INT 14,5 - Extended Communication Port Control (PS/2 & later systems)

	AH = 05
	AL = read or write modem control register
	   = 00 read modem control register
	   = 01 read modem control register
	DX = zero based serial port number (0-1) (0-3 for AT)

	BL = modem control register

		|7|6|5|4|3|2|1|0|	BL
		 | | | | | | | `---- 1 = activate data terminal ready
		 | | | | | | `----- 1 = activate request to send
		 | | | | | `------ OUT1
		 | | | | `------- OUT2
		 | | | `-------- 0 = normal, 1 = loopback test
		 `------------- reserved, forced to zero

	on return:
	AH = port status
	AL = modem status
	BL = UART modem control register

	- for status bits see INT 14,STATUS