An apparatus for displaying the display outputs of a plurality of simultaneously active computer processes in corresponding windows on a single screen includes a microcomputer, a display screen and display system software. The software represents a plurality of virtual terminals, one corresponding to each active process, for emulating the operation of real terminals communicating with the processes. Each virtual terminal maintains a display list comprising a set of instructions defining a display on a screen according to the output from the corresponding process. The software also includes a screen process for maintaining a subrectangle list comprising a set of instructions for allocating window portions of the screen to the displays defined by the separate display lists. A display list processor is provided for creating the windows on the screen according to the display and subrectangle lists.