Accelerated Graphics Port (AGP)

The Accelerated Graphics Port (often shortened to AGP) is a high-speed point-to-point channel for attaching a video card to a computer's motherboard, primarily to assist in the acceleration of 3D computer graphics. Since 2004 AGP has been progressively phased out in favor of PCI Express (PCIe). By mid-2009 PCIe cards dominated the market; AGP cards and motherboards were still produced, but OEM driver support was minimal.

 

Advantages over PCI

 
As computers became increasingly 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 framebuffer, 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.

 

Versions

 

AGP and PCI: 32-bit buses operating at 66 and 33 MHz respectively

 

Specification	Speed	Pumping	Rate (MB/s)	Frequency (MHz)	Voltage (V)
PCI	-	single	133	33	3.3
AGP 1.0	1×	single	266	66	3.3
AGP 1.0	2×	double	533	66	3.3
AGP 2.0	4×	quad	1066	66	1.5
AGP 3.0	8×	octuple	2133	66	0.8
AGP 3.5 *	8×	octuple	2133	66	0.8

 

Official extensions

AGP Pro

It is an official extension for cards that required more electrical power. It is a longer slot with additional pins for that purpose. AGP Pro cards were usually workstation-class cards used to accelerate professional computer-aided design applications employed in the fields of architecture, machining, engineering, simulations, and similar fields.
 

64-bit AGP

A 64-bit channel was once proposed as an optional standard for AGP 3.0 in draft documents, but it was dropped in the final version of the standard.

The standard allows 64-bit transfer for AGP8× Reads, Writes, and Fast Writes; 32-bit transfer for PCI operations.
 

Unofficial variations

A number of non-standard variations of the AGP interface have been produced by manufacturers.

 

Internal AGP interface

Ultra-AGP, Ultra-AGPII

It is an internal AGP interface standard used by SiS for the north bridge controllers with integrated graphics. The original version supports same bandwidth as AGP 8×, while Ultra-AGPII has maximum 3.2GB/s bandwidth.

 

PCI-based AGP ports

AGP Express

Not a true AGP interface, but allows an AGP card to be connected over the legacy PCI bus on a PCI Express motherboard. It is a technology used on motherboards made by ECS, intended to allow an existing AGP card to be used in a new motherboard instead of requiring a PCIe card to be obtained (since the introduction of PCIe graphics cards few motherboards provide AGP slots). An "AGP Express" slot is basically a PCI slot (with twice the electrical power) with an AGP connector. It offers backward compatibility with AGP cards, but provides incomplete support (some AGP cards do not work with AGP Express) and reduced performance—the card is forced to use the shared PCI bus at its lower bandwidth, rather than having exclusive use of the faster AGP.

AGI

The ASRock Graphics Interface (AGI) is a proprietary variant of the Accelerated Graphics Port (AGP) standard. However, it is not fully compatible with AGP, and several video card chipsets are known not to be supported.

AGX

The EpoX Advanced Graphics eXtended (AGX) is another proprietary AGP variant with the same advantages and disadvantages as AGI.

XGP

The Biostar Xtreme Graphics Port is another AGP variant, also with the same advantages and disadvantages as AGI and AGX.

 

PCIe based AGP ports

AGR

The Advanced Graphics Riser is a variation of the AGP port used in some PCIe motherboards made by MSI to offer limited backwards compatibility with AGP. It is, effectively, a modified PCIe slot allowing for performance comparable to an AGP 4×/8× slot, but does not support all AGP cards; the manufacturer published a list of some cards and chipsets that work with the modified slot.

   

Compatibility

 
AGP cards are backward and forward compatible within limits. 1.5 V-only keyed cards will not go into 3.3 V slots and vice versa, though "Universal" cards exist which will fit into either type of slot. There are also unkeyed "Universal" slots that will accept either type of card. When an AGP Universal card is plugged-into an AGP Universal slot, only the 1.5 V portion of the card is used. AGP Pro cards will not fit into standard slots, but standard AGP cards will work in a Pro slot. Motherboards equipped with a Universal AGP Pro slot will accept a 1.5 V or 3.3 V card in either the AGP Pro or standard AGP configuration, a Universal AGP card, or a Universal AGP Pro card. Some cards incorrectly have dual notches, and some  motherboards incorrectly have fully open slots, allowing a card to be plugged into a slot that does not support the correct signaling voltage, which may damage card or motherboard. Some incorrectly-designed older 3.3 V cards have the 1.5 V key.

 

Power consumption

 
Actual power supplied by an AGP slot depends upon the card used. The maximum current drawn from the various rails is given in the specifications for the various versions. For example, if maximum current is drawn from all supplies and all voltages are at their specified upper limits, an AGP 3.0 slot can supply up to 48.25 watts, this figure can be used to specify a power supply conservatively, but in practice a card is unlikely ever to draw more than 40 W from the slot, with many using less. AGP Pro provides additional power up to 110 W. Many AGP cards had additional power connectors to supply them with more power than the slot could provide.

 

Legacy use

 
By 2010 few new motherboards had AGP slots. No new motherboard chipsets were equipped with AGP support, but motherboards continued to be produced with older chipsets with support for AGP.

Graphics processors of this period use PCI-Express, a general-purpose (not restricted to graphics) standard that supports higher data transfer rates and full-duplex. To create AGP-compatible graphics cards, those chips require an additional PCIe-to-AGP bridge-chip to convert PCIe signals to and from AGP signals. This incurs additional board costs due to the need for the additional bridge chip and for a separate AGP-designed circuit board. Various manufacturers of graphics cards continued to produce AGP cards for the shrinking AGP user-base.

References

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17. AMD community forums