Digging deeper into the shader core

Many of the same patterns that lead designers of current hardware to their conclusions are still true today. For instance, pixels next to each other on the screen still tend to follow a very similar path through the hardware. This means that it still makes sense to process pixels in quads. As for changes, as hardware becomes more programmable, we are seeing a higher percentage of scalar data being used. In spite of the fact that much of the work done by graphics hardware is vector based, it becomes easier to schedule code if we are working with a bunch of parallel, independent, scalar processors. It is also more efficient to build separate units for texture addressing and filtering, and ATI has done this for quite some time now.

NVIDIA has finally decoupled the texture units from their shader hardware, enabling math and texturing to happen at the same time with no scheduling issues. They have also decided to implement their math hardware as a collection of scalar processors that can be used together to perform vector operations. NVIDIA calls the scalar processors Stream Processors (SPs), and they handle all the math performed in the shader core of G80.

It isn't surprising to see that NVIDIA's implementation of a unified shader is based on taking a pixel shader quad pipeline, and breaking up the vector units into 4 scalar units. Now, rather than 4 pixel quads, we see 16 SPs per "quad" or block of stream processors. Each block of 16 SPs shares 4 texture address units, 8 texture filter units, and an L1 cache.

G70 Pixel Shader Quad


G80 Stream Processor Block


The fact that these SPs are now independent and scalar gives NVIDIA the ability to keep more of them busy more of the time. This is very important as programmers start to write longer more complex shaders. Even while working with vectors, programmers need to use scalar values all the time to manipulate and evaluate data.

Each Stream Processor is able to complete one MAD and one MUL per clock cycle. While this is based on maximum throughput, we can reasonably expect to achieve this even though the hardware is pipelined. In spite of the 4 or 5 cycles (depending on precision) latency of a MUL in Conroe, SSE is now capable of one MUL per cycle throughput (as long as there are no stalls in the pipeline). Latency of operations in G80 could be even longer and sustain high throughput, as most of the time we are working with code that isn't riddled with dependencies.

The fact that each SP is capable of IEEE 754 single precision and can sustain high throughput for MAD and MUL operations while running any type of shader code makes this hardware very powerful and more general purpose than ever.

As a thread exits the SP, G80 is capable of writing the output of the shader to memory. The fact that SPs can do this at any time (except after pixel shaders) goes beyond the DX10 spec of just allowing for stream output after the Geometry Shader. On previous hardware, data would have to go through every stage of the pipeline until a value was finally written out to the frame buffer. Now, we can write data out at the end of anything but a pixel shader (as pixel shaders must send their output straight over to the ROPs for processing). This will be a great benefit to GPGPU (general purpose computing on graphics processing units).

G80: A Mile High Overview Branching, Early Z and Memory Interface
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  • JarredWalton - Wednesday, November 08, 2006 - link

    The text is basically complete, and minor spelling issues aren't going to change the results. Obviously, proofing 29 pages of article content is going to take some time. We felt our readers would be a lot more interested in getting the content now rather than waiting even longer for me to proof everything. I know the vast majority of readers don't bother to comment on spelling and grammar issues, but my post was to avoid the comments section turning into a bunch of short posts complaining about errors that will be corrected shortly. :) Reply
  • Iger - Wednesday, November 08, 2006 - link

    Pff, of course we would! If I would like to read a novel I would find a book! Results first - proofing later... if ever :) Thanks for the article! Reply
  • JarredWalton - Wednesday, November 08, 2006 - link

    Did I say an hour? Okay, how about I just post here when I'm done reading/editing? :) Reply
  • JarredWalton - Wednesday, November 08, 2006 - link

    Okay, I'm done proofing/editing. If you still see errors, feel free to complain. Like I said, though, try to keep them in this thread.

    --Jarred
    Reply
  • LuxFestinus - Thursday, November 09, 2006 - link

    Pg. 3 under <b>Unified Shaders</b>

    quote:

    <i>Until now, building a GPU with unified shaders would not have desirable, let alone practical, but Shader Model 4.0 lends itself well to this approach.</i>

    Should read as follows:
    <i>Until now, building a GPU with unified shaders would not have <b>been</b> desirable, let alone practical, but Shader Model 4.0 lends itself well to this approach.</i>

    Good try though.;)
    Reply
  • shabby - Wednesday, November 08, 2006 - link

    $600 for the gtx and $450 for the gts is pretty good seeing how much they crammed into the gpu, makes you wonder why the previous gen topped 650 bucks at times. Reply
  • dcalfine - Wednesday, November 08, 2006 - link

    How does the 8800GTX compare to the 7950GX2? Not just in FPS, but also in performance/watt? Reply
  • dcalfine - Wednesday, November 08, 2006 - link

    Ignore ^^^
    sorry


    Hot card by the way!
    Reply
  • neogodless - Wednesday, November 08, 2006 - link

    I know you touched on this, but I assume that DirectX 10 is still not available for your testing platform, Windows XP Professional SP2, and additionally no games have been released for that platform. Is this correct? If so...

    Will DirectX 10 be made available for Windows XP?
    Will you publish a new review once Vista, DirectX 10 and the new games are available?

    Can we peak into the future at all now?
    Reply
  • JarredWalton - Wednesday, November 08, 2006 - link

    DX10 will be Vista only according to Microsoft. What that means according to some game developers is that DX10 support is going to be somewhat slow, and it's also going to be a major headache because for the next 3-4 years they will pretty much be required to have a DX9 rendering solution along with DX10. Reply

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