Each window has a destination pointer that indicates where its output goes when it is set on the screen and output is written to it. The destination is a memory screen. By default, all windows use VID_SCRP, a pre-defined memory screen that maps to the physical screen. Thus when the window is put up, it physically shows on the screen. You can, however, change the destination of a window.
There are many good reasons for changing the destination pointer. You can use it to combine several windows on a single destination to format output. You can isolate overlapping windows for printing. Or you can use it to speed the update when you are putting several windows on the screen at once.
Changing a Window's Destination
Behavior of Windows with a Memory Screen Destination
Changing the Destination Back to the Video Screen
