Set Real Mode Interrupt Vector

Sets the current virtual machine's real mode interrupt
vector for the specified interrupt.

Call With:

   AX     = 0201H
   BL     = interrupt number
   CX:DX  = segment:offset of real mode interrupt handler

Returns:

   Carry flag= clear (this function always succeeds)

Notes:

o  The address passed in CX must be a real mode segment
   address, not a selector.  Consequently, the interrupt
   handler must either reside in DOS memory (i.e. below the
   1 MB boundary) or the client must allocate a real mode
   callback address.  See Int 31H Functions 0100H and 0303H.
o  If the interrupt being hooked is a hardware interrupt,
   the memory  that the interrupt handler uses must be
   locked.