AH = 5
        AL = 0 read modem control register
              Return: BL = modem control register (see below)
                      AH = status
        AL = 1 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: AH = status
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