The input and output devices supported by GDDM vary widely in their capabilities and in the datastreams required to drive them. The GDDM programming interface shields application programs from many of these device variations. This permits application programs to be written with a degree of device independence, which in turn permits these programs to use a range of different devices without modification to the programs.

The degree of device independence provided by GDDM varies, as follows:
• In some cases GDDM provides complete independence by emulating a function which a device may not have. For example, GDDM emulates graphics data on a 3279 display and image data on a 3472G display.
• In other cases, GDDM effectively ignores an application's requests for functions not available in the device, without rejecting the application calls. In general, it does this for functions that it cannot emulate and which, if ignored, would not normally affect a typical application program. For example, GDDM ignores requests for alphanumeric field highlighting on devices that do not support it.
• In other cases, GDDM is unable to emulate a function not available in a device. However, to ignore requests for the function would probably affect a typical application program adversely. In such a situation, GDDM responds to the function call with an error message. An example of this is a request to enable input from a mouse when the display device does not have one.

Subtopics:

• 4.1 Operator windows, partitions, primary, alternate, and dual screens
• 4.2 Device-specific saved pictures
• 4.3 Screen redraw
• 4.4 Programmed symbol sets (PS) and graphics text
• 4.5 Alphanumerics
• 4.6 Graphics colors
• 4.7 Color mixing
• 4.8 Graphics line types and widths
• 4.9 Graphics area shading
• 4.10 Graphics image
• 4.11 Graphics logical input devices
• 4.12 Image