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Int 1Bh                 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.


    Note
    DOS tests the state of the internal flag occasionally, depending
    on the state of the Control- Break Check (Int 21h, Function
    3300h). 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 0040h:0071h to
    signal that a Control-Break key has been entered.