In order to display text information on a computer screen we need some way to represent symbols and characters in the computer. Historically, all letters, numbers, and special symbols of the alphabet are coded in computers so each character is a unique 6- or 7-bit value called an ASCII byte. (The number of bits used depends on whether the lowercase is included in the character set.) In order to display the ASCII characters, circuits must be built that convert these bit-coded characters to images on the screen. Furthermore, the characters on the tv screen must be refreshed every 1/30 of a second in order to appear permanent to the eye.

In memory mapping a special area of the computer system's RAM is set aside to be the “screen” memory, that is, an area of memory devoted entirely to holding the ASCII characters that will appear on the screen display. Fig. 2-3 illustrates the memory-mapped concept.


Fig. 2-3. In memory-mapped video the screen memory is part of regular RAM.

As you can see, each location on the screen for a character corresponds to a specific byte number in the long chain of screen memory. Usually the memory is arranged in a simple fashion so each row of characters on the screen corresponds to a series of contiguous bytes in memory. It is important to understand that the part of the system's RAM used by the display can still be used like any other memory, but the computer will convert the bytes in the memory to equivalent ASCII characters and put them on the screen.


(A) The 5×7 ASCII character font from 2513 character generator ROM. Fig. 2-4. Popular character fonts.
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