INT 69 - ISR.COM v1.00 - SPECIFY INTERRUPT HANDLER
        AX = 6996h
        DS:DX -> interrupt handler or 0000h:0000h to disable
Return: AX = 9669h
Notes:  ISR (Interrupt Service Reflector) is a TSR by Rich Bono which permits
          a program to provide hardware interrupt handlers even while being
          debugged with a debugger that swaps interrupt vectors during
          debugging.
        the interrupt vector which is to be reflected is set at installation
          time and cannot be changed