Scroll page down

        Entry:
          AH    = 07h
          AL    = number of lines to scroll window (0 = blank whole window)
          BH    = attributes to be used on blanked lines
          CH,CL = row,column of upper left corner of window to scroll
          DH,DL = row,column of lower right corner of window
        Return:
          AX,SI,DI,BP destroyed by call