# Appendix B. VDG Display System Functions

## The Video Display Generator

OS-9 allows the VDG display to be used in alphanumeric, semigraphic, and graphics modes. There are many built-in functions to control the display, which are activated by used of various ASCII control characters. Thus, these functions are available for use by software written in any language using standard output statements (such as “PRINT” in BASIC). The Basic09 language has a Graphics Interface Module that can automatically generate these codes using Basic09 RUN statements.

The display system has two display modes: Alphanumeric (“Alpha”) mode and Graphics mode. The Alphanumeric mode also includes “semigraphic” box-graphics. The computer's display system uses a separate - memory area for each display mode so operations on the Alpha display do not affect the Graphics display, and visa-versa. Either display can be selected under software control.

8-bit characters sent to the display system are interpreted according to their numerical value, as shown in the chart below.

Character Range (Hex) | Mode/Used For
--- | ---
00 - 0E | Alpha Mode - cursor and screen control
0F - 1B | Graphics Mode - drawing and screen control
1C - 20 | Not used
20 - 5F | Alpha Mode - upper case characters
60 - 7F | Alpha Mode - lower case characters
80 - FF | Alpha Mode - Semigraphic patterns

The graphics and alphanumeric functions are handled by the OS-9 device driver module called “CCIO” or “KBVDIO”.