The A5X SoC

The ridiculousness of the new iPad begins at its heart: the A5X SoC.

The A5X breaks Apple's longstanding tradition of debuting its next smartphone SoC in the iPad first. I say this with such certainty because the A5X is an absolute beast of an SoC. As it's implemented in the new iPad, the A5X under load consumes more power than an entire iPhone 4S.

In many ways in the A5X is a very conservative design, while in others it's absolutely pushing the limits of what had been previously done in a tablet. Similar to the A5 and A4 before it, the A5X is still built on Samsung's 45nm LP process. Speculation about a shift to 32nm or even a move TSMC was rampant this go around. I'll admit I even expected to see a move to 32nm for this chip, but Apple decided that 45nm was the way to go.

Why choose 45nm over smaller, cooler running options that are on the table today? Process maturity could be one reason. Samsung has yet to ship even its own SoC at 32nm, much less one for Apple. It's quite possible that Samsung's 32nm LP simply wasn't ready/mature enough for the sort of volumes Apple needed for an early 2012 iPad launch. The fact that there was no perceivable slip in the launch timeframe of the new iPad (roughly 12 months after its predecessor) does say something about how early 32nm readiness was communicated to Apple. Although speculation is quite rampant about Apple being upset enough with Samsung to want to leave for TSMC, the relationship on the foundry side appears to be good from a product delivery standpoint.

Another option would be that 32nm was ready but Apple simply opted against using it. Companies arrive at different conclusions as to how aggressive they need to be on the process technology side. For example, ATI/AMD was typically more aggressive on adopting new process technologies while NVIDIA preferred to make the transition once all of the kinks were worked out. It could be that Apple is taking a similar approach. Wafer costs generally go up at the start of a new process node, combine that with lower yields and strict design rules and it's not a guarantee that you'd actually save any money from moving to a new process technology—at least not easily or initially. The associated risk of something going wrong might have been one that Apple wasn't willing to accept.

CPU Specification Comparison
CPU Manufacturing Process Cores Transistor Count Die Size
Apple A5X 45nm 2 ? 163mm2
Apple A5 45nm 2 ? 122mm2
Intel Sandy Bridge 4C 32nm 4 995M 216mm2
Intel Sandy Bridge 2C (GT1) 32nm 2 504M 131mm2
Intel Sandy Bridge 2C (GT2) 32nm 2 624M 149mm2
NVIDIA Tegra 3 40nm 4+1 ? ~80mm2
NVIDIA Tegra 2 40nm 2 ? 49mm2

Whatever the reasoning, the outcome is significant: the A5X is approximately 2x the size of NVIDIA's Tegra 3, and even larger than a dual-core Sandy Bridge desktop CPU. Its floorplan is below:


Courtesy: Chipworks

From the perspective of the CPU, not much has changed with the A5X. Apple continues to use a pair of ARM Cortex A9 cores running at up to 1.0GHz, each with MPE/NEON support and a shared 1MB L2 cache. While it's technically possible for Apple to have ramped up CPU clocks in pursuit of higher performance (A9 designs have scaled up to 1.6GHz on 4x-nm processes), Apple has traditionally been very conservative on CPU clock frequency. Higher clocks require higher voltages (especially on the same process node), which result in an exponential increase in power consumption.

ARM Cortex A9 Based SoC Comparison
  Apple A5X Apple A5 TI OMAP 4 NVIDIA Tegra 3
Manufacturing Process 45nm LP 45nm LP 45nm LP 40nm LPG
Clock Speed Up to 1GHz Up to 1GHz Up to 1GHz Up to 1.5GHz
Core Count 2 2 2 4+1
L1 Cache Size 32KB/32KB 32KB/32KB 32KB/32KB 32KB/32KB
L2 Cache Size 1MB 1MB 1MB 1MB
Memory Interface to the CPU Dual Channel LP-DDR2 Dual Channel LP-DDR2 Dual Channel LP-DDR2 Single Channel LP-DDR2
NEON Support Yes Yes Yes Yes

With no change on the CPU side, CPU performance remains identical to the iPad 2. This means everything from web page loading to non-gaming app interactions are no faster than they were last year:

SunSpider JavaScript Benchmark 0.9.1

Rightware BrowserMark

JavaScript performance remains unchanged, as you can see from both the BrowserMark and SunSpider results above. Despite the CPU clock disadvantage compared to the Tegra 3, Apple does have the advantage of an extremely efficient and optimized software stack in iOS. Safari just went through an update in improving its Javascript engine, which is why we see competitive performance here.

Geekbench has been updated with Android support, so we're able to do some cross platform comparisons here. Geekbench is a suite composed of completely synthetic, low-level tests—many of which can execute entirely out of the CPU's L1/L2 caches.

Geekbench 2
  Apple iPad (3rd gen) ASUS TF Prime Apple iPad 2 Motorola Xyboard 10.1
Integer Score 688 1231 684 883
Blowfish ST 13.2 MB/s 23.3 MB/s 13.2 MB/s 17.6 MB/s
Blowfish MT 26.3 MB/s 60.4 MB/s 26.0 MB/s -
Text Compress ST 1.52 MB/s 1.58 MB/s 1.51 MB/s 1.63 MB/s
Text Compress MT 2.85 MB/s 3.30 MB/s 2.83 MB/s 2.93 MB/s
Text Decompress ST 2.08 MB/s 2.00 MB/s 2.09 MB/s 2.11MB/s
Text Decompress MT 3.20 MB/s 3.09 MB/s 3.27 MB/s 2.78 MB/s
Image Compress ST 4.09 Mpixels/s 5.56 Mpixels/s 4.08 Mpixels/s 5.42 Mpixels/s
Image Compress MT 8.12 Mpixels/s 21.4 Mpixels/s 7.98 Mpixels/s 10.5 Mpixels/s
Image Decompress ST 6.70 Mpixels/s 9.37 Mpixels/s 6.67 Mpixels/s 9.18 Mpixels/s
Image Decompress MT 13.2 Mpixels/s 20.3 Mpixels/s 13.0 Mpixels/s 17.9 Mpixels/s
Lua ST 257.2 Knodes/s 417.9 Knodes/s 257.0 Knodes/s 406.9 Knodes/s
Lua MT 512.3 Knodes/s 1500 Knodes/s 505.6 Knodes/s 810.0 Knodes/s
FP Score 920 2223 915 1514
Mandelbrot ST 279.5 MFLOPS 334.8 MFLOPS 279.0 MFLOPS 328.9 MFLOPS
Mandelbrot MT 557.0 MFLOPS 1290 MFLOPS 550.3 MFLOPS 648.0 MFLOPS
Dot Product ST 221.9 MFLOPS 477.5 MFLOPS 221.5 MFLOPS 455.2 MFLOPS
Dot Product MT 438.9 MFLOPS 1850 MFLOPS 439.4 MFLOPS 907.4 MFLOPS
LU Decomposition ST 217.5 MFLOPS 171.4 MFLOPS 214.6 MFLOPS 177.9 MFLOPS
LU Decomposition MT 434.2 MFLOPS 333.9 MFLOPS 437.4 MFLOPS 354.1 MFLOPS
Primality ST 177.3 MFLOPS 175.6 MFLOPS 178.0 MFLOPS 172.9 MFLOPS
Primality MT 321.5 MFLOPS 273.2 MFLOPS 316.9 MFLOPS 220.7 MFLOPS
Sharpen Image ST 1.68 Mpixels/s 3.87 Mpixels/s 1.68 Mpixels/s 3.86 Mpixels/s
Sharpen Image MT 3.35 Mpixels/s 9.85 Mpixels/s 3.32 Mpixels/s 7.52 Mpixels/s
Blur Image ST 666.0 Kpixels/s 1.62 Kpixels/s 664.8 Kpixels/s 1.58 Kpixels/s
Blur Image MT 1.32 Mpixels/s 6.25 Mpixels/s 1.31 Mpixels/s 3.06 Mpixels/s
Memory Score 821 1079 829 1122
Read Sequential ST 312.0 MB/s 249.0 MB/s 347.1 MB/s 364.1 MB/s
Write Sequential ST 988.6 MB/s 1.33 GB/s 989.6 MB/s 1.32 GB/s
Stdlib Allocate ST 1.95 Mallocs/sec 2.25 Mallocs/sec 1.95 Mallocs/sec 2.2 Mallocs/sec
Stdlib Write 2.90 GB/s 1.82 GB/s 2.90 GB/s 1.97 GB/s
Stdlib Copy 554.6 MB/s 1.82 GB/s 564.5 MB/s 1.91 GB/s
Stream Score 331 288 335 318
Stream Copy 456.4 MB/s 386.1 MB/s 466.6 MB/s 504 MB/s
Stream Scale 380.2 MB/s 351.9 MB/s 371.1 MB/s 478.5 MB/s
Stream Add 608.8 MB/s 446.8 MB/s 654.0 MB/s 420.1 MB/s
Stream Triad 457.7 MB/s 463.7 MB/s 437.1 MB/s 402.8 MB/s

Almost entirely across the board NVIDIA delivers better CPU performance, either as a result of having more cores, having higher clocked cores or due to an inherent low-level Android advantage. Prioritizing GPU performance over a CPU upgrade is nothing new for Apple, and in the case of the A5X Apple could really only have one or the other—the new iPad gets hot enough and draws enough power as it is; Apple didn't need an even more power hungry set of CPU cores to make matters worse.

Despite the stagnation on the CPU side, most users would be hard pressed to call the iPad slow. Apple does a great job of prioritizing responsiveness of the UI thread, and all the entire iOS UI is GPU accelerated, resulting in a very smooth overall experience. There's definitely a need for faster CPUs to enable some more interesting applications and usage models. I suspect Apple will fulfill that need with the A6 in the 4th generation iPad next year. That being said, in most applications I don't believe the iPad feels slow today.

I mention most applications because there are some iOS apps that are already pushing the limits of what's possible today.

iPhoto: A Case Study in Why More CPU Performance is Important

In our section on iPhoto we mentioned just how frustratingly slow the app can be when attempting to use many of its editing tools. In profiling the app it becomes abundantly clear why it's slow. Despite iPhoto being largely visual, it's extremely CPU bound. For whatever reason, simply having iPhoto open is enough to eat up an entire CPU core. 

Use virtually any of the editing tools and you'll see 50—95% utilization of the remaining, unused core. The screenshot below is what I saw during use of the saturation brush:

The problem is not only are the two A9s not fast enough to deal with the needs of iPhoto, but anything that needs to get done in the background while you're using iPhoto is going to suffer as well. This is most obvious when you look at how long it takes for UI elements within iPhoto to respond when you're editing. It's very rare that we see an application behave like this on iOS, even Infinity Blade only uses a single core most of the time, but iPhoto is a real exception.

I have to admit, I owe NVIDIA an apology here. While I still believe that quad-cores are mostly unnecessary for current smartphone/tablet workloads, iPhoto is a very tangible example of where Apple could have benefitted from having four CPU cores on A5X. Even an increase in CPU frequency would have helped. In this case, Apple had much bigger fish to fry: figuring out how to drive all 3.1M pixels on the Retina Display.

Battery Life, Charging & Thermals The GPU & Apple Builds a Quad-Channel Memory Controller
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  • Jamezrp - Wednesday, March 28, 2012 - link

    Didn't want to cause Verizon too much trouble? Heh, very funny. I am amazed at how the iPad ends up being an amazingly good Wi-Fi hotspot. It almost seems like business users should opt to get an iPad for that function alone. I know plenty of people who would be happy to keep it in their bag, with the hotspot feature enabled constantly, while travelling about. Even for the price there is nothing even close that can compare.

    Plus, you know, you get the tablet too.
    Reply
  • supertwister - Wednesday, March 28, 2012 - link

    "It’s a quantum leap from the noisy, 0.7MP mess that was the iPad 2 camera."

    Interesting choice of word considering a quantum is the smallest possible division for a quantity...
    Reply
  • omion - Wednesday, March 28, 2012 - link

    Quantum leap:
    (n) an abrupt change, sudden increase, or dramatic advance

    The phrase comes from the ability of particles to make a sudden jump between two energy levels. It is a leap (of any amount) between two quantization levels, not a leap of the smallest possible amount.
    Reply
  • drwho9437 - Wednesday, March 28, 2012 - link

    A large fraction of the die doesn't seem to have a known use? Wondering what could be taking up all that area if not GPU, CPU and memory interfaces/caches... Most other I/O would have small footprints... Reply
  • tipoo - Wednesday, March 28, 2012 - link

    The 4S had a larger than usual die for its voice cancellation features that were needed to make Siri work well, the iPad does't have that but it does have voice dictation so some space is probably for that. Reply
  • PeteH - Wednesday, March 28, 2012 - link

    A big chunk of it is probably the ISP they talked about when the 4S debuted. Reply
  • Lucian Armasu - Wednesday, March 28, 2012 - link

    So this is how I assumed. The new iPad is in fact slower than the iPad 2, if games actually start using the 2048x1536 resolution for their apps, which everyone seems to be encouraging them to do. But once they do that the graphics will either look poorer, or they will be slower than they were on the old resolution, even on an iPad 2.

    Add that to the fact that apps are much bigger in size with the retina resolution, and the CPU is the same as last year. The new display might look great, but it's obvious that the new iPad is absolutely a step-back in terms of performance, whether it's GPU or CPU we're talking about. Hardly worth an upgrade, especially for iPad 2 owners.
    Reply
  • xype - Thursday, March 29, 2012 - link

    Blah blah blah performance blah not worth it.

    I don’t give a shit about theoretical performance that I might be getting if DNA folding software was available for tablets. I really, really give a shit about being able to read website and ebook text without my eyes straining after an hour.

    One would think that 10 years after "No wireless. Less space than a nomad. Lame." and Apple raking in millions and billions of profit, those Geek Metrics™ that people are so fond of here (nothing wrong with that, it’s interesting stuff!), would be recognized as completely and utterly worthless to the average population. But apparently not.

    The iPad was never ment to replace PCs and Consoles as a hardcore gaming device, and it was never ment as a render farm server replacement. It would be really nice if people realized that, at some point. In the next 5 years, perhaps.
    Reply
  • tipoo - Thursday, March 29, 2012 - link

    It seems a bit like the 3GS-4 transition, it used the same GPU despite higher resolution and so performed worse at native, although in this case the CPU is unchanged and the GPU is "only" 2x better for 4x the pixels. Developers got around that on the 4 by making games for the old resolution and using upscaling mode. I'd imagine they will do the same here once games hit the limits of the GPU at native. Games like Infinity Blade 2 also use separate resolutions for things like the menus vs shadows vs terrain textures. Reply
  • darkcrayon - Thursday, March 29, 2012 - link

    I guess if the only thing you bought an iPad for were games, and you could only consider a game to be worthwhile if it were drawn directly at 2048x1536, you'd have a point. But of course the new iPad could play games at the "iPad 2" resolution at much higher detail, or at a slightly resolution with the same detail, etc.

    It doesn't make sense to say it's a step backward in performance overall- it simply has the option to display much higher resolution graphics that the old model didn't have. The iPad 2 displays 2048 x 1536 text at "0 mhz" so to speak. It's not like you are losing anything by having the option of ultra high resolution if the type of game (or app) can use it within the hardware capabilities.
    Reply

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