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Function 06h (6)         Direct Console Character I/O

    Gets a character from the standard input device, if one is available;
    or outputs a character to the standard output device. Does not echo
    characters read or check for Ctrl-Break or Ctrl-C.

 1. To input a character

       On entry:      AH         06h
                      DL         FFh

       Returns:       AL         Character
                      ZF         0    If a character is available
                      ZF         1    If no character is available

 2. To output a character

       On entry:      AH         06h
                      DL         Character

       Returns:       Nothing

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       Notes:         If this function is used for input and no character
                      is ready, the Zero Flag is set; if a character is
                      ready, the Zero Flag is cleared and the character
                      value is returned in AL. Input is not echoed to the
                      standard output device.

                      A 0 returned in AL means that one of the "special"
                      characters (a function key or a cursor key, for
                      example) was input. A second call to Function 06h
                      will then return the extended code of the special
                      character. See the Key Codes table for a list of
                      special keys and their extended codes.

                      This function does not issue an INT 23h in response
                      to Ctrl-Break or Ctrl-C.