Expects: AH    02H
DX    COMM port number (0 to 3)
                                                                  
Returns: AH    0    = character received successfully
else = an error occurred; see BIOS COM status codes
AL    character that came in
                                                                  
Info: Receives one character from the specified COM port.  If AH is 00
on return, the received character is in AL.

Notes: The return status code in AH cannot indicate a time-out error
since bit 7 is used to flag any error.  Thus, after any error,
you should use INT 14H 03H to get the full story on what went
wrong.

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INT 14H 02H: Receive a Character