INT 10 - TopView - UPDATE SCREEN FROM SHADOW BUFFER
        AH = FFh
        CX = number of consecutive changed characters
        ES:DI -> first changed character in shadow buffer
Notes:  avoid CX=0000h
        DESQview will discontinue the automatic screen updating initiated by
          AH=FEh after this call
        not supported (ignored) by DESQview/X 1.0x