What Is Accelerated Graphics Port (AGP)?

Accelerated Graphics Port, often abbreviated as AGP, is a standard type of connection for internal video cards.

Generally, Accelerated Graphics Port refers to the actual expansion slot on the motherboard that accepts AGP video cards, as well as to the types of video cards themselves.

Accelerated Graphics Port Versions

There are three common AGP interfaces:

AGP Version Comparison Table
Interface	Clock Speed	Voltage	Speed	Transfer Rate
AGP 1.0	66 MHz	3.3 V	1X and 2X	266 MB/s and 533 MB/s
AGP 2.0	66 MHz	1.5 V	4X	1,066 MB/s
AGP 3.0	66 MHz	0.8 V	8X	2,133 MB/s

The transfer rate is basically the bandwidth, and is measured in megabytes.

The 1X, 2X, 4X, and 8X numbers indicate the bandwidth speed in relation to the speed of AGP 1.0 (266 MB/s). For example, AGP 3.0 runs at eight times the speed of AGP 1.0, so its maximum bandwidth is eight times (8X) that of AGP 1.0.

Microsoft has named AGP 3.5 Universal Accelerated Graphics Port (UAGP), but its transfer rate, voltage requirement, and other details are identical to AGP 3.0.

What Is AGP Pro?

AGP Pro is an expansion slot that's longer than that of AGP and has more pins, providing more power to the AGP video card. It may be useful for power-intensive tasks, like very advanced graphics programs. 

You can read more about AGP Pro in the AGP Pro Specification PDF.

Differences Between AGP and PCI

AGP was introduced by Intel in 1997 as a replacement of the slower Peripheral Component Interconnect (PCI) interfaces. AGP provides a direct line of communication to the CPU and RAM, which in turns allows for quicker rendering of graphics.

One major improvement that AGP has over PCI interfaces is how it works with RAM. Called AGP memory, or non-local memory, AGP is able to access the system memory directly instead of relying solely on the memory of the video card.

AGP memory allows AGP cards to avoid having to store texture maps (which may use lots of memory) on the card itself because it stores them in system memory instead. This means not only that the overall speed of AGP is improved versus PCI, but also that the size limit of texture units is no longer determined by the amount of memory in the graphics card.

A PCI graphics card receives information in "groups" before it can use it, instead of all at once. For example, while a PCI graphics card will gather the height, length, and width of an image at three different times, and then combine them together to form an image, AGP can get all of that information simultaneously. This makes for quicker and smoother graphics than what you'd see with a PCI card.

A PCI bus normally runs at a speed of 33 MHz, allowing it to transfer data at 132 MB/s. Using the table from above, you can see that AGP 3.0 is able to run at over 16 times that speed to transfer data much quicker, and even AGP 1.0 exceeds the speed of PCI by a factor of two.

While AGP replaced PCI for graphics, PCIe (PCI Express) has been replacing AGP as the standard video card interface, having nearly completely replaced it by 2010.

AGP Compatibility

Motherboards that support AGP will either have a slot available for an AGP video card or will have onboard AGP.

AGP 3.0 video cards can be used on a motherboard that supports AGP 2.0 only, but it will be limited to what the motherboard supports, not what the graphics card supports. In other words, the motherboard will not allow the video card to perform better just because it's an AGP 3.0 card; the motherboard itself isn't capable of such speeds (in this scenario).

Some motherboards that use only AGP 3.0 might not support the older AGP 2.0 cards. So, in a reverse scenario from the above, the video card might not even function unless it's capable of working with a newer interface.

Universal AGP slots are available that support both 1.5 V and 3.3 V cards, as well as universal cards.

Some operating systems, like Windows 95, don't support AGP due to a lack of driver support. Other operating systems, like Windows 98 through Windows XP, require a chipset driver download for AGP 8X support.

Installing an AGP Card

Installing a graphics card into an expansion slot should be a pretty simple process. If you're having problems with a video card that has already been installed, consider reseating the card. This goes for AGP, PCI, or PCI Express.