This vector (0:006c) is taken by BIOS when it senses a press of the
Ctrl-Break key.

INT 1bH initially points to an IRET, but DOS sets it to point to local
code that sets an internal DOS flag.  DOS tests every once in a while and
acts upon it, depending upon the state of DOS break-testing.

See DOS Fn 33H and INT 23H for descriptions of how DOS handles Ctrl-Break.

A flag in the BIOS Data Area (0:0471) gets set upon the first occurrence
of Ctrl-Break, but it never gets reset unless you do it yourself.  You
might use this flag as a way to determine if a child process was aborted
via Ctrl-Break.

See Also: DOS Interrupts
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INT 1bH: Keyboard Break