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  • SharkyForums.Com - Print: is AGP 8x a sham?

    is AGP 8x a sham?
    By ilsie December 15, 2000, 02:02 PM

    I'm confused.

    If I'm correct in thinking, the reason AGP 4x gives you almost no performance increase over AGP 2x is thus:

    AGP runs natively at 66MHz. Double pump that, you effectively get a 133MHz bus rate. Thus, AGP is transferring/receiving addresses/data to the Northbridge at 133MHz. The Northbridge, on the other hand, is talking to system memory at a MAX speed of 133MHz, given the technology of memory today. (Not talking about DDR, of course) Even with this, there are some lost cycles, etc. since the AGP bus doesnt get exclusive rights to the memory bus. (Yes, I know it gets it's own allocated memspace when it needs it, but it still has to share the memory bus with the processor.)

    So... AGP 4x is basically useless until DDR memory/motherboards begin to show up on the market, at which point we should theoretically see a large performance increase, at least in texture streaming.

    My question is, why is everybody hyped about AGP 8x? If I'm right, this wont even give you a performance increase over AGP 2x with our current systems, and will again give us no performance increase over AGP 4x until we have effective memory bus speeds of 532MHz.

    Sounds like a marketing scam to me.

    By ilsie December 15, 2000, 02:05 PM

    P.S. If I have been misinformed/misinterpreting any information in this post, please let me know.

    By cracKrock December 15, 2000, 02:27 PM

    No, it may not be a marketing scam. I sort of see it like the Ultra ATA interface. IDE hard drives still can't transfer faster than 30MB/sec, but it gives them headroom for the technology to improve. (30MB/sec is based on my own benchmarks.)

    By Moridin December 15, 2000, 03:36 PM

    All this is correct AFAIK. You are missing one thing though. Most software doesn’t push anywhere near enough triangles to the video card to saturate AGP 1X let alone AGP 4X. I saw an article a while ago comparing the performance of a video card (Voodoo ??) which had both PCI and AGP versions. The PCI card was within 10% of AGP 4X on every common consumer app.

    The only thing that could make use of the available bandwidth right now is AGP texturing, but nobody is doing that since most video card makers are putting plenty of RAM on the card and AGP texturing is much slower.

    That doesn’t mean that 4X and 8X won't be important in the future though, they should increase in importance as the amount of detail in games and apps increase.

    It nice to have a problem solved before it becomes a bottleneck for a change.

    By Marsolin December 15, 2000, 07:10 PM

    AGP8X is not a sham, although it may not provide much real world performance increase for most users. It will provide 2 GB/s of bandwidth. Current graphics cards uses methods that allow them to store textures in local memory and, as a result, do not require the bandwidth that AGP provides.

    Perhaps I should state that differently. It's not that they don't need it, but that it's not enough. It has been much documented on games like Quake 3 that fps tops out at high resolution due memory bandwidth limitations.

    Take the nVidia Quadro2 MXR that is used with some SGI workstations. It has a memory bandwidth of 2.93 GB/s. It is also dedicated bandwidth that they now will always be available. AGP is controlled by another chip and must be fed from the CPU or system memory, and is more likely to typically operate at a lower percentage of its peak bandwidth.

    Ideal solutions would result in a mix that streams certain features over AGP, while relegating the most critical to the local graphics memory. As AGP bandwidth increases more features can be left to it, freeing local memory for higher resolutions.

    Of course there are further generations of graphics ports in development, but for now AGP8X is the future. I've heard rumors of Serial AGP to follow that, but I don't know any details.

    I should also mention that I don't work in graphics. This is just my take from presentations and specs that I have read.


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