Interfacing with the video system

Interfacing with the video system

A large amount of data moves between the video card and the PC's CPU and RAM to create each frame of the video display. What you see on the monitor is actually a series of still images displayed very quickly. Each frame requires a great deal of information to be sent from the PC to the video card. The pathway that the video information data travels over must have more bandwidth than any other peripheral device interface on the PC. This is why either the PCI (Peripheral Components Interconnect) or the AGP (Accelerated Graphics Port) interfaces are used for most modern PC video systems. Yes, there are some ISA (Industry Standard Architecture) video cards still hanging around.

Warning Avoid the myth that the number of bits used on the video card's internal bus is also the number of bits used for the video card's interface. A 128-bit video card most likely uses a 32-bit interface. The width of the interface is 16 bits (ISA/EISA cards), 32 bits (VL-Bus, PCI, or AGP), or 64 bits (PCI).

The two most popular video system interfaces in use today are:

l Peripheral Component Interconnect (PCI): All Pentium-class motherboard chipsets support the PCI interface bus. PCI is commonly used for 2-D graphics cards, sound cards, network interface cards, and other expansion cards that attach directly to the motherboard. Of course, a PCI card slot is also required.

l AGP (Accelerated Graphics Port): The AGP interface was designed specifically for use as a video system interface. AGP runs twice as fast as the PCI interface and creates a high-speed link between the video card and the PC's processor. AGP also has a direct link to system RAM, which makes it possible for the video system to use system RAM for calculations and temporary storage.

An AGP video card will only fit in an AGP slot.

Because of its faster transfer rates, AGP is quickly replacing the PCI interface as the interface of choice.

In fact, AGP has evolved into several versions, each designated as a multiple of the original standard's speed. AGP 1X has a data transfer rate of 266 Mbps (compared to PCI's 133 Mbps), AGP 2X supports 533 Mbps, and AGP 4X transfers data at 1.07 Gbps (and no, I don't know what happened to AGP 3X).

One reason for the increased speed is that AGP is a port and supports only one device (the video card), and PCI is a true bus structure over which the PC communicates with a number of devices.