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How Graphics Cards Work

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.

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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 is 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.
 

 

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) 34,800 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 (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.
 

 

Graphics Coprocessors
A simple graphics card, like the one described previously, is called a frame buffer. The card simply holds a frame of information that is sent to the screen. The computer's microprocessor does the job of updating every byte of video memory.
The problem with frame buffers is that, on complex graphics operations, the microprocessor ends up spending all of its time updating video memory and can't get any other work done. For example, if a 3-D image contains 10,000 polygons, the microprocessor has to draw and fill each polygon in the video memory, 1 pixel at a time. This takes a while.

Modern graphics cards have evolved to take some or all of this load off the microprocessor. A modern card contains its own high-power central processing unit (CPU) that is optimized for graphics operations. Depending on the graphics card, this CPU will be either a graphics coprocessor or a graphics accelerator.

Think of a coprocessor as a co-worker, and an accelerator as an assistant. The coprocessor and the CPU work simultaneously, while the accelerator receives instructions from the CPU and carries them out.

In the coprocessor system, the graphics card driver software sends graphics-related tasks directly to the graphics coprocessor. The operating system sends everything else to the CPU.

With a graphics accelerator, the driver software sends everything to the computer's CPU. The CPU then directs the graphics accelerator to perform specific graphics-intensive tasks. For example, the CPU might say to the accelerator, "Draw a polygon with these three vertices," and the accelerator would do the work of painting the pixels of the polygon into video memory.

More and more complex graphics operations have moved to the graphics coprocessor or accelerator, including shading, texturing and anti-aliasing.

As graphics cards and coprocessors continue to evolve, the capabilities become more and more amazing. Modern cards can draw millions of polygons per second. These features make it possible to create extremely realistic games and simulations.
 

 

More on Graphics Card Components
There are several components on a typical graphics card:

  • Graphics Processor - The graphics processor is the brains of the card, and is typically one of three configurations:
  • Graphics Co-processor: A card with this type of processor can handle all of the graphics chores without any assistance from the computer's CPU. Graphics co-processors are typically found on high-end video cards.
  • Graphics Accelerator: In this configuration, the chip on the graphics card renders graphics based on commands from the computer's CPU. This is the most common configuration used today.
  • Framebuffer: This chip simply controls the memory on the card and sends information to the digital-to-analog converter (DAC) (see below). It does no processing of the image data and is rarely used anymore.
  • Memory - The type of RAM used on graphics cards varies widely, but the most popular types use a dual-ported configuration. Dual-ported cards can write to one section of memory while it is reading from another section, decreasing the time it takes to refresh an image.
  • Graphics BIOS - Graphics cards have a small ROM chip containing basic information that tells the other components of the card how to function in relation to each other. The BIOS also performs diagnostic tests on the card's memory and input/output (I/O) to ensure that everything is functioning correctly.
  • Digital-to-Analog Converter (DAC) - The DAC on a graphics card is commonly known as a RAMDAC because it takes the data it converts directly from the card's memory. RAMDAC speed greatly affects the image you see on the monitor. This is because the refresh rate of the image depends on how quickly the analog information gets to the monitor.
  • Display Connector - Graphics cards use standard connectors. Most cards use the 15-pin connector that was introduced with Video Graphics Array (VGA) (see next page to learn about VGA).
  • Computer (Bus) Connector - This is usually Accelerated Graphics Port (AGP). This port enables the video card to directly access system memory. Direct memory access helps to make the peak bandwidth four-times higher than the Peripheral Component Interconnect (PCI) bus adapter card slots. This allows the central processor to do other tasks while the graphics chip on the video card accesses system memory.

 

Graphics Card History and Standards
The first graphics cards, introduced in August of 1981 by IBM, were monochrome cards designated as Monochrome Display Adapters (MDAs). The displays that used these cards were typically text-only, with green or white text on a black background. Often, the graphics card had a printer port, since the printer would print the same data shown on the low-resolution "green" screen. Color for IBM-compatible computers appeared on the scene with the 4-color Hercules Graphics Card (HGC), followed by the 8-color Color Graphics Adapter (CGA) and 16-color Enhanced Graphics Adapter (EGA). During the same time, other computer manufacturers, such as Commodore, were introducing computers with built-in graphics adapters that could handle a varying number of colors.

When IBM introduced the Video Graphics Array (VGA) in 1987, a new graphics standard came into being. A VGA display could support up to 256 colors (out of a possible 262,144-color palette) at resolutions up to 720x400. Perhaps the most interesting difference between VGA and the preceding formats is that VGA was analog, whereas displays had been digital up to that point. Going from digital to analog may seem like a step backward, but it actually provided the ability to vary the signal for more possible combinations than the strict on/off nature of digital. Of course, the way we manipulate digital display data has changed significantly since the days of CGA and EGA. Now, graphics-card manufacturers are able to provide all-digital display solutions that can support the same number of colors that analog adapters can.

Over the years, VGA gave way to Super Video Graphics Array (SVGA). SVGA cards were based on VGA, but each card manufacturer added resolutions and increased color depth in different ways. Eventually, the Video Electronics Standards Association (VESA) agreed on a standard implementation of SVGA that provided up to 16.8-million colors and 1280x1024 resolution. Most graphics cards available today support Ultra Extended Graphics Array (UXGA). UXGA can support a palette of up to 16.8-million colors and resolutions up to 1600x1200 pixels.

Graphics cards adhere to industry standards so that you can choose from a variety of cards for your PC. Even though any card you can buy today will offer higher colors and resolution than the basic VGA specification, VGA mode is the de facto standard for graphics and is the minimum on all cards. In addition to including VGA, a graphics card must be able to connect to your computer. While there are still a number of graphics cards that plug into an Industry Standard Architecture (ISA) or Peripheral Component Interconnect (PCI) slot, most current graphics cards use the Accelerated Graphics Port (AGP).

 

 

 

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Last modified: 11/30/03