ACCELERATED GRAPHICS PORT (AGP)
The latest addition to most computer systems was the inclusion of the accelerated graphics port (AGP), until the PCI Express interface became available for video. The AGP operates at the bus clock frequency of the microprocessor. It is designed so that a transfer between the video card and the system memory can progress at a maximum speed. The AGP can transfer data at a maximum rate of 2G bytes per second. This port probably will never be used for any devices other than the video card, so we do not devote much space to its coverage. Because PCI Express video cards use 8 lanes, data transfer occurs at a rate of 4 GBps for the x16 PCI Express video card.
Figure 15–22 illustrates the interface of the AGP to a Pentium 4 system and the placement of other buses in the system. The main advantage of the AGP bus over the PCI bus is that the AGP can sustain transfers (using the 8X compliant system) at speeds up to 2G bytes per second. The 4X system transfers data at rates of over 1G byte per second. The PCI bus has a maximum transfer speed of about 133M bytes per second. The AGP is designed specifically to allow high- speed transfers between the video card frame buffer and the system memory through the chip set.
