The Xbox 360 GPU: ATI's Xenos

On a purely hardware level, ATI's Xbox 360 GPU (codenamed Xenos) is quite interesting. The part itself is made up of two physically distinct silicon ICs. One IC is the GPU itself, which houses all the shader hardware and most of the processing power. The second IC (which ATI refers to as the "daughter die") is a 10MB block of embedded DRAM (eDRAM) combined with the hardware necessary for z and stencil operations, color and alpha processing, and anti aliasing. This daughter die is connected to the GPU proper via a 32GB/sec interconnect. Data sent over this bus will be compressed, so usable bandwidth will be higher than 32GB/sec. In side the daughter die, between the processing hardware and the eDRAM itself, bandwidth is 256GB/sec.

At this point in time, much of the bandwidth generated by graphics hardware is required to handle color and z data moving to the framebuffer. ATI hopes to eliminate this as a bottleneck by moving this processing and the back framebuffer off the main memory bus. The bus to main memory is 512MB of 128-bit 700MHz GDDR3 (which results in just over 22GB/sec of bandwidth). This is less bandwidth than current desktop graphics cards have available, but by offloading work and bandwidth for color and z to the daughter die, ATI saves themselves a good deal of bandwidth. The 22GB/sec is left for textures and the rest of the system (the Xbox implements a single pool of unified memory).

The GPU essentially acts as the Northbridge for the system, and sits in the middle of everything. From the graphics hardware, there is 10.8GB/sec of bandwidth up and down to the CPU itself. The rest of the system is hooked in with 500MB/sec of bandwidth up and down. The high bandwidth to the CPU is quite useful as the GPU is able to directly read from the L2 cache. In the console world, the CPU and GPU are quite tightly linked and the Xbox 360 stands to continue that tradition.

Weighing in at 332M transistors, the Xbox 360 GPU is quite a powerful part, but its architecture differs from that of current desktop graphics hardware. For years, vertex and pixel shader hardware have been implemented separately, but ATI has sought to combine their functionality in a unified shader architecture.

What's A Unified Shader Architecture?

The GPU in the Xbox 360 uses a different architecture than we are used to seeing. To be sure, vertex and pixel shader programs will run on the part, but not on separate segments of the hardware. Vertex and pixel processing differ in purpose, but there is quite a bit of overlap in the type of hardware needed to do both. The unified shader architecture that ATI chose to use in their Xbox 360 GPU allows them to pack more functionality onto fewer transistors as less hardware needs to be duplicated for use in different parts of the chip and will run both vertex and shader programs on the same hardware.

There are 3 parallel groups of 16 shader units each. Each of the three groups can either operate on vertex or pixel data. Each shader unit is able to perform one 4 wide vector operation and 1 scalar operation per clock cycle. Current ATI hardware is able to perform two 3 wide vector and two scalar operations per cycle in the pixel pipe alone. The vertex pipeline of R420 is 6 wide and can do one vector 4 and one scalar op per cycle. If we look at straight up processing power, this gives R420 the ability to crunch 158 components (30 of which are 32bit and 128 are limited to 24bit precision). The Xbox GPU is able to crunch 240 32bit components in its shader units per clock cycle. Where this is a 51% increase in the number of ops that can be done per cycle (as well as a general increase in precision), we can't expect these 48 piplines to act like 3 sets of R420 pipelines. All things being equal, this increase (when only looking at ops/cycle) would be only as powerful as a 24 piped R420.

What will make or break the difference between something like a 24 piped R420 and the unified shaders of the Xbox GPU is how well applications will lend themselves to the adaptive nature of the hardware. Current configurations don't have nearly the same vertex processing power as they do pixel processing power. This is quite logical when we consider the fact that games have many more pixels displayed than vertices. For each geometry primitive, there are likely a good number of pixels involved. Of course, not all titles will need the same ratio of geometry to pixel power. This means that all the ops per clock could either be dedicated to geometry processing in truly polygon intense scenes. On the flip side (and more likely), any given clock cycle could see all 240 ops being used for pixel processing. If game designers realize this and code their shaders accordingly, we could see much more focused processing power dedicated to a single type of problem than on current hardware.

ATI is predicting that developers will use lots of very small triangles in Xbox 360 games. As engines like Epic's Unreal Engine 3 have shown incredible results using pixel shaders and normal maps to augment low geometric detail, we can't tell if ATI is trying to provide the chicken or the egg. In other words, will we see many small triangles on Xbox 360 because console developers are moving in that direction or because that is what will run well on ATI's hardware?

Regardless of the paths that lead to this road, it is obvious that the Xbox 360 will be a geometry power house. Not only are all 3 blocks of 16 shaders able to become vertex shaders, but ATI's GPU will be able to handle twice as many z operations if a z only pass is performed. The same is true of current ATI and NVIDIA hardware, but the fact that a geometry only pass can now make use of shader hardware to perform 48 vector and 48 scalar operations in any given clock cycle while doing twice the z operations is quite intriguing. This could allow some very geometrically complicated scenes.

How Many Threads? Inside the Xenos GPU
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  • Darkon - Friday, June 24, 2005 - link

    #49

    WTF are you talking ?

    The Cell does general-purpose processing although not as good as 360 cpu.


    And Anand I suggest you do some more research on cell
    Reply
  • Alx - Friday, June 24, 2005 - link

    Someone explain to me how Sony will support 1080p please. If developers make the games run at acceptable framerate at that resolution, most people running them at 720p and 480i will be wasting at least half of PS3's rendering power.

    On the other hand if XBOX360 game devs make their games run just fast enough at 720p, that'll give them far more resources to work with than those poor Sony game devs.
    Reply
  • Shinei - Friday, June 24, 2005 - link

    That's not necessarily true, #48. The Cell processor doesn't do general-purpose processing, so it can't do decoding on its own--and as far as I know, even pressed DVDs have to be decoded by some kind of processor. (Of course, I know next to nothing about video equipment, so I could be wrong...) Reply
  • arturnow - Friday, June 24, 2005 - link

    Another difference between RSX and G70 is hardware video decoder - PureVideo, i'm sure RSX doesn't need that which saves transistors count Reply
  • freebst - Friday, June 24, 2005 - link

    Actually, in response to 31 there is no 1080p 60 frame/sec signal. the only HD signals are 1080 30p, 24p, 60i, 720 60p, 30p, 24p. Reply
  • BenSkywalker - Friday, June 24, 2005 - link

    Why the support for lower resolutions? I'm a bit confused by this- I can't see why anyone who isn't a fanatic loyalist wouldn't want to see the highest resolution possible supported by the consoles. The XBox(current) supports 1080i and despite the extreme rarity in which it is used- it IS used. Supporting 1080p x2 may seem like overkill, but think of the possibilities in terms of turn based RPGs or strategy games(particularly turn based) where 60FPS is very far removed from required.

    The most disappointing thing about the new generation of consoles is MS flipping its customers off in terms of backwards compatability. Even Nintendo came around this gen and MS comes up with some half done emulation that works on some of 'the best selling' games. Also, with their dropping production of the original XB already it appears they still have an enormous amount to learn about the console market(check out sales of the original PS after the launch of the PS2 for an example).
    Reply
  • Warder45 - Friday, June 24, 2005 - link

    errr #31 not 37 Reply
  • Warder45 - Friday, June 24, 2005 - link

    #37 is right on the money. There is a good chance that there will be no HDTV that can accept a 1080p signal by the time the PS3 comes out.

    It seems less like Sony future proofing the PS3 and more like Sony saying we have bigger balls then MS. Not to say MS is exempt from doing the same.
    Reply
  • IamTHEsnake - Friday, June 24, 2005 - link

    Excellent article Anand and crew.

    Thank you for the very informative read.
    Reply
  • masher - Friday, June 24, 2005 - link

    > "Collision detection is a big part of what is commonly
    > referred to as “game physics.” ..."

    Sorry, collision detection is computational geometry, not physics.

    > "However it is possible to structure collision detection for
    > execution on the SPEs, but it would require a different
    > approach to the collision detection algorithms... "

    Again, untrue. You walk the tree on the PPE, whereas you do the actual intersection tests on the SPs. The SPs are also ideally suited to calculating the positions of each object (read: real physics) and updating the tree accordingly.
    Reply

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