40-column text mode
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40-column text mode is part of the category Display Hardware Supported.
About Text Modes
In a text mode the individual pixels that make up the display image cannot be set individually. Instead, the screen is broken up into character cells where each cell can be set to one of the available predefined letters or symbols. This limitation allows high resolution text while needing less memory and allows faster screen updates. Since it was not possible to have control over individual pixels, the only type of graphics that could be displayed was ASCII art. The number of colors available, the display resolution, and the font used for the characters depended on what display adapter and monitor were being used; the font was typically stored in ROM and could not be modified by software (although some cards did allow font modifications).
40-column text on the IBM PC
With the IBM MDA adapter or Hercules Monochrome, 40-column text modes would display the same as 80-column text however only the leftmost 40 columns (the left side of the screen) would be available. Systems with a color graphics card such as CGA or EGA included a font for 40-column text so the text would fill the full width of the screen. With a CGA card connected to a composite monitor or t.v., 40-column text was typically preferred as it was much easier to read due to artifacting.
See also: 80-column text mode.