A PCI (Peripheral Component Interconnect) and AGP (Accelerated Graphics Port) are two different technologies used to connect expansion cards ù such as video, sound, and graphics cards ù to your PC. The main difference between the two is speed, particularly when it comes to processing graphics. Gone are the days of simple words and numbers ù these days we expect our business, entertainment and educational software to impress us with fancy images, charts, icons, textures, and 3-D graphics. Software developers, aware of our expectations for graphics intensive applications, are continually pushing the technology envelope by incorporating larger and more complex images into their programs. As programs become more graphics intensive, they require more bandwidth and memory to display each screen and image. If bandwidth and memory resources are limited, a bottleneck occurs, causing the software, and generally the PC, to slow to a crawl while the graphics are processed.

In the past, several attempts have been made to reduce the bottlenecks associated with graphics processing. A significant breakthrough occurred in 1993 when Intel introduced the PCI bus. The PCI standard uses a technique called bus mastering, which allows the CPU and expansion cards to process information simultaneously. The bus operates at a bandwidth of 66MHz, and PCI cards can communicate with the PC using 32- or 64-bit data. PCI provides fast communication between the CPU and peripherals, but peripheral devices have to compete with each other for bandwidth. The PCI bus is currently the highest performing general I/O bus used in PCs, and it provides adequate acceleration and processing features for most games, video and multimedia applications. PCI is able to handle 2-D images and general business graphics quite competently, but it can be challenged by intense 3-D graphics. ThatÆs where AGP comes in.

To create 3-D images, the graphics controller must be able to manage texture data and z-buffer information. Texture data produces the digital representation of the surface of an object and generates properties such as transparency, which makes the object look more realistic; z-buffer information provides depth, which also increases realism. Both of these data sets are memory intensive, and unfortunately, both compete for the same memory space. Intel introduced AGP in 1996 in an attempt to solve this dilemma.

The AGP specification is based on the PCI 2.1 specification, but unlike PCI, AGP is designed solely for use with graphics cards. It's not intended to replace the PCI interface as the general I/O interface bus; its primary purpose is to deliver high-performance graphics, including 3-D imaging. AGP has the ability to quadruple the theoretical bandwidth of current PCI buses, and has the potential to perform even higher. This increased performance is achieved by introducing a dedicated point-to-point channel that gives the graphics controller direct access to main system memory. In addition, the AGP channel is 32 bits wide and runs at 66MHz, which translates into a total bandwidth of 266MBps. AGP also supports two fast modes, 2x and 4x, which have throughputs of 533MBps and 1.07GBps respectively. Features such as texturing and pipelining further enhance the graphics processing ability of AGP. Texturing, also called Direct Memory Execute mode, allows texture data to be stored in main memory. Pipelining is a process that enables the graphics card to send several instructions together instead of sending one at a time.

AGP improves the overall performance of a PC in several ways:

1. Graphics operations are faster because they donÆt have to share bus bandwidth with other peripherals.

2. Peripheral devices are also faster because they donÆt have to share the PCI bus with the bandwidth intensive graphics operations. AGP operates concurrently with, and independent from, most transactions on the PCI bus. Since the AGP bus is handling all graphics tasks, the PCI bus is free to serve devices such as disk controllers, modems and network cards.

3. The quality of the 3-D graphics created using AGP is very high, and since they are extremely realistic, the quality of both 2-D and 3-D software is improved.