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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 1000:  Multimedia
      9  III  Video Cards
           9  Video Card Interfaces

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Accelerated Graphics Port (AGP)

The Accelerated Graphics Port, or AGP, is yet another expansion slot technology developed specifically for video cards.

Unlike VLB and PCI however, no other expansion cards are using AGP technology. The Intel Corporation, as a modification of their PCI architecture, developed the AGP bus.

Like a PCI, AGP has a 32-bit wide data bus. However instead of a theoretical frequency of half a system bus speed (PCI), the AGP bus can theoretically run at the same frequency as the system bus.

Most chipsets on motherboards limit the frequency of the AGP to 66MHz in practice. Even with this limitation, the AGP bus has a practical bandwidth of 254.3 MB/s.

One significant difference of AGP from its PCI cousin is that it has a direct peer-to-peer connection with the system memory.

It does not share any of its bandwidth with any other devices. AGP has been developed further and now offers a 2X mode that doubles its bandwidth to 508.6 MB/s. This doubling of the bandwidth is achieved by sending data on the rising and falling edge of a single clock cycle, much like the technology used in DDR SDRAM.

[spacer]4X AGP and Beyond

Of course a 4X mode had to be developed that doubles the bandwidth again to an amazing 1017 MB/s. Many new motherboards support 4X AGP, and new video cards that run in this mode are starting to find their way into the market. Unfortunately, because it has a direct connection with the system memory, it is currently limited to the bandwidth of the memory bus. You will recall from Chapter 0011, that the memory bus is a 64-bit wide data bus currently running at 133MHz. This leaves a memory bus bandwidth of 1,014 MB/s, slightly under 4X AGP. Hopefully with new DDR SDRAM technologies becoming commonplace in the market, the full potential of a 4X AGP video card will be realized. However if history has taught us anything, as soon as this becomes a reality we will have 8X AGP architecture.



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