INT 23 - Control-Break Exit Address
no input data
- not an interrupt but a pointer to a routine that is called when a
DOS function detects a Ctrl-Break or Ctrl-C has been pressed
resulting in a "Break condition"
- a Break condition is detected if DOS's internal Break flag is
found set by INT 1B or the next word in the BIOS keyboard buffer
is a scan code representing one of the Break key combinations.
If a matching scan code is found, the keyboard buffer head pointer
is incremented by 2 (which effectively removes the ^C keycode), the
^C is printed and then INT 23 is called.
- disabling the abort is easy, but to disable the ^C from appearing
you must either not use any of the DOS I/O functions that check
for Break or you must trap the Ctrl-C keypress from within INT 9.
Another method is to scan the BIOS keyboard buffer and delete any
Break key combinations.
- Ctrl-Break empties the keyboard buffer by resetting the keyboard
head and tail pointers then places a 0000h at the queue head. It
then sets an internal "Break" flag. DOS subfunctions that check
for Break see this and then issue INT 23h. DOS does not detect
INT 1B using the keyboard buffer, but uses it's own internal flag.
- Alt-Keypad-3 and Ctrl-2 also result in this interrupt
- Ctrl-C places 2E03h in the BIOS keyboard buffer while Ctrl-2
places 0300h and Alt-Keypad-3 places 0003h; none of these key
combinations empty the keyboard like Ctrl-Break but all result
in a ^C being displayed; note that all three produce scan codes
- since DOS checks only the first word of the keyboard buffer
Ctrl-C, Ctrl-2 and Alt-Keypad-3 are only detected if they are the
first word in the buffer while Ctrl-Break is detected via the
internal flag and takes effect as soon as it's detected.
- do not execute this interrupt directly
- see also INT 9