The Electronic Labyrinth

Graphic Display Standards

Every computer display is composed of a rectangular array of pixels (picture elements). The more pixels, the more detail may be shown in a given amount of space. This is termed the resolution. Each pixel may display a given number of distinct colours; this is the colour depth.

In order to work together, operating systems, graphics boards, and monitors support a number of standard video modes. As hardware has improved, and users have become more demanding, video modes have tended towards higher resolutions and greater colour depth. This requires a larger amount of dedicated memory, either on the computer motherboard, or on the graphics card.

In some modes, the software addresses each pixel individually; these are graphics modes. In order to speed processing, DOS systems often address pixels in character boxes of a fixed size; these are termed text modes. Each box is allocated a symbol from the ASCII character set.

For example, mode 18 supports a resolution of 640x480, for a total of 307,200 addresses. Using a character block of 16x8, 30 lines of 80 characters each may be displayed. As there are now only 2,400 distinct addresses, performance is greatly enhanced. This explains the phenomenon familiar to DOS users: software takes a lot longer to display a graphic image than a screen full of text.

There have been several DOS graphics standards. Each is backwards compatible with those which have gone before. The following list includes the maximum resolution and colour depth for each standard:

Resolutions beyond SVGA are commonly available, though standardization is poor. Various "standards" (eg. EVGA, 8514/A, XGA, and XGA-2) support 1024x768 resolution at different colour depths. The trend in the last two years has been towards a proprietary mix of display modes driven by specialized graphics accelerators. These have been designed to optimize performance for particular applications (AutoCAD) or environments (Microsoft Windows).

On Apple computers, all displaying is done in graphics mode. This permits a greater number of screen characters, though with a performance penalty. The lower resolution of Mac displays tends to render fuzzier images. Graphics standards are not as important, since there is a far smaller third-party market. Most users buy an improved graphics card in a bundle with a new monitor from the same manufacturer.

© 1993-2000 Christopher Keep, Tim McLaughlin, Robin Parmar.
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