Apparatus for extending windows using z buffer memory

A computer system which has a display memory 14, 16 for storing information to be presented on an output display, and a full screen bitmapped window identification memory 20 for storing information regarding window position on an output display, and uses circuitry 18, 22 for comparing incoming information with information stored in the window identification memory 20 to determine whether the incoming information should appear in a particular window of the output display, and also includes a second full screen bitmapped memory 42 normally utilized for storing information indicative of other than window position on an output display utilizes circuitry for selectively (26) storing information in the second memory 42 regarding window position on the output display when additional windows are required. The second memory is normally used as a Z-buffer memory.