1.In a multi-window system for setting a plurality of windows on the screen for independent display in each window, a novel display control method is disclosed in which a visible region of a given window is determined from the positions and overlapped relations between the windows on the display screen, and display data is selectively applied to the visible region. The visible region is checked for crossing between an object window and another window making up a reference window on the screen, and if they cross each other, the object window is divided into subregions, while the subregion not overlapped with the reference window is regarded as a new object region, which is collated with another reference window and subdivided. This process of collation and division is repeated. A display output is thus applied sequentially to the subregions which are found to be a visible region (abstract). In the multi-window system, the respective windows set on the display screen correspond to logical or virtual display screens set on a memory which is referred to by a data processing system. Data stored in a part of this virtual screen is extracted, and developed as a bit map in a predetermined region on a frame memory corresponding to an actual display screen so as to be read-out as the contents to be displayed in the window. Data representing the position and size of each window on the actual screen and the correspondence between the window and an extraction region on the virtual screen are registered in a specific area alloted to that window on a window control table, so that the operator can, by inputting a screen operation command into the data processing system, change and control the contents of the display in any desired window on the display screen by reference to the window control table (col. 1, ln 29-46).