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How PCI Express Works

        Tech | Buses

PCI Express and Advanced Graphics
NVIDIA SLI link card
NVIDIA SLI link card
Photo courtesy NVIDIA

We've established that PCIe can eliminate the need for an AGP connection. A x16 PCIe slot can accommodate far more data per second than current AGP 8x connections allow. In addition, a x16 PCIe slot can supply 75 watts of power to the video card, as opposed to the 25watt/42 watt AGP 8x connection. But PCIe has even more impressive potential in store for the future of graphics technology.

With the right hardware, a motherboard with two x16 PCIe connections can support two graphics adapters at the same time. Several manufacturers are developing and releasing systems to take advantage of this feature: NVIDIA Scalable Link Interface (SLI): With an SLI-certified motherboard, two SLI graphics cards and an SLI connector, a user can put two video cards into the same system. The cards work together by splitting the screen in half. Each card controls half of the screen, and the connector makes sure that everything stays synchronized.

ATI CrossFire: Two ATI RadeonĀ® video cards, one with a "compositing engine" chip, plug into a compatible motherboard. ATI's technology focuses on image quality and does not require identical video cards, although high-performance systems must have identical cards. Crossfire divides up the work of rendering in one of three ways:

  • splitting the screen in half and assigning one half to each card (called "scissoring")
  • dividing up the screen into tiles (like a checkerboard) and having one card render the "white" tiles and the other render the "black" tiles
  • having each card render alternate frames

Alienware Video Array: Two off-the-shelf video cards combine with a Video Merger Hub and proprietary software. This system will use specialized cooling and power systems to handle all the extra heat and energy from the video cards. Alienware's technology may eventually support as many as four video cards.

Since PCI, PCI-X and PCI Express are all compatible, all three can coexist indefinitely. So far, video cards have made the fastest transition to the PCIe format. Network and sound adapters, as well as other peripherals, have been slower in development. But since PCIe is compatible with current operating systems and can provide faster speeds, it is likely that it will eventually replace PCI as a PC standard. Gradually, PCI-based cards will become obsolete.

For more information about PCI Express and related topics, check out the links on the next page.


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