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INT 1Bh (27)             Keyboard Break
    Called when a Control-Break key sequence is encountered.

  --------------------------------------------------------------------------

    When the machine is powered on, the interrupt handler for this
    interrupt just IRETs back to the caller. DOS, however, installs a
    Ctrl-Break handler that sets an internal flag to signal that a
    keyboard Break condition has occurred.

       Notes:         DOS tests the state of the internal flag
                      occasionally, depending on the state of the Control-
                      Break Check (INT 21h, Function 33h). If the flag is
                      on during a test, then the string ". followed by
                      a Carriage-Return/ Line-Feed combination, is
                      displayed on the screen, the DOS internal flag is
                      reset, and INT 23h (Control-Break Exit Address) is
                      called.

                      Note that INT 09h (Keyboard) sets the byte at
                      0:0471h to signal that a Control-Break key has been
                      entered.