Character-mapped display screens, such as the IBM monochrome monitor, can display only characters. (Note: As described in the next section, a character-mapped display screen may be able to display graphics if a video adapter card is plugged into the motherboard.) The patterns of pixels used to represent the standard characters displayed one a monitor (the alphabetic characters, numbers, and special symbols) in character-mapped displays are drawn from prerecorded templates (guides) stored in a video display ROM chip.
When the user’s software sends a request to display, for example, the letter at a specific location, the template for that pixel pattern is looked up in the video display ROM chip. The electron gun then uses this pattern when it fires at the phosphor in the appropriate character box. The screen of a personal computer has 25 lines with 80 characters per line; this means that there are 2000 positions on the screen where a predefined character can be placed.