More Detail on the Xbox 360 GPU

ATI has been working on the Xbox 360 GPU for approximately two years, and it has been developed independently of any PC GPU. So despite what you may have heard elsewhere, the Xbox 360 GPU is not based on ATI's R5xx architecture.

Unlike any of their current-gen desktop GPUs, the 360 GPU supports FP32 from start to finish (as opposed to the current FP24 spec that ATI has implemented). Full FP32 support puts this aspect of the 360 GPU on par with NVIDIA's RSX.

ATI was very light on details of their pipeline implementation on the 360's GPU, but we were able to get some more clarification on some items. Each of the 48 shader pipelines is able to process two shader operations per cycle (one scalar and one vector), offering a total of 96 shader ops per cycle across the entire array. Remember that because the GPU implements a Unified Shader Architecture, each of these pipelines features execution units that can operate on either pixel or vertex shader instructions.

Both consoles are built on a 90nm process, and thus ATI's GPU is also built on a 90nm process at TSMC. ATI isn't talking transistor counts just yet, but given that the chip has a full 10MB of DRAM on it, we'd expect the chip to be fairly large.

One thing that ATI did shed some light on is that the Xbox 360 GPU is actually a multi-die design, referring to it as a parent-daughter die relationship. Because the GPU's die is so big, ATI had to split it into two separate die on the same package - connected by a "very wide" bus operating at 2GHz.

The daughter die is where the 10MB of embedded DRAM resides, but there is also a great deal of logic on the daughter die alongside the memory. The daughter die features 192 floating point units that are responsible for a lot of the work in sampling for AA among other things.

Remember the 256GB/s bandwidth figure from earlier? It turns out that that's not how much bandwidth is between the parent and daughter die, but rather the bandwidth available to this array of 192 floating point units on the daughter die itself. Clever use of words, no?

Because of the extremely large amount of bandwidth available both between the parent and daughter die as well as between the embedded DRAM and its FPUs, multi-sample AA is essentially free at 720p and 1080p in the Xbox 360. If you're wondering why Microsoft is insisting that all games will have AA enabled, this is why.

ATI did clarify that although Microsoft isn't targetting 1080p (1920 x 1080) as a resolution for games, their GPU would be able to handle the resolution with 4X AA enabled at no performance penalty.

ATI has also implemented a number of intelligent algorithms on the daughter die to handle situations where you need more memory than the 10MB of DRAM on-die. The daughter die has the ability to split the frame into two sections if the frame itself can't fit into the embedded memory. A z-pass is done to determine the location of all of the pixels of the screen and the daughter die then fetches only what is going to be a part of the scene that is being drawn at that particular time.

On the physical side, unlike ATI's Flipper GPU in the Gamecube, the 360 GPU does not use 1T-SRAM for its on-die memory. The memory on-die is actually DRAM. By using regular DRAM on-die, latencies are higher than SRAM or 1T-SRAM but costs should be kept to a minimum thanks to a smaller die than either of the aforementioned technologies.

Remember that in addition to functioning as a GPU, ATI's chip must also function as a memory controller for the 3-core PPC CPU in the Xbox 360. The memory controller services both the GPU and the CPU's needs, and as we mentioned before the controller is 256-bits wide and interfaces to 512MB of unified GDDR3 memory running at 700MHz. The memory controller resides on the parent die.

Index Scratching the Surface of NVIDIA's RSX
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  • finbarqs - Saturday, May 21, 2005 - link

    perhaps that's why they were very *hush* *hush* regarding their PS3 GPU. For all we know, the Sony demo's is very possible on the XGPU Reply
  • Illissius - Saturday, May 21, 2005 - link

    It'd have been nice to have some specs for current desktop GPUs in there for comparison purposes... how many shader ops/sec can they do? 16 + 6 = 22? Reply
  • ksherman - Saturday, May 21, 2005 - link

    DAmn they cant count! they just explained what the transisters are doing, and last i checked 232million+100million != 232million! Reply
  • Low Roller - Saturday, May 21, 2005 - link

    IGN posted an update to their Xbox 360 specs:

    "Lastly, we were sent updated spec numbers on the Xbox's numbers, and we spoke with Microsoft's Vice President of hardware, Todd Holmdahl, about the Xbox 360's final transistor count.

    Another bit of information sent our way is the final transistor count for Xbox 360's graphics subset. The GPU totals 332 million transistors, which is spit between the two separate dies that make up the part. The parent die is the "main" piece of the GPU, handling the large bulk of the graphics rendering, and is comprised of 232 million transistors. The daughter die contains the system's 10MB of embedded DRAM and its logic chip, which is capable of some additional 3D math. The daughter die totals an even 100 million transistors, bringing the total transistor count for the GPU to 232 million."

    http://xbox360.ign.com/articles/617/617951p3.html
    Reply
  • Low Roller - Friday, May 20, 2005 - link

    OK, I did a bit more hunting around on the transistor count in the Xbox 360's GPU, and the only thing I've found comes from Microsoft's Major Nelson.(Xbox Live Director of Programming)

    According to his blog, the Xbox 360's GPU has 330 million transistors.

    http://www.majornelson.com/2005/05/20/xbox-360-vs-...

    I'm not sure how credible either IGN's or Major Nelson's figures are on this, as their not even close to each other.
    Reply
  • ksherman - Friday, May 20, 2005 - link

    WOAAA, so Sony is manufacturing nVidia's GPU? really weird.. hope sony got a discount on the price of the core then... Reply
  • Calin - Friday, May 20, 2005 - link

    Just remember that video chipsets developers have multiple teams working on different generations - they have one chip in early development and one in late development/near production. This means two teams at least (to keep up with the 6 month product launches that were the norm starting from some 3 years ago) Reply
  • Cygni - Friday, May 20, 2005 - link

    With the memory controller functions ALSO on the 360 GPU, 150 million is pretty darn out of reach, im thinking. The 360 GPU almost sounds like an integrated single chip north bridge. It will be interesting to crack an Xbox360 open and get a peak at the insides when it ships. Should be interesting.

    I couldnt help but wonder if perhaps this is a tile based arch... any info on that? If so, it would answer a few of my questions about the system.
    Reply
  • Anand Lal Shimpi - Friday, May 20, 2005 - link

    Low Roller

    Jarred is correct, the IGN figures have to be wrong. Remember that R420, a 16 pipe design, was already around 160M transistors. The Xbox 360 GPU has 48 pipes, although they are unified shader pipes. Add in the additional logic to deal with handling both vertex and pixel instruction streams and you are already dealing with a GPU that is larger than the R420.

    Not to mention the 10MB of embedded DRAM, which will not be tiny.

    Take care,
    Anand
    Reply
  • IceWindius - Friday, May 20, 2005 - link

    I think im gonna give up on the upgrade race for PC's, im so tired of it. Think i'll go buy Xbox360, PS3 and come out money ahead. Reply

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