INT 14h,  04h (4)         Advanced Port Control                         Some

     Advanced port control (not supported on all systems)

       On entry:      AH         05h
                          AL         Subservice (0 - 1)
                                     00h :  Get modem register
                             Returns:    bl bit 0 = Data Terminal Ready (DTR)
                                             bit 1 = Request To Send (RTS)
                                             bit 2 = Out 1
                                             bit 3 = Out 2
                                             bit 4 = Loop modem

                                     01h  Set modem register
                             Call with:  bl = value for register (see above)