How Graphics Cards Work

You can better understand the essence of a graphics card by looking at the simplest possible one. This card would be able to display only black or white pixels, and it would do that on a 640x480-pixel screen.

Here are the three basic components of a graphics card and what they do:

• Memory: The first thing that a graphics card needs is memory. The memory holds the color of each pixel. In the simplest case, since each pixel is only black or white, you need just 1 bit to store each pixel's color (See How Bits and Bytes Work for details.). Since a byte holds 8 bits, you need (640/8) 80 bytes to store the pixel colors for one line of pixels on the display. You need (480 X 80) 38,400 bytes of memory to hold all of the pixels visible on the display.

• Computer Interface: The second thing a graphics card needs is a way for the computer to change the graphics card's memory. This is normally done by connecting the graphics card to the card bus on the motherboard. The computer can send signals through the bus to alter the memory.

• Interface: The next thing that the graphics card needs is a way to generate the signals for the monitor. The card must generate color signals that drive the cathode ray tube (CRT) electron beam, as well as synchronization signals for horizontal and vertical sync (See How Television Works for details.). Let's say that the screen is refreshing at 60 frames per second. This means that the graphics card scans the entire memory array 1 bit at a time and does this 60 times per second. It sends signals to the monitor for each pixel on each line, and then sends a horizontal sync pulse; it does this repeatedly for all 480 lines, and then sends a vertical sync pulse.

The basic parts of a graphics card are computer interface, memory and video interface.

When a graphics card handles color, it does it in one of two ways. A true-color card devotes 3 or 4 bytes per pixel (4 bytes allows an extra byte for an "alpha channel"). On a 1600x1200-pixel display, this adds up to about 8-million bytes of video memory.

The other alternative is to use 1 byte per pixel and then use these bytes to index a Color Look-Up Table (CLUT). The CLUT contains 256 entries with 3 or 4 bytes per entry. The CLUT gets loaded with the 256 true colors that the screen will display.

The table above provides an example of a CLUT. Each pixel is assigned a byte value that is 8 bits (1 byte) in length, with 256 possible values. The byte value corresponds to a color value taken from a larger palette that is 24 bits (3 bytes), which is about 16.8-million possible colors.