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
- 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