chip cache to hold tiles in to work on, and while they don’t need an entire frame’s worth of cache (which on iPad Pro would be over 21MB), they do need enough cache to hold a single tile. It’s much harder to estimate GPU L2 cache size from a die shot (especially with Apple’s asymmetrical design), but I wouldn’t be surprised of A9X’s GPU L2 cache is greater than A9’s or A8X’s.

Building A9X Big: 147mm2, Manufactured By TSMC

Finally, let’s talk about the construction and fabrication of the A9X SoC itself. Chipworks’ previous analysis shows that the A9X is roughly 147mm2 in die size, and that it’s manufactured by TSMC on their 16nm FinFET process.

At 147mm2 the A9X is the second-largest of Apple’s X-series tablet SoCs. Only the A5X, the first such SoC, was larger. Fittingly, it was also built relative to Apple’s equally large A5 phone SoC. With only 3 previous tablet SoCs to use as a point of comparison I’m not sure there’s really a sweet spot we can say that Apple likes to stick to, but after two generations of SoCs in the 120mm2 to 130mm2 range, A9X is noticeably larger.

Some of that comes from the fact that A9 itself is a bit larger than normal – the TSMC version is 104.5mm2 – but Apple has also clearly added a fair bit to the SoC. The wildcard here is what yields look like for Apple, as that would tell us a lot about whether a 147mm2 A9X is just a large part or if Apple has taken a greater amount of risk than usual here.

A9X continues to be the largest 16nm FinFET ASIC we know to be in mass production at TSMC (we’ll ignore FPGAs for now), and while this will undoubtedly change a bit later this year once the next-generation discrete GPUs come online, I don’t think you’ll find a better example of how the contract chip manufacturing market has changed in a single generation. 4 years ago it would be GPUs leading the charge, but now it’s phone SoCs and a rather sizable tablet SoC that are first out of the gate. After almost a decade of catching up, SoCs have now reached the bleeding edge for chip fabrication, enabling rapid performance growth, but also inheriting the risks of being the leader. I won’t dwell on this too much, but I’m immensely curious about both what A9X yields are like as the largest FinFET ASIC at TSMC, and just how much of TSMC’s FinFET capacity Apple has been consuming with the production of A9 and A9X.

Finally, it's also interesting to note just how large A9X is compared to other high performance processors. Intel's latest-generation Skylake processors measure in at ~99mm2 for the 2 core GT2 configuration (Skylake-Y 2+2), and even the 4 core desktop GT2 configuration (Intel Skylake-K 4+2) is only 122mm2. So A9X is larger than either of these CPU cores, though admittedly as a whole SoC A9X contains a number of functional units either not present on Skylake or on Skylake's Platform Controller Hub (PCH). Still, this is the first time that we've seen an Apple launch a tablet SoC larger than an Intel 4 core desktop CPU.

Introduction and Design SoC Analysis: On x86 vs ARMv8
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  • xerandin - Saturday, January 23, 2016 - link

    In what way did Microsoft saw Surface Pro parts off of other products? You know what's better than that "most stable, secure, and highest quality mobile OS?" For most people, that would be Microsoft Windows--even if they love to complain about it, you can't deny Microsoft's ubiquity in the Professional space (and home userspace, too, but we're trying to keep this in the professional sector, right?)

    I've heard a few people at work say that the sysadmins love Macs (it was a Director obsessed with his Macbook telling me this), but I can't seem to find any of these supposed Mac-lovers. It could have something to do that they're a nightmare to administer for most sysadmins running a domain (which covers the vast majority of sysadmins), or the fact that Mac users tend to be just as inept and incapable as Windows users, so you get to have another pain-in-the-ass group of users to deal with on a system that just isn't very nice to administer.

    If you take off your hipster glasses for a moment and actually use computers in the world we live in, there's no way around the reality that Microsoft wins in the Professional space--including their beautifully made, super-powerful Surface Pro.

    And no, I'm not a Microsoft shill, I just can't stand Apple fanboys with more money than sense.
    Reply
  • gistya - Sunday, January 24, 2016 - link

    Xerandin, you have no idea what you are talking about. 90% of the professional software development studios I work with are almost solely Mac based, for the simple reason that they barely need an IT department at all, in that case.

    Windows is technical debt, plain and simple. It's legacy cruft stuck to the face of the world. The only IT guys that hate Macs are the idiot ones who don't know bash from tsch and couldn't sudo themselves out of a wet paper box. Then there are the smart ones who know that a shift to such a low maintenance platform would mean their department would get downsized.

    But so many companies are stuck on crap like SAP, NovellNetware, etc., that Microsoft could literally do nothing right for 10 years and still be a powerhouse. Oh wait.

    Apple hardware is worth every extra cent it costs, and then some; if you make such little money that $500 more on a tool that you'll professionally use 8-12 hours a day for three years is a deal-breaker, then I feel very sorry for you.

    But personally I think it's more than worth it to have the (by far) best screen, trackpad, keyboard, case, input drivers, and selection of operating systems. I have five different OS's installed right now including three different versions of windows (the good one, and then the most recent one, and the one that my last job still uses, which does not receive security patches and gets infected with viruses after being on a website for 10 seconds).

    As for the iPad Pro, all of you fools just don't understand what it is, or why the pencil is always sold out everywhere, or what the difference is. As a software developer I can tell you that there is the most extreme difference; and that more development for iOS is being done now than ever before, and is being done at an accelerating pace. This is just version 1.0 of the large-size, pro type model for Apple, and those of us who did not buy it yet and who are still waiting for that killer app, are basically saying that well, once that app comes out, then heck yes we'll buy it. Do you seriously believe that no company will rise up to capitalize on that obviously large market? Someone will, and frankly lets hope it's not Adobe.

    I regularly see iPad pros now in the hands of the professional musicians and producers I work with, and they are most certainly using them for professional applications. That's a niche to be sure, but everyone who thinks that the surface pro 4 (a mildly crappy laptop with a touchscreen that makes a bad, thick tablet and an underpowered, overheated laptop) is even remotely in the same category of device, is utterly smoking crack.
    Reply
  • doggface - Sunday, January 24, 2016 - link

    I think it might be you smoking the crack there mate. Cor, what a rant. Microsoft are pretty safe in enterprise and it has everything to do with managing large networks(1000s not 10s of computers.. Please, direct me to Apple's answer to sccm, please show me Apple's answer to exchange. Please show me an Apple only environment running 1000 different apps outside ofGoogle and Apple HQ. Just aint haopening. Reply
  • Constructor - Sunday, January 24, 2016 - link

    IBM (yes, the IBM!) has just announced that they will switch over to Macs a while ago. And they're neither the first nor the last. Springer (a major german media company) had done that a while ago already for similar reasons (removing unproductive friction and cutting the actual cost of ownership due to less needed user support).

    I know that many people had imagined that Windows would be the only platform anyone would ever need to know, but that has always just been an illusion.
    Reply
  • damianrobertjones - Monday, January 25, 2016 - link

    Link to the article please! They're THINKING of using Apple for mobile use... . Reply
  • Constructor - Monday, January 25, 2016 - link

    Nope. They're massively ramping up Mac purchases as well, with a target of 50-75% Macs at IBM. They are already full steam ahead with it:
    http://www.i4u.com/2015/08/93776/ibm-purchase-2000...
    (Okay: The official announcements don't have that aspect, but an internal video interview with IBM's CIO leaked to YouTube makes it rather explicit even so.)

    One of the motivators is apparently that despite higher sticker prices the total cost of ownership is lower for Macs (which is not news any more, but having IBM arriving at that conclusion still says something).
    Reply
  • Oxford Guy - Wednesday, January 27, 2016 - link

    Gartner said in 1999 that Macs were substantially cheaper in TCO. Reply
  • mcrispin - Tuesday, January 26, 2016 - link

    doggface, you speak with confidence where that confidence isn't deserved. I've managed deployments of OS X way over 10k, there are plenty of places with deployments this high. JAMF Casper Suite is the "SCCM of Apple", I don't need an "Apple" replacement for Exchange, 0365 and Google are just fine for email. There are plenty of non-Apple/Google Apps for OS X and iOS. You are seriously misinformed about the reality of the OS X marketplace. Shame that. Reply
  • Ratman6161 - Wednesday, January 27, 2016 - link

    mcrispin...sure you can find individual shops that have done big MAC deployments. but its anecdotal evidence. Its like looking at one neigborhood in my city and from that conclude that trailer parks are the norm in my city of 15,000.

    Take a look at this Oct 2015 article on Mac market share (I'm assuming you dont consider Apple Insider to be a bunch of Microsoft shills?) http://appleinsider.com/articles/15/10/08/mac-gain...

    While touting how Mac is gaining market share they show a chart where in Q32015 they were at 7.6%. The chart is by company and even smaller Windows PC vendors Asus and Acer are at 7.1 and 7.4 respectively. Throw in Lenovo, HP and Dell at 20.3%, 18.5%, 13.8% and the 25.3% "others" (and others are not MAC's because Apple is the only company with those).

    So IBM is doing 50 - 75% Mac's? OK Ill take your word for that but so what? In the larger scheme of things Apple still has only 7.6% and selling some computers to IBM isn't going to siginificantly change that number. Also, don't forgot that some companies (that compete with Microsoft in various areas) will not use a Microsoft product no mater how good it was.

    No matter how you look at it, Windows is the main stream OS for busineses world wide. Touting the exceptions to that doesn't cange the truth of it.
    Reply
  • Constructor - Wednesday, January 27, 2016 - link

    Of course most workplace computers now are PCs. The thing is just that Macs are making major inroads there as well.

    $25 billion in Apple's corporate sales are already very far removed from your theory (and that's even without all the smaller shops who are buing retail!).
    Reply

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