The original IBM PC fully supported dual-monitor configurations, but unlike today, it was an expensive setup and required separate video cards. When the IBM PC was introduced, the only available combination was an MDA (Monochrome Display Adapter) connected to an IBM 5151 Monochrome Display, and a CGA (Color Graphics Adapter) connected to an IBM 5153 Color Display.
At any given time, only one display was considered “active”, and the initial active display was determined by switches on the
motherboard. Beginning with PC DOS 2.00, you could use
MODE commands to change the active display:
MODE CO40selects the Color Display and sets it to 40-column mode
MODE CO80selects the Color Display and sets it to 80-column mode
MODE MONOselects the Monochrome Display, which supports only 80-column mode
The initial active display for the machine below is the Monochrome Display. Third-party software with dual-display support could write to both displays simultaneously, but most text-based applications only wrote to the active display. Programs that used color graphics, on the other hand, might use the Color Display whether or not it was currently active.