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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  • doobydoo - Saturday, March 31, 2012 - link

    Lucian Armasu, you talk the most complete nonsense of anyone I've ever seen on here.

    The performance is not worse, by any stretch of the imagination, and lets remember that the iPad 2 runs rings around the Android competition graphically anyway. If you want to run the same game at the same resolution, which wont look worse, at all (it would look exactly the same) it will run at 2x the FPS or more (up-scaled). Alternatively, for games for which it is beneficial, you can quadruple the quality and still run the game at perfectly acceptable FPS, since the game will be specifically designed to run on that device. Attempting anything like that quality on any other tablet is not only impossible by virtue of their inferior screens, they don't have the necessary GPU either.

    In other words, you EITHER have a massive improvement in quality, or a massive improvement in performance, over a device (iPad 2) which was still the fastest performing GPU tablet even a year after it came out. The game developers get to make this decision - so they just got 2 great new options on a clearly much more powerful device. To describe that as not worth an upgrade is quite frankly ludicrous, you have zero credibility from here on in.
    Reply
  • thejoelhansen - Wednesday, March 28, 2012 - link

    Hey Anand,

    First of all - thank you so much for the quality reviews and benchmarks. You've helped me build a number of servers and gaming rigs. :)

    Secondly, I'm not sure I know what you mean when you state that "Prioritizing GPU performance over a CPU upgrade is nothing new for Apple..." (Page 11).

    The only time I can remember Apple doing so is when keeping the 13" Macbook/ MBPs on C2Ds w/ Nvidia until eventually relying on Intel's (still) anemic "HD" graphics... Is that what you're referring to?

    I seem to remember Apple constantly ignoring the GPU in favor of CPU upgrades, other than that one scenario... Could be mistaken. ;)

    And again - thanks for the great reviews! :)
    Reply
  • AnnonymousCoward - Wednesday, March 28, 2012 - link

    "Retina Display" is a stupid name. Retinas sense light, which the display doesn't do. Reply
  • xype - Thursday, March 29, 2012 - link

    GeForce is a stupid name, as the video cards don’t have anything to do with influencing the gravitational acceleration of an object or anything close to that.

    Retina Display sounds fancy and is lightyears ahead of "QXGA IPS TFT Panel" when talking about it. :P
    Reply
  • Sabresiberian - Thursday, March 29, 2012 - link

    While I agree that "Retina Display" is a cool enough sounding name, and that's pretty much all you need for a product, unless it's totally misleading, it's not an original use of the phrase. The term has been used in various science fiction stories and tends to mean a display device that projects an image directly onto the retina.

    I always thought of "GeForce" as being an artist's licensed reference to the cards being a Force in Graphics, so the name had a certain logic behind it.

    ;)
    Reply
  • seapeople - Tuesday, April 03, 2012 - link

    Wait, so "Retina Display" gets you in a tizzy but "GeForce" makes perfect sense to you? You must have interesting interactions in everyday life. Reply
  • ThreeDee912 - Thursday, March 29, 2012 - link

    It's basically the same concept with Gorilla Glass or UltraSharp displays. It obviously doesn't mean that Corning makes glass out of gorillas, or that Dell will cut your eyes out and put them on display. It's just a marketing name. Reply
  • SR81 - Saturday, March 31, 2012 - link

    Funny I always believed the "Ge" was short for geometry. Whatever the case you can blame the name on the fans who came up with it. Reply
  • tipoo - Thursday, March 29, 2012 - link

    iPad is a stupid name. Pads collect blood from...Well, never mind. But since when are names always literal? Reply
  • doobydoo - Saturday, March 31, 2012 - link

    What would you call a display which had been optimised for use by retinas?

    Retina display.

    They aren't saying the display IS a retina, they are saying it is designed with retinas in mind.

    The scientific point is very clear and as such I don't think the name is misleading at all. The point is the device has sufficient PPI at typical viewing distance that a person with typical eyesight wont be able to discern the individual pixels.

    As it happens, strictly speaking, the retina itself is capable of discerning more pixels at typical viewing distance than the PPI of the new iPad, but the other elements of the human eye introduce loss in the quality of the image which is then ultimately sent on to the brain. While scientifically this is a distinction, to end consumers it is a distinction without a difference, so the name makes sense in my opinion.
    Reply

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