INT 6B - NPC NCSI EXTENDED SERIAL I/O - SET/RETRIEVE SERVICE ADDRESS            
	AH = 14h
	AL = virtual circuit number
	ES:BX -> buffer for/containing service address
Return: ???
Note:	this function is also supported by TelAPI, which only supports
	  retrieving the address
SeeAlso: AH=13h,AH=15h,AH=21h