Get Extended Processor Exception Handler Vector
(Real Mode)

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

Call With:

   AX     = 0211H
   BL     = exception number (00H-1FH)

Returns:

   if function successful
   Carry flag= clear
   CX:(E)DX= selector
   :offset of exception handler

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

Notes:

o  CX:(E)DX does not specify a real-mode segment:offset.
   The reason is that this function allows a client to get
   the address of the 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.