What a Video Card Does

In the early days of personal computers, the graphics controller received nothing but text elements-letters, numbers, and a few other odd characters and symbols-so it didn't have much work to do. If you've been working with computers for a while, you might remember those old monochrome monitors that displayed everything in green or white or orange on a black background.

When color monitors and graphical user interfaces like Windows came along, the CPU suddenly had to handle a lot more graphics information. So accelerators were introduced that could offload much of that burden from the CPU. Instead of sending the graphics controller every detail of every pixel, an accelerator card already knew, for example, what many elements of a window looks like; the device driver would tell the accelerator, "Place a window in this location, with this information inside." The accelerator would then draw the window. This allowed the CPU to devote more of its time to other activities, while the video accelerator created images for the monitor far more quickly.

Today, Windows and other modern operating systems send much more complex data to the video subsystem, including text in a variety of typefaces, windows, icons and other graphic elements, still pictures, and moving images with thousands of shades of color and even the illusion of three dimensions on a flat screen. As the graphics controller has taken over more and more processing from the CPU, it has evolved into a second small computer system that assists the main processor. Graphics controllers have become a specialized part of the larger computer industry, dominated by a small handful of companies.

Figure 10.1 shows how a video controller moves graphic information from the computer's CPU to the display monitor. Like the computer's motherboard, the graphics card has a BIOS that contains instructions and timing information and also tests the card's inputs, outputs, and onboard memory in response to an instruction from the system BIOS. The graphics processor receives information from the computer's chipset and uses the BIOS settings to convert that information to a video image. After the operating system begins, the video device driver takes over control of the image from the BIOS.

If the controller is connected to an analog monitor, the video image passes through a digital-to-analog converter (a DAC or RAMDAC) on its way to the monitor. If the controller is connected to a digital monitor, the image goes directly to the monitor through a DV-I (digital video interactive) interface. Like the computer's CPU, the graphics controller uses RAM on the graphics card to hold incoming information from the CPU before the graphics processor can work on it and outbound images on the way to the monitor.

Figure 10.1: The structure of a graphics controller is similar to the overall operation of the computer.

The monitor renews the on-screen image 60 or more times per second, but the graphics controller produces pixels more quickly than that, so the controller stores both the current image and the image under construction in part of the RAM mounted on the video card. The portion of video RAM that holds completed images is called the frame buffer.

Like the computer's CPU, video processors can handle data at very high speeds, which often produces a lot of heat on the surface of the processor chip. Therefore, most graphics controllers have heat sinks and fans to move heated air away from the surface of the printed circuit card. Unfortunately, the fans on many graphics cards are louder than CPU fans, so the video card is often one of the noisiest components inside the computer. If this is a problem in your office or home, you can replace the fan with a silent heat sink that uses copper fins or a heat pipe, like the ones made by Zalman and Thermaltake.