INT 79 U - AutoCAD Device Interface - DIGITIZER - GET DIGITIZER STATUS
        AX = 0003h
Return: AX = button state
            0002h no button pressed
                BX = X coordinate
                CX = Y coordinate
            0003h Pick button (0,A,B,C,D) pressed
                BX = X coordinate
                CX = Y coordinate
            0005h other button (1-9) pressed
                BX = number of button - 1
                CX = X coordinate
                DX = Y coordinate
Note:   these return values are valid for the Houston Instruments HiPad Plus
          9236 digitizer with a 16-button cursor
SeeAlso: AX=0002h