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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  • PS3 Masterbater 5 - Tuesday, January 09, 2007 - link

    I WOULD JUST LIKE TO SAY THAT IF PS3 HAD A HOLE IN IT I WOULD INSERT MY PENIS IN AND MAKE SWEET LOVE TO IT BECAUSE IT IS THE GREATEST THING EVER. NINTENDO WII CAN SUCK MY HUGE COCK BECAUSE ITS A LITTLE BITCH AND IT IS THE POOR MANS PS3. IF NINTENDO WII WAS A MAN IT WOULD HAVE A VERY SMALL PENIS AND STILL BE A VIRGIN YOU GUYS ARE SO JEALOUS THAT I HAD THE FIRST PS3 EVER AND I WILL DOMINATE ANYONE IN "RESISTANCE : FALL OF MAN"
    Reply
  • Wizzdo - Friday, January 14, 2011 - link

    Definitely a bigger head below than above! Reply
  • steveyoung123456789 - Friday, December 09, 2011 - link

    your a virgin pussy and if i ever find out where you live i will kick your ass!!!!!!!!!!!! Reply
  • steveyoung123456789 - Friday, December 09, 2011 - link

    Btw your a psycho for wanting to fuck a gaming cousel... smh.... queef!! Reply
  • SilverTrine - Friday, November 17, 2006 - link

    The GPU in the Ps3 is more than enough for what its intended for. Theres no magic in GPUs they're just specialized processors.

    In the Xbox360 the GPU carries more of the processing load. Remember the unified ram that the GPU uses in the Xbox360 is 700mhz fast.

    The GPU in the Ps3 also has 700mhz ram. However the Cell processor has access to XDR ram running at a whopping 3.2ghz! In the Ps3 system the Cell with the superfast XDR ram will do more of the grunt work and rely less on the GPU.

    Saying the GPU in the Xbox360 somehow gives the system is a mistake. What would you rather have doing processing work a GPU running relatively slow with 700mhz ram or a extremely fast Cell processor with 3.2ghz XDR ram?

    However utilizing this on the Ps3 will require more specialized programming, the Xbox360 because its fairly conventional will be able to tap more of its power sooner than the Ps3.
    Reply
  • tipoo - Wednesday, August 06, 2014 - link

    Uhh, "mhz fast" doesn't matter an iota. The bandwidth of that XDR RAM was still 25GB/s to the Cell, it just works in a different way than GDDR, needs a higher clock speed for similar bandwidth. The clock speed was no advantage. And the RSX could only get data back at 15GB/s from the Cell going to the XDR. Reply
  • theteamaqua - Tuesday, July 12, 2005 - link

    http://theconsolewars.blogspot.com/2005/05/xbox-36...
    i just wan people to know that how bias this site is, i mean this guy has no idea what he is talking about
    Reply
  • jwix - Wednesday, July 06, 2005 - link

    #77 I wouldn't say Anand's article was "full of shit." I would say it was a bit sensationlist, as stated in the Arstechnica article. What surprised me more than anything was that Anand would post such an article, then remove it so quickly. That's not his style.
    Bottom line though - these consoles will offer nothing new or innovative in the way of gameplay. I think I'll stick with my PC and Nintendo DS for now.
    Reply
  • steveyoung123456789 - Friday, December 09, 2011 - link

    get a life Reply
  • calimero - Wednesday, July 06, 2005 - link

    http://arstechnica.com/news.ars/post/20050629-5054...

    btw Anand article was "full of shit" (sorry but that is the right phrase) and it's not odd that Anand pull it. It's quite embarassing for Anand; someone already told: one thing is to write test of CPU speed and speed of graphics card in games... and another to analyse CPU architecture.
    Reply

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