INT 14 - Digiboard DigiCHANNEL PC/X - SPECIAL CHARACTER FLAG/COUNTER
        AH = 1Ah
        BX = subfunction
            00h return pointer to special character flag byte
            01h return pointer to special character counter word
        DX = port number (00h-03h) (04h-43h for XAPCM232.SYS)
Return: ES:BX -> special character flag or counter
Notes:  flag is FFh if one or more special characters are in the receive
          buffer; it is 00h and the counter is invalid if no special characters
          are in the receive buffer
        counter (if valid) contains the number of characters in the receive
          buffer up to and including the last-received special character