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Unaccelerated and Accelerated Video Cards
The video card is only one part of the equation that determines what you see on your screen. It is in a way the "middle man", working between the processor and the monitor. The monitor, of course, is what actually provides the display that you see. The processor computes and thus determines what you are going to see. A conventional video card does the job of translating what the processor produces into a form that the monitor can display.
Older video cards did this translation only; they were rather dumb in that they could only take what the processor created and send it to the monitor. The processor did all of the work of deciding what would be displayed. This was fine for older environments like DOS, and especially for text-based output where the amount of information involved was small. When graphical operating systems like Windows became the norm, suddenly large amounts of data were being shuffled around on the screen, and the CPU was spending a lot of time moving windows around, and drawing boxes and cursors and frames. As a result the processor would often get bogged down and performance would decrease--dramatically.
To clear this bottleneck companies began making cards called accelerators; in fact, Windows drove this effort so much that they were often called Windows accelerators. These were video cards that added smarts to enable them to do much of the video calculating work that had been previously done by the processor. With an accelerator, when the system needs to draw a box on the screen, it doesn't compute where all the pixels need to be and what color, it sends a request to the video card saying "draw a window at these locations" and the video card does it. The processor can then go on to do more useful work. The accelerator, for its part, can be highly customized and tailored to this specific job, and therefore be far more efficient at it than the processor.
This offloading of video calculation work has led to a many-fold increase in the power of the video subsystem in a modern PC. Virtually all modern video cards incorporate acceleration, some of it quite sophisticated. In essence, the video card becomes a coprocessor, working with the main CPU. Continuing the trend, new 3D accelerators are becoming more common, which offload the (tremendously time-consuming) work of 3D animation from the processor as well.
Next: The Video Chipset