Int 15 Fn 87  - Hunter 16 - Set Int 72h Vector                             [b]

   AH = 87h
   DS:DX = new service routine
Desc:  set the INT 72h vector which is called in 2 cases:
     - when the machine is about to turn off or reboot, INT 72h is called
       with AH=0
     - when the machine is powering up INT 72h is called with AH=01h

Note:  the actual INT 72h vector must also be changed with INT 21/AH=25h