Function 0Bh            Check Keyboard Status

    Checks to see if a character is available from the standard input
    device. Checks for Ctrl-Break and Ctrl-C.

        Entry   AH = 0Bh

        Return  AL = FFh   One or more characters available
                AL = 00h   If no character is available

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    This function is used to see if a character is available at the
    standard input device (usually the keyboard), before transferring
    control to one of the waiting keyboard input services (function
    07h or 08h).

    If DOS encounters a Ctrl-Break or Ctrl-C, it executes an Int 23h.