The graphics card plays an essential role in the
PC.
It takes the digital information that the computer produces and turns it into
something human beings can see.
On most computers, the graphics card converts
digital information to analog information for display on the monitor; on laptops,
the data remains digital because laptop displays are digital.
If you look at the screen of a typical PC very closely, you can see that all
of the different things on the screen are made up of individual dots. These dots
are called pixels, and each pixel has a color.
On some screens (for
example, on the original Macintosh), the pixels could have just two colors --
black or white. On some screens today, a pixel can be one of 256 colors.
On many
screens, the pixels are full-color (also known as true color) and
have 16.8 million possible shades. Since the human eye can only discern about
10-million different colors, 16.8 million colors is more than enough for most
people.
The goal of a graphics card is to create a set of signals that display the
dots on the computer screen.
What's a Graphics Card?
A modern graphics card is a circuit board with memory
and a dedicated processor.
The processor is designed specifically to handle the intense computational
requirements of displaying graphics. Most of these graphics processors have
special command sets for graphics manipulation built right into the chip.
Graphics cards are known by many names, such as:
Video cards
Video boards
Video display boards
Graphics boards
Graphics adapter cards
Video adapter cards
Today's graphics cards are computing systems in their own right. But these
cards started out as very simple devices. By understanding the evolution of
graphics cards, you can begin to see why they are so powerful today.
Graphics Card Basics
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. 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.
Video 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. 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.
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.
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