AGP - Advantages Over PCI

Advantages Over PCI

As computers increasingly became graphically oriented, successive generations of graphics adapters began to push the limits of PCI, a bus with shared bandwidth. This led to the development of AGP, a "bus" dedicated to graphics adapters.

The primary advantage of AGP over PCI is that it provides a dedicated pathway between the slot and the processor rather than sharing the PCI bus. In addition to a lack of contention for the bus, the direct connection allows for higher clock speeds. AGP also uses sideband addressing, meaning that the address and data buses are separated so the entire packet does not need to be read to get addressing information. This is done by adding eight extra 8-bit buses which allow the graphics controller to issue new AGP requests and commands at the same time with other AGP data flowing via the main 32 address/data (AD) lines. This results in improved overall AGP data throughput.

In addition, to load a texture, a PCI graphics card must copy it from the system's RAM into the card's Video memory, whereas an AGP card is capable of reading textures directly from system RAM using the graphics address remapping table, which reapportions main memory as needed for texture storage, allowing the graphics card to access them directly. The maximum amount of system memory available to AGP is defined as the AGP aperture.