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Function 0Bh (11)        Check Standard Input Status

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

       On entry:      AH         0Bh

       Returns:       AL         FFh       If a character is available
                                 00h       If no character is available

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    This function can be 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 01h,
    Function 07h, and Function 08h).

       Notes:         The function returns FFh if a character is available
                      and 00h if none is available.

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