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AGP Graphics Cards
The demands of fast-paced video games require more speed and greater bandwidth than the PCI Bus can provide. Thus, the AGP slot was born, providing a dedicated interface to transfer graphics data only.
The letters AGP stand for Accelerated Graphics Port, and it is the term used to describe a dedicated, point-to-point interface that connects a video card directly to the system's memory and processor.
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AGP was first introduced by Intel in 1996, and is based off of their previous work in developing the PCI bus. Despite being based on PCI technology, the AGP and PCI slots on a motherboard are not interchangeable, so an AGP card can not be installed into a PCI slot, and vice versa.
The initial release of AGP saw a sizeable performance boost over PCI, and the few revisions to the standard helped increase this even more as years went by. Other than having a dedicated path to the system's memory and processor, several other design features help AGP outperform PCI when it comes to graphics performance.
AGP allows the operating system to store texture maps in the system's memory which allows for more space, and perhaps faster access, rather than being limited to the use of graphics card memory only. Graphics art address remapping table, also known as GART, is a term used to describe a process that maps physical memory as virtual memory for the storage of texture maps. Basically, GART takes the system memory it is allowed to use to store texture maps and re-addresses it so that the system thinks these maps are now actually being stored in the frame buffer, or virtual memory. This might not sound like anything special, but this re-addressing requires that the texture map be written to memory only once and it is locked into place right where the AGP card can find it quickly. |
AGP can be broken down into different groups based on revisions to the specification (AGP 1.0, AGP 2.0, and AGP 3.0), as well as by the general speeds (1x, 2x, 4x, and 8x). There is overlap between the various categories, with AGP 1.0 supporting 1x and 2x, AGP 2.0 supporting 1x, 2x, and 4x, and AGP 3.0 supporting 4x and 8x. For a complete break down of all the combinations available, please visit this page.
Before taking a look at the specifications of AGP, let's have a refresher as to what was available on PCI prior to the birth of AGP. The standard PCI bus has a width of 32-bit, operates at 33 MHz, provides a maximum bandwidth of 132 MB/s (which has to be shared by all devices connected), and operates on 3.3V (or 5V on the original standard).
Given the timeline of the evolution of these cards, AGP 8x cards dominate today's marketplace. Finding some cards that are backwards compatible is possible, but the tricky part may be ensuring that the slot on the motherboard will accept them. Comparing the connector on this 128MB Apollo GeForce FX6600 GT card, to the connector on this 64MB Hercules 3D Prophet Ultra II card, and to the connector on this 256MB Chaintech GeForce FX5200 card shows that the first one is obviously different than the second two. The Apollo card is 8x only, the Hercules card is 4x/2x compatible, and the Chaintech card is 8x/4x, which results in different notches in the connector.
AGP 1.0 only features a 3.3V connection, the release of AGP 2.0 saw the availability of both a 3.3V and 1.5V connector, and AGP 3.0 uses the same 1.5V, but only requires 0.8V for signaling. In order to protect cards of different voltages/formats, special keyed connectors were designed so that only the correct card could be installed on any motherboard. A universal connector was eventually released for AGP 1.0/2.0 which allowed cards of either voltage to be installed. For a schematic of the various connectors, please visit this page. Although AGP 3.0 can share in the use of a universal connection, many motherboards now only support 4x/8x cards based on the AGP 3.0 standard.
Another specification for AGP was released between 2.0 and 3.0, and was referred to as AGP Pro. AGP Pro was intended to be the new standard to meet the demands of high powered graphics workstations, but it never really garnered widespread acceptance. Speeds of 1x, 2x, and 4x were supported with AGP Pro, and it utilized either a 3.3V, 5V, or a universal connector, similar to AGP 2.0. But, the AGP Pro connector was not the same size as the 'standard' AGP connector (see schematic at link above), meaning there were now three more possible connections to consider. An AGP Pro connection is longer than a standard AGP connection, and depending on the connector type, it could accept AGP 1.0 and 2.0 cards.
Modern motherboards supporting AGP will specify what type of card is compatible with the board, so the guess work is eliminated when trying to match one with the other. For example, this Socket 754 Chaintech motherboard specifies that it has one AGP 4x/8x slot and this Biostar LGA 775 motherboard specifies that it has one 8x AGP slot.
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