EXPANDING A ROM FOR CHARACTER GRAPHICS

The most basic way to add graphics features to the computer is by defining a new set of graphics characters in a larger ROM. A graphics character is the same size as a regular text character. As shown in Fig. 2-9A a 256-byte ROM can be used instead of the 64-byte 2513. The ROM has eight address inputs, labeled A0-A7. The A7 line is used to switch in a set of 128 graphics characters or a set of 128 text characters (128 + 128 = 256).

This is the approach used in computers like the PET. In the PET a symbol of the graphics character is scribed onto the keyboard key just above the letter on the key. The graphics characters act just like any text symbol and appear when you strike the key and at the same time a special graphics upper-case key.

The format for the graphics characters is usually defined by the design engineer and is burned into the ROM. Therefore you will find no exact standard here, except of course the standard created by the number of PETs sold (estimated at 25,000 in 1978). However, within most graphics character sets you will find what are called block graphics characters (Fig. 2-9B) and line graphics characters (Fig. 2-9C). These characters can be used to build up complex figures and shapes but these feats of construction require some ingenuity on the part of the programmer.