Accelerated Graphics Port
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AGP_slot.jpg
The Accelerated Graphics Port (also called Advanced Graphics Port) is a high-speed point-to-point channel for attaching a single device (generally a graphics card) to a computer's motherboard, primarily to assist in the acceleration of 3D computer graphics. Many classify AGP as a type of computer bus, but this is something of a misnomer since buses generally allow multiple devices to be connected, while AGP does not. Some motherboards have been built with multiple independent AGP slots. AGP is slowly being phased out by PCI-Express.
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Initial development
AGP dynamically allocates the PC's normal RAM to store the screen image and to support texture mapping, z-buffering and alpha blending. RAM used in this manner is referred to as the AGP Aperture.
AGP originated from Intel, and it was first built into the 440LX chipset for the Pentium II microprocessor in 1997. AGP cards generally slightly exceed PCI cards in length and can be recognized by a typical "hook" at the inner end of the connector, which does not exist on PCI cards. AGP became common in mainstream systems in 1998, but PCI video cards are still being produced today, as many home PCs are sold without an AGP slot. They are also necessary if a user wants to install more than one video card at a time (since AGP-motherboards only have one AGP slot.)
The first version of AGP, now called AGP 1.0 or AGP 1x, uses a 32-bit channel operating at 66 MHz with 3.3 V signaling. This results in a maximum data rate for an AGP 1x slot of 266 megabytes per second. In comparison, a standard 32-bit 33 MHz PCI bus is limited to 133 MB/s, which is shared among all the cards using the PCI bus.
Newer versions of AGP
As of 2004, newer versions of AGP increase the transfer rate from two to eight times. Available versions include:
- 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 megabytes per second; 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 megabytes per second; 1.5 V signaling;
- AGP 8x, using a 32-bit channel operating at 66 MHz octuple pumped to an effective 533 MHz resulting in a maximum data rate of 2133 megabytes per second; 0.8 V signaling.
In addition, AGP Pro cards of various types exist. They require more power and are often longer than standard AGP card (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.
Implementation details
AGP allows for efficient use of frame buffer memory, thereby improving 2D graphics performance as well.
AGP provides a coherent memory management design which allows reading scattered data from system memory in rapid bursts. AGP was designed to reduce the overall cost of creating high-end graphics subsystems by using existing system memory. However, general system memory, although cheap, performs much slower than dedicated on-card graphics RAM, and both mid-range and high-end graphics cards rely on their own high-speed RAM for performance. Cheap low-end graphics cards with little on-board RAM benefited from AGP early in the life-cycle of the technology, but the lowered cost of memory since about 2000 has led to even low-end cards having 64MB or 128MB of dedicated RAM, and therefore graphics now rarely utilize system RAM.
External links
- Advanced Graphics Port information (http://web.archive.org/web/20040205095311/http://www.gcsextreme.com/agpfaq.htm)
- AGP expansion slots (http://www.motherboards.org/articles/tech-planations/920_4.html)
- AGP compatibility (http://www.ertyu.org/~steven_nikkel/agpcompatibility.html) (with pictures)
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
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