The Video Controller

Published: 16th April 2010
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The quality of the images that a monitor can display is defined more by the capabilities of another device, called the video controler, or video card, than by those of the monitor itself.

The video controller is an intermediary device between the CPU and the monitor. The controller contains the memory and other circuitry necessary to send information to the monitor for display on the screen.

The video controller uses it's own special memory to maintain the image that it receives from the CPU and sends to the monitor. In fact, the memory on a video controller is designed to be shared by the CPU and the controler. For this reason, it's called dual-port memory.

You could say that the dual port memory actually has two doors into each byte. The CPU goes through the front doors and set each byte, then the video controller comes along and opens all the back doors at once to let the bytes flow out to the monitor.

All microcomputers have a video controller, so what we've said so far applies to whatever monitor you are using or plan to use. If you use Macintosh, this is generally everything you need to know unless you are working with high quality graphics. However, several types of video controllers are available for the PC world.

The most important thing to know about PC video controllers is that most of them can operate in several modes. Some modes take advantage of the monitor's maximum resolution, whereas others don't even approach the maximum. More important, some modes can display only text, whereas others can display graphics.

Unlike the Macintosh, which has always been able to display graphics, many early PCs could display only text. Generally people didn't mind the limitation because at the time most software was text based. People used microcomputers primarily for word processing and creating spreadsheets, both of which worked well in text mode. Even today, many popular DOs based programs are designed to operate in text mode.

In text mode, the video controller divides the screen into a number of columns an rows for displaying whole characters. Although there are several text modes, the most common one displays 80 columns of characters on 25 rows. An 80 by 25 character screen can display up to 2000 characters at a time. Text modes can only display the alphabetic characters, numbers and other symbols that appear on a standard keyboard. Plus a range of graphc characters that are part of the PC caracter set.

In graphics mode, the controller can direct the monitor's phosphor guns to shoot beams at any point on the screen; there's much more information with which to contend than in text mode. There are 2000 character positions on the screen in a text mode; in a 640 x 480 pixel graphics mode, the same screen has 307,200 pixels.

Some video modes require more memory than many PCs have. That's an other reason video controllers come with their own dedicated memory.

Over the years, the PC's graphics capabilities have improved a great deal. The first video controller available with the PC was COlor Graphics Adapter ( CGA ). By modern standards, CGA resolution was rough. Demand for increased resolution led IBM to develop the Enhanced Grraphics Adapter ( EGA ) and other the Video Graphics Array ( VGA ). Today, CGA and EGA are standards of the past. Almost every PC sold comes with at least VGA output. Many are packaged with another adapter, the Super VGA ( SVGA ). IBM computers come with that company's extended Graphics Array ( XGA ), which is similar to SVGA.



This article was written by Stephan D. The owner an operator of the ePagini web site.


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