AGP 8X A Closer Look - AGP 8x A Closer Look

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Lets take a look at a chart provided by nVidia showing the subsystem signal flow and where AGP 4X is falling short.

 

Chart provided by NVIDIA Corporation

Notice how all buses except PCI are pushing quite a bit more bandwidth than the AGP 4X bus. Moving from 1GB/sec. to 2.1GB/sec. will even out the flow of data between itself and other system and memory throughput rates. Resulting in higher performance overall, and getting us much closer to that real-time experience we are looking for.

Moving on, 8X AGP brings a few more advancements to the table in addition to twice the amount of bandwidth. These include:

• Direct Access to System Memory
• New signaling scheme with few inverted signals and a lower voltage swing.
• Power-on calibration scheme to optimize signal quality.
• Dynamic Bus Inversion to reduce the SSO noise.
• Isochronous mode operation to enable continuous data transfer for applications such as video streaming.

Isochronous Mode? Another buzzword we haven't heard before. Quite simply, Isochronous Mode provides predictable and uninterrupted data flow. This may not sound like much, but AGP 1.0 and 2.0 specs just could not deliver the amount of bandwidth that current applications need, such as video broadcasting and streaming video. AGP 3.0 makes it possible.

Now lets get a bit more technical for a moment and introduce clocking changes that AGP 3.0 brings about. The common clock frequency of the AGP spec remain the same, at 66MHz. However, we are now seeing source synchronous signals "strobed" at 8X the common clock frequency. Thus AGP 8X.

Below is a clocking sample of AGP 8X Side Band Addressing showing three consecutive SBA requests.

1. To initiate a new memory transaction, an AGP Master may need to generate anywhere from one to all four request Types. A complete request bundle including Types 4, 3, 2, and 1 now fits into one, common clock cycle in AGP8X. Such a request requires two common clock cycles in AGP4X and four common clock cycles in AGP2X.

2. While it is possible to initiate multiple transactions2 in one common clock cycle, AGP enforces a rule of at most one transaction per common clock cycle. The first Type request of a transaction may start at any rising SB_STBF edge within a common clock cycle at the AGP3.0 Master. The Master must insert NOPs into any unused request slots.

3. The two strobes SB_STBF and SB_STBS alternately use the L->H edge to latch the SBA data. The illustration above shows the one strobe as the inverse of the other. However, the specification does not require this. The only requirement is for each strobe to provide an assertion edge centered in each alternate SBA transfer window.

Obviously AGP 8X is letting us negotiate more transactions per clock cycle, and this is in turn providing us better video subsystem performance.

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Compatibility
Another excellent feature of moving to AGP 8X is it's inherent backwards compatibility with both AGP 4X and AGP 2X. Currently, all video card manufacturers with an AGP 8X based card are keeping their hardware compatible with AGP 2X and 4X specs. Consequently, if your using an 8X capable card, it will typically utilize the highest available speed supported by your system. Good news for those of you who wish to keep their current systems, but want to take advantage of the latest video technology. You may not completely benefit from these capabilities with an older system, but a performance increase will be garnished nevertheless, and when you do upgrade your system, you'll be ready to go.

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Conclusion - The Real Deal
The technology behind the new AGP 8X Specification is actually quite complicated, and more than I can simply highlight to you in this article today. But my purpose here was not to fill you full of technical gibberish or introduce you to marketing buzzwords. More to bring a general understanding of how the new specification will be implemented, and what gains we can expect to see over the aging 4X specification. Plainly put, do I believe AGP 8X is a revolutionary step in the right direction? Yes, it is, and that step will ultimately improve our overall visual experience. Will we as consumers benefit from this technology in the way we see and experience graphics and video applications? Yes. Will the AGP 8X provide twice the amount of raw gaming and/or application performance seen in AGP 4X? No, it won't even come close at this point in time. To AGP 8X's defense however, the technology is not being billed as the savior of graphic solutions. Nor are there any claims that AGP 8X will deliver twice the performance of an AGP 4X video card. Still, we have to take into consideration that current applications simply are not optimized for the newer spec, or drivers, and even hardware for that matter. I think as time passes, benefits from the technology will eventually materialize to a greater point than where they are now, but by that time, we may very well be gearing up for AGP 4.0 / AGP 16X.
 

We'd like to thank you for joining us here today. If you would like to discuss this article, please head into the forums for a chat.

I'd also like to give personal thanks to NVIDIA Corporation, Intel Corporation and XFX a Division of PINE, for providing the technical information by which I was able to reference in order to write this review.

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