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
POST A COMMENT

93 Comments

View All Comments

  • tipoo - Wednesday, August 6, 2014 - link

    That was such bullshit. RSX was worth under 200Gflops, Cell about the same and much harder to extract that much from, Xenos was over 200, Xenon was around 100. Nothing was near the terraflop range except in marketing bullspeak. Reply
  • LanceVance - Saturday, June 25, 2005 - link

    #59

    "Does every xbox game needs to be playable? No."
    "How many of you still play your old games? Market research shows not a lot."

    Backwards compatibility is a feature. It's just like any other feature on a mass market consumer product. Some people value it and others don't.

    You clearly don't value that and won't consider it when making consumer decisions.

    Other people clearly DO value that and you are trying to persuade them not to. That's none of your business. If people value a specific feature they have every right to consider it in their buying behaviors.

    Sorry, I know this post has now gone way off topic from the original article of technical analysis. Beautiful article; but any open forum on such a political topic is doomed to degenerate into this.
    Reply
  • knitecrow - Saturday, June 25, 2005 - link

    software emulation is difficult and takes a lot of man power to get right.

    The main advantage for microsoft was that they didn't have to stuck with poor business and engineering design choices of the past.

    Does every xbox game needs to be playable? No. I don't care for games like "big rigs"
    I abviously want halo to work, but also the lesser known good titles on xbox ... like Panzer Dargoon Orta, Kingdom Under Fire, Otogi 1 & 2 and so on.

    How many of you still play your old games? Market research shows not a lot.I have an 80+ games library for the xbox. I don't mind.
    Reply
  • BenSkywalker - Saturday, June 25, 2005 - link

    milomnderbnder21-

    They are flipping consumers off. Sony and Nintendo at this point are both adding costs to their consoles to incorporate hardware to make certain that their systems have full compatability with the prior generation. MS has decided that you and I are not good enough to offer that same assurance. They have decided to save themselves a couple of dollars and render useless all of their games they can't get running on their new platform so they can save a few dollars. They will take a shot at software hacks- if they don't work we are out of luck. They are also stopping production of current gen XBox hardware. I have about thirty games for my XB currently, what do I do with them if my XB dies and they aren't supported by XB360(which there is no assurance they will)? I keep my legacy hardware around, back to my 2600, and my games.
    Reply
  • Olaf van der Spek - Saturday, June 25, 2005 - link

    > Compared to the built in 5X CAV DVD drive in the Xbox, the hard drive offered much faster performance. With the Xbox 360, the performance demands on the hard drive are lessened, the console now ships with a 12X CAV DVD-DL drive.

    Aren't all read-only DVD drives dual-layer?
    Reply
  • Starglider - Saturday, June 25, 2005 - link

    I'm a game programmer and I take issue with the statement on page 4 that BSP collision detection benefits from branch prediction. It doesn't; it's one of the rare types of code where the branches are effectively impossible to predict. The algorithm /does/ benefit heavily from speculative execution, but as I understand it neither the XBox360 or the PS3 are capable of this. As such this is one area where PC style processors have an advantage; neither console is going to beat a modern PC at SuperPi. Reply
  • devilzblood - Friday, June 24, 2005 - link

    altho this is my first post at anandtech, i have been reading ur articles from the geforce 3 launch.
    newayz.....im posting here coz im wondering if neone knos this.....what degrees do Anand and Derek hold?? they seem to be such a bank of information, i never thought it was humanly possible to know so much about computers..needless to say im impressed by u people..and all i would like to say about the article is that it was an informative and enjoying read
    Reply
  • milomnderbnder21 - Friday, June 24, 2005 - link

    #46

    MS is by no means "flipping off it's supporters" with regards to backwards compatibility. They have flat out stated that it is there goal that EVERY Xbox game be compatible on the 360, but they simply cannot guarentee it. In any case, look for a majority of them to be so. And if they can't get everything working, I'm not going to miss outlaw golf on my 360...
    Reply
  • MDme - Friday, June 24, 2005 - link

    #50

    Sony WILL support 1080p. They are supporting it so that they can BRAG about it. It's all about the hype, even if they only have 1 game supporting it, they will BRAG about it. heck, if you really think about it, if sony played a video (H.264) at 1080p then ran the game at 720p they will still claim, WE HAVE SUPPORT FOR 1080p. It's all marketing. Even X360 can claim this.
    Reply
  • finbarqs - Friday, June 24, 2005 - link

    bla bla bla, which one is better? Reply

Log in

Don't have an account? Sign up now