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

containing '03'

- 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~