A graphics data processor which includes the capability of determining whether a defined pixel location in a graphics display is within a window in an X Y coordinate system. The respective X and Y coordinates of the selected pixel are separately compared with the window limits. The window limits are preferable expressed as the X and Y coordinates of two diagonally opposite vertexes of a rectangular window. The results of this comparison are preferable available in two forms. In a first embodiment a single data processing instruction enables the generation of a digital data word which indicates the relation of the pixel to the window. This digital word includes a separate indication of the relationship of the pixel to the vertical and horizontal window limits. This indication can be used to generate a "trivial rejection" in determining whether a line or line segment passes through the window by ANDing the results for two points on the line. In a second embodiment the window compare capability is employed to determine whether or not a destination pixel is within the window. This is useful in array move instructions in which an entire array of pixels is moved to a location in the display. The array move may be aborted if a window violation is found or the move may be modified to plot to the display only those pixels within the window.