Accelerated Graphics Port

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Image:AGP slot.jpg The Accelerated Graphics Port (also called Advanced Graphics Port) 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 slowly being phased out in favour of PCI Express.

1 Versions of AGP
2 Hardware and Software Adoption
3 Compatibility
4 AGP vs PCI
5 Use today
6 Historical Remarks
7 References
8 External links

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Versions of AGP

Intel released the first version of AGP, appropriately titled “AGP specification 1.0,” in 1997. It included both the 1x and 2x speeds. As of 2004, newer versions of AGP increase the transfer rate from two to eight times. Available versions include:

AGP 1x, using 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, using 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, using 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, using 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 the professional computer-aided design applications employed in the fields of architecture, machining, engineering, and similar fields.

There are two 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 3.3V cards incorrectly have the 1.5V key, which may result in burnt mainboard if installed in a AGP 4X/8X slot.

64 bit AGP, using a 64 bit channel. Used in high end professional graphic cards.
AGP Express. This is not a true AGP interface, but rather a way to allow an AGP card to be connected over the PCI bus. 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 PCI Express graphics card as well (most new motherboards do not provide AGP slots, only PCI Express slots). An "AGP Express" slot is basically two PCI slots wired together 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 ASRock Graphics Interface (AGI) is a proprietary variant of the Accelerated Graphics Port (AGP) standard. Its purpose is to provide AGP-support for those of Asrock's motherboards that use chipsets lacking native AGP-support. However, it's not fully compatible and several videocard chipsets are known to not be supported. The performace and the quality of that interface is somehow questionable.

AGX. 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.

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Hardware and Software Adoption

The AGP slot first appeared on x86 compatible system boards based on Socket 7 Pentium and Slot 1 Pentium II processors.
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.
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 4.00.900 C.
Microsoft first introduced AGP support into Windows NT 4.0 Service Pack 3.
Linux support for AGP enhanced fast data transfers was first added in 1999 with the implementation of the Agpgart kernel device.
Intel introduced AGP support with the LM440LX 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 LM440LX chipset included the well established PIIX4 south bridge from Intel's 430TX Socket 7 offering.
First generation Pentium II systems were based around the 440FX chipset which did not provide AGP support.
A second feature introduced by Intel for the first time with the LM440LX chipset was support for SDRAM, which provided for the extra bandwidth that AGP video cards consumed.
Early video chipsets featuring AGP support: Rendition V2200 AGP, NVidia Riva 128, Intel i740, ATI Rage Pro, 3DFx Voodoo
Whereas the V2200 was a PCI design internally with an AGP interface bolted on, the ATI Rage Pro and the Intel i740 were explicitly designed to exploit the new AGP feature set.
Many of these early AGP video cards were designed to run under the Microsoft Direct3D API.

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Compatibility

AGP cards are backwards and forwards compatible within limits. 1.5 V 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.

It should be noted that 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 architectures may not work due to firmware issues.

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AGP vs PCI

As computers became increasingly graphically-oriented, the graphics card became far more important than other PCI devices, and, thus, the AGP slot was developed. AGP slots are superior to PCI for graphics cards because they provide a dedicated pathway between the slot and the processor, allowing for faster communication between the two. In addition, AGP uses sideband addressing, meaning that addressing for packets is carried outside of the packet, meaning that 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. 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 produced is that firstly they can be used in nearly any PC, as very few modern desktop PCs do not have PCI slots - though some motherboards with built-in graphics adaptors lack an AGP slot. 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 cannot be done with AGP cards, as very few (if any) motherboards are equipped with more than one AGP slot.

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Use today

Although AGP is still incorporated into many motherboards today, it is gradually being phased out in favour of PCI Express. PCI Express allows much faster data transfer and also supports other devices. So far, 2006 has seen many motherboards available with PCIe slots, and most graphics cards are available in both AGP and PCIe versions. At this rate, it is expected that AGP will be almost totally replaced in the next several years.

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Historical Remarks

Subtleties of the AGP memory interface semantics caused great consternation at AMD when an exotic bug manifested itself in Linux 2.4 kernels running on AMD's then-new line of Athlon processors.<ref>The AMD AGP Linux Kernel Patch - AMD concerned about sour AGP/Athlon interactions</ref>

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