AGP Bus Tutorial

Operation Modes

The operation modes of the AGP bus refer to the amount of data that are transferred by clock cycle. The 1x AGP is capable of transferring only one datum per clock tick. Since the AGP bus works at 66 MHz (66.66 MHz to be exact), transferring 32 bits of data at a time, the maximum transfer rate of the 1x AGP bus is 266 MB/s, twice as much as that of the PCI bus. The 2x AGP bus works transferring two data per clock cycle, which results in a transfer rate of 533 MB/s. The second version of the AGP bus introduced the 4x operation mode, which allows transferring four data per clock cycle, obtaining then a transfer rate of 1,066 MB/s. The newest video cards allow operating in the 8x mode, which has a transfer rate of 2,133 MB/s, sixteen times higher than that of the PCI bus!

Mode Clock Number of bits Data per clock cycle Transfer Rate
x1 AGP 66 MHz 32 bits 1 266 MB/s
x2 AGP 66 MHz 32 bits 2 533 MB/s
x4 AGP 66 MHz 32 bits 4 1,066 MB/s
x8 AGP 66 MHz 32 bits 8 2,133 MB/s

AGP Bus Tutorial

Figure 2: Comparison between the transfer rates of the PCI bus and the AGP bus.

Besides working at high transfer rates, the AGP bus also allows the video card to use the RAM memory of the PC as an extension of its video memory, for the storage of textures and the z-buffering (responsible for the depth vector in 3D images), which considerably increases its performance, since the memory bus is faster than the AGP bus. That resource is known as DIME (Direct Memory Execute) or AGP Texturing and is not supported by all AGP video cards.

Author: Cassio Lima

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