Function 06h            Direct Console I/O

    Reads a character (if one is available) from standard input or
    outputs a character to standard output. Does not echo characters
    read or check for Ctrl-C.

    Character input:
        Entry   AH = 06h
                DL = FFh

        Return  ZF = 1  No character available
                        (AL undefined, but usually zero)
                ZF = 0  AL = Character


    Character output:
        Entry   AH = 06h
                DL = Character (< FFh)

        Return  AL = Character (undocumented)

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    If no character is available when reading from standard input,
    this function does not wait but returns immediately.

    This function is typically used by programs that must be able to
    read and write any character or control code. Ctrl-C is returned
    as ASCII 03h.

    If the character read from the keyboard is an extended key code
    (for example, if the user presses one of the function keys), this
    function returns 00h and the program must call the function again
    to get the second byte of the extended key code.