Int 14 Fn E9  - Telapi - "telportosn" - Get Session Number For Com Port    [N]

   AH = E9h
   DX = serial port number (0000h-0003h = COM1-COM4)

Return: AX >= 0000h session number
   AX < 0000h error code (see #0323)

See Also: AH=E0h,AH=E8h,AH=EAh,AX=FF00h