INT 14 - SERIAL I/O - EXTENDED COMMUNICATION PORT CONTROL (CONVERTIBLE,PS)
        AH = 05h
        AL = 00h read modem control register
              Return: BL = modem control register (see below)
                      AH = status
        AL = 01h write modem control register
              BL = modem control register
                  bit 0: data terminal ready
                  bit 1: request to send
                  bit 2: OUT1
                  bit 3: OUT2
                  bit 4: LOOP
                  bits 5-7 reserved
              Return: AX = status
        DX = port number
SeeAlso: AH=1Fh