Int 6B Fn 14  - Npc Ncsi Extended Serial I/o - Set/retrieve Service Addre  [S]

   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

See Also: AH=13h,AH=15h,AH=21h