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INT 14h,  06h    Raise/lower DTR

      Entry:  DX         Port number

              AL         DTR state to be set (01h = Raise, 00h = Lower)

      Exit:   None

    This function is used to control the DTR line to the modem. AL = 00h
    means lower DTR (disable the modem), and AL = 01h means to raise DTR
    (enable the modem). No other function (except Init) should alter DTR.