Set Real Mode Interrupt Vector


                 This function sets the value of the current task's real
                 mode interrupt vector for the specified interrupt.

            To Call

                 AX = 0201h
                 BL = Interrupt number
                 CX:DX = Segment:Offset of real mode interrupt handler

            Returns

                 If function was successful:
                 Carry flag is clear.

                 If function was not successful:
                 Carry flag is set.

            Programmer's Notes

                 o    The address  passed in  CX must  be  a  real  mode
                      segment, not a selector.
                 o    If  the  interrupt  being  hooked  is  a  hardware
                      interrupt then  you must lock the segment that the
                      interrupt handler  runs in  as well  as any memory
                      the handler may touch at interrupt time.
                 o    The address contained in CX:DX must be a real mode
                      segment:offset, not a selector:offset.  This means
                      that the  code  for  the  interrupt  handler  must
                      either reside  in DOS  addressable memory  or  you
                      must use  a real mode call-back address.