Set Extended Processor Exception Handler Vector
(Real Mode)

Sets the address of the client's protected mode handler for
the specified real mode exception.

Call With:

   AX     = 0213H
   BL     = exception/fault number (00H-1FH)
   CX:(E)DX= selector:offset of exception handler

Returns:

   if function successful
   Carry flag= clear

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8021H   invalid value (BL not in range 00H-1FH)
           8022H   invalid selector

Notes:

o  CX:(E)DX does not specify a real-mode segment:offset.
   The reason is that this function allows a client to set
   the address of an exception handler which will receive
   control in protected mode when the specified exception
   occurs in real mode (i.e. the host will provide an
   implied mode switch for the purposes of servicing the
   exception, then return to real mode after  the handler
   exits).
o  Real mode exceptions are sent to the primary client of
   the virtual machine. (See Appendix A: Glossary for
   definition of primary client.)
o  Refer to the rules for descriptor usage in Appendix D.