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Graphics Card Interface

Graphics Beginners' Guide, Part 2: Graphics Technology
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All data communicated between the graphics card and the rest of the computer travels through the graphics card's slot, or interface. There are three types of graphics interfaces currently in use: PCI, AGP, and PCI Express. Different graphics interfaces allow for different amounts of bandwidth, and more bandwidth means better performance. It is important to note that contemporary graphics cards are only capable of using so much bandwidth. At a certain point, if this interface bandwidth is sufficient, it no longer remains a bottleneck.

The slowest graphics card bus, the PCI bus (Peripheral Components Interconnect), is detrimental to graphics card performance. AGP (Accelerated Graphics Port) is much better, but even the AGP 1.0 and AGP 2x specifications can limit performance. However, once we reach AGP 4x, we begin to approach the practical limit of bandwidth required for contemporary graphics cards. The AGP 8x specification has twice the bandwidth of the AGP 4x specification (2.16 GB/s), but there is a negligible difference in performance between these two standards.

The newest and highest-bandwidth interconnect is the PCI Express bus. New graphics cards typically use the PCI Express x16 specification that combines 16 separate PCI Express links (or lanes) to reach as much as 4 GB/s bandwidth. This is twice the bandwidth of AGP 8x. PCI Express offers this bandwidth both for uploading data to the computer and downloading to the graphics cards. However, the AGP 8x specification is still so adequate that we have not seen an example of a PCI Express graphics card performing better than an AGP 8x model (assuming all other hardware and parameters are equal). For example, an AGP version of a GeForce 6800 Ultra will perform identically to a PCI Express 6800 Ultra.

PCI Express is the preferred graphics card slot today and will be around for several years. The highest performance parts are not manufactured for the aging AGP 8x bus anymore, and PCI Express solutions tend to be cheaper and easier to find now than their AGP counterparts.

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