INT 1B - BIOS Ctrl-Break Handler Address

- holds address of the BIOS Ctrl-Break interrupt handler

- called by ~INT 9~ after Ctrl-Break key is pressed; INT 9 sets the

BIOS keyboard head and tail pointers to the buffer start and

places a zero word in the buffer

- the BIOS initially sets this value to point to a simple ~IRET~ but

DOS re-vectors this to its own code, usually 2 lines of code that

sets an internal flag (to 3) and then returns via IRET.

- DOS checks this flag on entry to many of its subfunctions. If the

flag is set, it invokes ~INT 23~.

- pointing this address to a null function with and IRET disables

Ctrl-Break aborts

- if the INT 1B code chooses to retain control of the system, it

must issue an EOI for any interrupt pending on the ~8259~ and reset

all I/O devices

- should be terminated via an IRET

- should not be called directly by user application