AGP Graphics Card Interface

The Accelerated Graphics Port (AGP) heralded the dawn of the PC gaming era.

The AGP port was added to computer systems manufactured in the late 90's and early 2000's that used PCI or PCI-X architectures for the specific function of supplying enhanced speeds to discrete graphics cards. By utilizing enhanced point-to-point communication protocols, AGP increased 3D imaging and image processing dramatically.

How Does AGP Work?

The biggest problem with PCI was the bottleneck at the Central Processing Unit (CPU) - the brain of the computer. In the PCI architecture, all the data from all the different expansion cards connected to the motherboard arrived at the CPU simultaneously. The CPU handled each piece of data in turn, performed the appropriate calculations, and responded with the appropriate instructions.

The chief problem was that graphics cards required a lot of attention to provide a seamless flow of graphical data. The longer the graphics card was forced to sit and wait for instructions from the CPU, the lower the maximum frame rates in any 3D application.

AGP solved this problem by opening two pipelines. One ran straight to the CPU, giving the graphics card priority access to higher computer functions. The second pipe ran straight to the main system RAM, allowing the graphics card direct access to the system memory. This essentially allowed the graphics card to use system memory as a slightly slower stockpile of video memory.

The results were spectacular, as anyone who remembers the emergence of AGP will relate. Maximum frame rates for all 3D applications doubled with the advent of AGP, and were then doubled again with each successive revision.

However, even as PC gamers celebrated their newfound prize, PCI-Express was in the works, and would soon emerge to kick the chair out from under AGP.

AGP Graphic Card History

AGP was introduced by Intel in late 1996, shortly after the release of the first line of Pentium microprocessors. The dawning realization that PC users were purchasing much hungrier graphics cards sparked the innovation of AGP, and AGP quickly became a mainstay of anyone who used graphics-intensive applications and hardware.

The revisions to AGP over the years increased its speed dramatically. These revisions worked to increase the overall clock speed of the bus connection, increase the rate of data flow, and reduce the voltage required by the connection.

AGP uses a technique called pumping to increase the amount of data that can travel through the bus connection to the graphics card. Essentially, pumping enables the connection to send data multiple times per clock cycle. These multiples eventually increased to 8 - meaning that an AGP 8x bus connection allows 8 streams of data per clock cycle through the connection to the graphics card.

Revision history and associated performance characteristics of AGP generations are as follows:

Pumping was the key to speed, since increasing the number of data transfers per cock cycle multiplied the performance and efficiency of the entire bus connection.

AGP Video Card Compatibility

All AGP cards are backward and forward compatible to a certain degree. Graphics cards that require the 1.5 volt charge of AGP 2.0 will not operate in the 3.3 volt slot of either AGP 1.0 or 1.5, and vice versa.

However, many AGP-universal cards were manufactured throughout the early 2000's. These cards will utilize any format of AGP architecture, however they can only function at the speed of the bus connection itself, regardless of the operating speed of the card.

Modifications and Extensions

Due to the overwhelming success of AGP, many different forms of adaptations, some supported and some unsupported, flooded the market during the late 90's and early 2000's as users and manufacturers alike attempted to find a way to make their favorite graphics cards work with the new architecture:

AGP Pro

An adaptation for cards that required increased electrical input, AGP Pro offers a longer slot with more available connectors to provide increased current. AGP Pro cards were typically found in industrial or engineering applications on high-end design systems.

64-bit AGP

A 64-bit structure was originally planned for AGP 3.0, but the final release kept the old 32-bit architecture. With the introduction of 64 bit AGP, the AGP port operated at the increased rate while standard PCI slots continued to operate at 32 bits.

Ultra-AGP and Ultra-AGP II

Developed exclusively for the SiS north bridge controller cards, Ultra-AGP provides the same bandwidth as AGP 3.0, while Ultra-AGPII has maximum bandwidth of 3.2 gigs per second.

AGP Express

Not exactly an AGP interface, AGP Express was a workaround that allowed an AGP graphics card to be connected to a PCI-Express motherboard.

AGI

The ASRock Graphics Interface (AGI) was developed exclusively for ASRock motherboards which lacked native AGP support.

AGX

Similar to AGI, the AGX was designed as a workaround for EpoX motherboards.

XGP

Another workaround, this time for motherboard manufacturer Biostar Xtreme.

AGR

The Advanced Graphics Riser is an AGP modification used in some MSI-manufactured PCIE motherboards that provided backwards AGP compatibility.