Extended communication port control (PS)

        Entry:
          AH = 05h
          AL = 0 Read modem control register
                 Return: BL = modem control register (see below)
                         AH = status
          AL = 1 Write modem control register
                 Return: 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
                         AH = status