The GPU

3D rendering is a massively parallel problem. Your GPU ultimately has to determine the color value of each pixel which may not remain constant between frames, at a rate of dozens of times per second. The iPad 2 had 786,432 pixels in its display, and by all available measures its GPU was more than sufficient to drive that resolution. The new iPad has 3.14 million pixels to drive. The iPad 2's GPU would not be sufficient.

When we first heard Apple using the term A5X to refer to the new iPad's SoC, I assumed we were looking at a die shrunk, higher clock version of the A5. As soon as it became evident that Apple remained on Samsung's 45nm LP process, higher clocks were out of the question. The only room for improving performance was to go wider. Thankfully, as 3D rendering is a massively parallel problem, simply adding more GPU execution resources tends to be a great way of dealing with a more complex workload. The iPad 2 shocked the world with its dual-core PowerVR SGX 543MP2 GPU, and the 3rd generation iPad doubled the amount of execution hardware with its quad-core PowerVR SGX 543MP4.

Mobile SoC GPU Comparison
  Adreno 225 PowerVR SGX 540 PowerVR SGX 543MP2 PowerVR SGX 543MP4 Mali-400 MP4 Tegra 2 Tegra 3
SIMD Name - USSE USSE2 USSE2 Core Core Core
# of SIMDs 8 4 8 16 4 + 1 8 12
MADs per SIMD 4 2 4 4 4 / 2 1 1
Total MADs 32 8 32 64 18 8 12
GFLOPS @ 200MHz 12.8 GFLOPS 3.2 GFLOPS 12.8 GFLOPS 25.6 GFLOPS 7.2 GFLOPS 3.2 GFLOPS 4.8 GFLOPS
GFLOPS @ 300MHz 19.2  GFLOPS 4.8 GFLOPS 19.2 GFLOPS 38.4
GFLOPS
10.8 GFLOPS 4.8 GFLOPS 7.2 GFLOPS
GFLOPS As Shipped by Apple/ASUS - - 16 GFLOPS 32 GFLOPS - - 12
GFLOPS

We see this approach all of the time in desktop and notebook GPUs. To allow games to run at higher resolutions, companies like AMD and NVIDIA simply build bigger GPUs. These bigger GPUs have more execution resources and typically more memory bandwidth, which allows them to handle rendering to higher resolution displays.

Apple acted no differently than a GPU company would in this case. When faced with the challenge of rendering to a 3.14MP display, Apple increased compute horsepower and memory bandwidth. What's surprising about Apple's move is that the A5X isn't a $600 desktop GPU, it's a sub 4W mobile SoC. And did I mention that Apple isn't a GPU company?

That's quite possibly the most impressive part of all of this. Apple isn't a GPU company. It's a customer of GPU companies like AMD and NVIDIA, yet Apple has done what even NVIDIA would not do: commit to building an SoC with an insanely powerful GPU.

I whipped up an image to help illustrate. Below is a representation, to-scale, of Apple and NVIDIA SoCs, their die size, and time of first product introduction:

If we look back to NVIDIA's Tegra 2, it wasn't a bad SoC—it was basically identical in size to Apple's A4. The problem was that the Tegra 2 made its debut a full year after Apple's A4 did. The more appropriate comparison would be between the Tegra 2 and the A5, both of which were in products in the first half of 2011. Apple's A5 was nearly 2.5x the size of NVIDIA's Tegra 2. A good hunk of that added die area came from the A5's GPU. Tegra 3 took a step in the right direction but once again, at 80mm^2 the A5 was still over 50% larger.

The A5X obviously dwarfs everything, at around twice the size of NVIDIA's Tegra 3 and 33.6% larger than Apple's A5. With silicon, size isn't everything, but when we're talking about similar architectures on similar manufacturing processes, size does matter. Apple has been consistently outspending NVIDIA when it comes to silicon area, resulting in a raw horsepower advantage, which in turns results in better peak GPU performance.

Apple Builds a Quad-Channel (128-bit) Memory Controller

There's another side effect that you get by having a huge die: room for wide memory interfaces. Silicon layout is a balancing act. You want density to lower costs, but you don't want hotspots so you need heavy compute logic to be spread out. You want wide IO interfaces but you don't want them to be too wide because then you'll cause your die area to balloon as a result. There's only so much room on the perimeter of your SoC to get data out of the chip, hence the close relationship between die size and interface width.

Most mobile SoCs are equipped with either a single or dual-channel LP-DDR2 memory controller. Unlike in the desktop/notebook space where a single DDR2/DDR3 channel refers to a 64-bit wide interface, in the mobile SoC world a single channel is 32-bits wide. Both Qualcomm and NVIDIA use single-channel interfaces, with Snapdragon S4 finally making the jump to dual-channel this year. Apple, Samsung, and TI have used dual-channel LP-DDR2 interfaces instead.

With the A5X Apple did the unthinkable and outfitted the chip with four 32-bit wide LP-DDR2 memory controllers. The confirmation comes from two separate sources. First we have the annotated A5X floorplan courtesy of UBMTechInsights:

You can see the four DDR interfaces around the lower edge of the SoC. Secondly, we have the part numbers of the discrete DRAM devices on the opposite side of the motherboard. Chipworks and iFixit played the DRAM lottery and won samples with both Samsung and Elpida LP-DDR2 devices on-board, respectively. While both Samsung and Elpida do a bad job of updating public part number decoders, both strings match up very closely to 216-ball PoP 2x32-bit PoP DRAM devices. The part numbers don't match up exactly, but they are close enough that I believe we're simply looking at a discrete flavor of those PoP DRAM devices.


K3PE4E400M-XG is the Samsung part number for a 2x32-bit LPDDR2 device, K3PE4E400E-XG is the part used in the iPad. I've made bold the only difference.

A cross reference with JEDEC's LP-DDR2 spec tells us that there is an official spec for a single package, 216-ball dual-channel (2x32-bit) LP-DDR2 device, likely what's used here on the new iPad.


The ball out for a 216-ball, single-package, dual-channel (64-bit) LPDDR2 DRAM

This gives the A5X a 128-bit wide memory interface, double what the closest competition can muster and putting it on par with what we've come to expect from modern x86 CPUs and mainstream GPUs.

The Geekbench memory tests show no improvement in bandwidth, which simply tells us that the interface from the CPU cores to the memory controller hasn't seen a similar increase in width.

Memory Bandwidth Comparison—Geekbench 2
  Apple iPad (3rd gen) ASUS TF Prime Apple iPad 2 Motorola Xyboard 10.1
Overall Memory Score 821 1079 829 1122
Read Sequential 312.0 MB/s 249.0 MB/s 347.1 MB/s 364.1 MB/s
Write Sequential 988.6 MB/s 1.33 GB/s 989.6 MB/s 1.32 GB/s
Stdlib Allocate 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
Overall 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

Although Apple designed its own memory controller in the A5X, you can see that all of these A9 based SoCs deliver roughly similar memory performance. The numbers we're showing here aren't very good at all. Even though Geekbench has never been good at demonstrating peak memory controller efficiency to begin with, the Stream numbers are very bad. ARM's L2 cache controller is very limiting in the A9, something that should be addressed by the time the A15 rolls around.

Firing up the memory interface is a very costly action from a power standpoint, so it makes sense that Apple would only want to do so when absolutely necessary. Furthermore, notice how the memory interface moved from being closer to the CPU in A4/A5 to being adjacent to the GPU in the A5X. It would appear that only the GPU has access to all four channels.

The A5X SoC A Word on Packaging & Looking Forward
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  • mr_ripley - Friday, March 30, 2012 - link

    It's a shame some people argue that against the workers when over a hundred of them have committed suicide over the working conditions. How can you still say that they are being offers a better deal here??

    On the other hand, it is also unfair that Apple is being singled out here. The world of Chinese manufacturing is a dirty one and all major corporations have a part in it. I'd trust Apple over most other companies to make a difference in that regard, and I'm happy to see something is being done in that regard. Ever heard McDonalds CEO touring the slaughterhouse of the meat packing companies??
    Reply
  • name99 - Friday, March 30, 2012 - link

    Reporting suicides as a number not as a rate shows you to be either a fool or a deliberate liar. How many people, over how many years, comprise the pool from which this suicide number is drawn? Everything I have read says that the actual suicide rate is not only lower than the average rate for China, it is lower than the average rate for the US. Reply
  • mr_ripley - Friday, March 30, 2012 - link

    In 2010, 18 workers attempted sucide, 14 succeeded. To me even one in a whole year is not acceptable. If you think that is ok I hope that statistic turns out to be you!! Reply
  • name99 - Saturday, March 31, 2012 - link

    The argument was NOT that suicide is a tragedy, it was a claim that FoxConn employees specifically tied to Apple production have such lousy lives that they commit suicide in higher numbers that other people around the world.

    You have done NOTHING to prove this claim; all you have done is bring up a very different issue.
    Reply
  • mr_ripley - Saturday, March 31, 2012 - link

    There is no disputing the fact that these deths are related to working conditions. I'm pretty sure this has been well established and documented. However, I did say in my previous post that Apple is unfairly singled out. It could have been any other company.

    Comparison between suicide rates is irrelevant. Higher sucide rates elsewhere does not justify this problem. Again the fact remains that many people have died and it is directly related to the working conditions.

    Apple happens to be in a position to directly influence their lives and make it better, after all they profit in billions from the work these people do. Corporations typically place little value over human life and living conditions (IBM sold equipment to the Nazis to track the Jews in concentration camps). Somehow, I feel Apple is different.
    Reply
  • doobydoo - Sunday, April 01, 2012 - link

    Dude, sorry but you're talking no sense at all.

    First of all, pretty much any product you want to buy, electronics wise, uses parts from China where conditions are far worse on average, than Apples factories. So if you actually factored working conditions into the product review, it would look favourable for Apple.

    Secondly, your argument that comparison between suicide rates is irrelevant, is absurd. Higher suicide rates where legislation is such that no jobs suffer such terrible conditions that suicide is the only option, such as is the case here, prove that even if working conditions are refined, you still get some depressed people. Your argument, therefore, is with the people who committed suicide. You say it is 'directly related to the working conditions' but where have you evidenced this, at all? You simply haven't. The fact that the suicide rates at Apple factories are lower than some American ones further backs up my point on this.

    Every company is in a position to change lives and make them better. You too, are in a position to do this. But guess what. You, just like companies, can do WHATEVER YOU LIKE with your OWN MONEY and have NO OBLIGATION WHATSOEVER to solve the worlds problems. Apple already has amongst the best conditions of factories in China. The amount of profit they make is absolutely irrelevant, if you say Apple should be putting money into this then a lot more manufacturers should also put a lot more money into this. It's very easy to decide what other people 'should' do with their money now, isn't it?

    Corporations don't have to adhere to moral values - they are not people. They are there solely to make money. Nothing else. Don't confuse them with people. And I hope you donate every single spare penny to charity and spend every spare second of your time working to build homes in the 3rd world. Oh wait, you're on here crying that other people should do it instead.

    Get a hold of yourself you illogical fool.
    Reply
  • mr_ripley - Sunday, April 01, 2012 - link

    Like I have said before it is a shame some people argue with great zeal against others who are suffering and devalue human life. Fortunately, Tim Cook is not one of them.

    If scores of people killing themselves citing poor working conditions is not enough proof what is? If your claim that there are work environments in America that have higher suicide rates because of working conditions is true that needs to be investigated as well and rectified.

    You give charity to people who are in need and cannot earn for themselves. If you think giving someone fair amount of compensation for hard work is charity you are delusional.

    If working in those factories is such a pleasent experience I suggest you try it out for yourself. Maybe the experience might broaden your perspective.

    Although, I don't see the point I will attempt to educate you. Legally, a corporation is considered as a person, that is right just like a live human being. Regardless of that corporations are run by people and actions of a corporation reflect upon the morality of the people running them.

    I will stop here as there is no point in continuing but you can respond with more insults and accusations of what I do or have done which frankly is no concern of yours.
    Reply
  • PeteH - Monday, April 02, 2012 - link

    I've not seen a single report of people killing themselves and citing "poor working conditions" as the reason. Can you provide a link?

    There have been reports of people killed because of unsafe working conditions, but that's a different issue. Maybe you're confusing the two.
    Reply
  • mr_ripley - Tuesday, April 03, 2012 - link

    Here's a Wikipedia link: you can read some of the circumstances and judge for yourself.

    They may not have said it in so many words but it is clear they were unhappy with ther work environment.

    Imagine your boss coming and beating you up because you lost an iPhone prototype!!!
    Reply
  • mr_ripley - Tuesday, April 03, 2012 - link

    http://en.wikipedia.org/wiki/Foxconn_suicides Reply

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