Accelerated Graphics Port Definition

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The Accelerated Graphics Port (also called Advanced Graphics Port, often shortened to AGP) is a high-speed point-to-point channel for attaching a graphics card to a computer's motherboard, primarily to assist in the acceleration of 3D computer graphics. Some motherboards have been built with multiple independent AGP slots. AGP is currently being phased out in favor of PCI Express.

Advanced Graphics Port (AGP) Advantages over PCI

As computers became increasingly graphically oriented, successive generations of graphics adapters began to push the limits of the PCI bus, leading to the development of the AGP bus, dedicated to graphics adapters. Most motherboards manufactured since the late 1990s include either an on-board integrated AGP adapter, or a physical AGP slot into which a separate AGP-based graphics card can be inserted.

For the needs of modern graphics adapters, the AGP bus is superior to PCI because it provides a dedicated pathway between the slot and the processor, allowing for faster communication between the two. AGP also uses sideband addressing, meaning that addressing for packets is carried outside of the packet, so the entire packet does not need to be read to get addressing information. 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 (GART). GART reapportions main memory for texture storage, allowing the graphics card to access them directly.

The two main reasons graphics cards with the PCI interface are still produced is that, first, they can be used in nearly any PC; because while some motherboards with built-in graphics adapters lack an AGP slot, few, if any, modern desktop PCs do not have PCI slots. Secondly, a user with an appropriate operating system can use several PCI graphics cards (or several PCI graphics cards in combination with one AGP card) simultaneously � to give many different video outputs (for the use of many screens). This is almost impossible with AGP 1.0 (early AGP 1x and 2x) and AGP 2.0 (AGP 4x) cards, because they do not support more than one AGP Master (video card) per AGP Target (chipset interface); AGP 3.0 (AGP 8x) does support more than one AGP Master per AGP Target, but nonetheless few PC motherboards are equipped with more than one AGP slot. Some server-class computers support having multiple AGP slots in a single system: the HP AlphaServer GS1280 has up to 16 AGP slots, the AlphaServer ES80 up to 4 AGP slots, and the AlphaServer ES47 up to 2 AGP slots.

History of AGP (Advanced Graphics Port)

The AGP slot first appeared on x86 compatible system boards based on Socket 7 Pentium and Slot 1 Pentium II processors. Intel introduced AGP support with the i440LX Slot 1 chipset in mid-October 1997 and a flood of products followed from all the major system board vendors; this chipset was discontinued by Intel on December 8, 2000. The i440LX chipset included the well established PIIX4 south bridge from Intel's 430TX Socket 7 offering.

The first Socket 7 chipsets to support AGP were the VIA Apollo VP3, SiS 5591/5592, and the ALI Aladdin V. FIC demonstrated the first Socket 7 AGP system board in November 1997 as the FIC PA-2012 based on the VIA Apollo VP3 chipset, followed very quickly by the EPoX P55-VP3 also based on the VIA VP3 chipset which was first to market.

Early video chipsets featuring AGP support included the Rendition V�rit� V2200, 3dfx Voodoo Banshee, NVIDIA RIVA 128, 3Dlabs PERMEDIA 2, Intel i740, ATI Rage series, Matrox Millennium II, and S3 ViRGE GX/2. Some early AGP boards used graphics processors built around PCI and were simply bridged to AGP. This resulted in the cards benefiting little from the new bus, with the only improvement used being the 66 MHz bus clock. Examples of such cards were the Voodoo Banshee, V�rit� V2200, Millennium II, and S3 ViRGE GX/2. Intel's i740 was explicitly designed to exploit the new AGP feature set. In fact it was designed to texture only from AGP memory, making PCI versions of the board difficult to implement. Local board RAM had to emulate AGP memory.

Microsoft first introduced AGP support into Windows 95 OEM Service Release 2 (OSR2 version 1111 or 950B) via the USB SUPPLEMENT to OSR2 patch. After applying the patch the Windows 95 system became Windows 95 version 4.00.950 B. The first Windows NT-based operating system to receive AGP support was Windows NT 4.0 Service Pack 3, when Microsoft first introduced it in 1997. Linux support for AGP enhanced fast data transfers was first added in 1999 with the implementation of the AGPgart kernel module.

Versions of AGP (Advanced Graphics Port)

Intel released the first version of AGP; appropriately titled "AGP specification 1.0," in 1997. It included both the 1x and 2x speeds. Specification 2.0 documented AGP 4X and 3.0 documented 8X. Available versions include:

AGP 1x - A 32-bit channel operating at 66 MHz resulting in a maximum data rate of 266 megabytes per second (MB/s), doubled from the 133 MB/s transfer rate of PCI bus 33 MHz / 32-bit; 3.3 V signaling.
AGP 2x - A 32-bit channel operating at 66 MHz double pumped to an effective 133 MHz resulting in a maximum data rate of 533 MB/s; signaling voltages the same as AGP 1x.
AGP 4x - A 32-bit channel operating at 66 MHz quad pumped to an effective 266 MHz resulting in a maximum data rate of 1066 MB/s (1 GB/s); 1.5 V signaling
AGP 8x - A 32-bit channel operating at 66 MHz, strobing eight times per clock, delivering an effective 533 MHz resulting in a maximum data rate of 2133 MB/s (2 GB/s); 0.8 V signaling.

In addition, AGP Pro cards of various types exist. They require more power and are often longer than standard AGP cards (though they only connect to one AGP slot). These cards are usually used to accelerate professional computer-aided design applications employed in the fields of architecture, machining, engineering, simulations, and similar fields. The AGP Pro slot has a few extra pins which supply extra power to the graphics card. However, AGP Pro has not become widely accepted. Instead, modern desktop grade power-hungry video cards are equipped with a separate power source, either through a Molex or peripheral plug for additional power. Generally, there are two types of AGP Pro slots, regarding necessary power: 50 W or 110 W, with or without molex connector on the graphic card (although standard AGP slot is confined to 42 W of power through slot itself).

There are three versions of the AGP physical interface, for 3.3 V and 1.5 V cards respectively. The 1.5 V version has a key further away from the external connector, while the 3.3 V version is the opposite. But some poorly designed older 3.3 V cards incorrectly have the 1.5 V key, which may result in burnt mainboard if installed in a AGP 4X/8X slot. The third physical interface is universal, allowing installation in both 1.5 V and 3.3 V motherboards.

AGP version 3.5 is only publicly mentioned by Microsoft under Universal Accelerated Graphics Port (UAGP), which specifies mandatory supports of extra registers once marked optional under AGP 3.0. Upgraded registers include PCISTS, CAPPTR, NCAPID, AGPSTAT, AGPCMD, NISTAT, NICMD. New required registers include APBASELO, APBASEHI, AGPCTRL, APSIZE, NEPG, GARTLO, GARTHI.

Variations of AGP (Advanced Graphics Port)

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

64 bit AGP - A 64-bit channel. Used in high end professional graphic cards. It was once proposed as an optional standard for AGP 3.0 in draft documents, but was dropped in the final version of the standard.
AGP Express - Not a true AGP interface, but rather a way to allow an AGP card to be connected over the legacy PCI bus on a PCI Express motherboard. It is a technology found on ECS motherboards, and is used as a selling point for AGP card owners who want a new motherboard but do not want to be forced to buy a PCIe graphics card as well (most new motherboards do not provide AGP slots, only PCIe slots). An "AGP Express" slot is basically a PCI slot (with the electrical power of two) in the AGP form factor. While it offers backward compatibility with AGP cards, its disadvantages include 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 EpoX Advanced Graphics eXtended (AGX) is also a proprietary variant of the Accelerated Graphics Port (AGP) standard. It shares the same problems with the AGI port explained above. User manuals even recommend not using AGP 8X ATI cards with AGX slots.
Ultra-AGP - 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 8x, while Ultra-AGPII has maximum 3.2GB/s bandwidth.
XGP - The Biostar Xtreme Graphics Port is also a variant of the Accelerated Graphics Port (AGP) standard. It is similar to the two standards above, in that it supports AGP cards with chipsets that do not support AGP. Also like the above, it has support issues with many AGP cards.
AGR - The Advanced Graphics Riser is a variation of the AGP port used in some PCIe motherboards to offer a limited backwards compatibility with AGP. It is, effectively, a modified PCI slot with no direct interconnection with the CPU or memory, and thus slower even compared to an AGP 1x slot. Its actual compatibility with AGP cards is also limited, while motherboard manufacturers usually publish a specific compatibility list.

AGP 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" slots exist which accept either type of card. AGP Pro cards (rarely used) will not fit into standard slots, but standard AGP cards will work in a Pro slot. Some newer cards like nVidia's GeForce 6-series or ATI's X800-series only have keys for 1.5 V to prevent them from installing in older mainboards without 1.5 V support. The graphic cores can only handle 0.8 V for AGP8x and 1.5 V for AGP4x and will be damaged by 3.3 V slots. Some of the last modern cards with 3.3 V support were the nVIDIA GeForce FX5000-series and the ATI Radeon 9500/9700/9800(R350) (but not 9600/9800(R360)).

However it is important to check voltage compatibility as some cards incorrectly have dual notches and some motherboards incorrectly have fully open slots. Inserting a card into a slot that does not support the correct signaling voltage may cause damage.

There are some proprietary exceptions to this rule. For example, Apple Power Macintosh computers with the Apple Display Connector have an extra connector which delivers power to the attached display. Additionally, moving cards between computers of various CPU architectures may not work due to firmware issues.

AGP Use Today

As of late 2006, few new motherboards feature AGP slots. PCI Express allows much faster data transfer, is full-duplex, and also supports other devices. So far, 2006 has seen many motherboards introduced with PCIe slots, with some gamer-oriented configurations offering dual and Quad PCIe x16 slots.

Many newer graphics cards are available exclusively in PCIe, with AGP versions of the same cards becoming correspondingly rare. The current top of the range AGP cards are the ATi X1950XT and nVidia 7950GT, but nVidia is planning to release an AGP card for its DX10 8 series. While AGP still makes up a large fraction of the installed PC base, it is expected to be almost totally replaced in the next few years. A survey by Valve Corporation in November 2006 found approximately a 50:50 split between AGP and PCI Express amongst Steam gamers.

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