Int 14 Fn 13  - PC-mos/386 V5.01 $serial.sys V5.04 - Register A Port With  [S]

   AH = 13h
   DX = port number

Return: AH = status
       FFh port number invalid
       else
       ES:BX -> BYTE flag (00h buffer empty, FFh buffer contains data)

See Also: AH=17h"PC-MOS"