Allocate Real Mode Callback Address

Returns a unique real mode segment:offset, known as a "real
mode callback,"  that will transfer control from real mode
to a protected mode procedure.  Callback addresses obtained
with this function can be passed by a protected mode program
to a real mode application, interrupt handler, device
driver, or TSR, so that the real mode program can call
procedures within the protected mode program or notify the
protected mode program of an event.

Call With:

   AX     = 0303H
   DS:(E)SI= selector:offset of protected mode procedure to
           call
   ES:(E)DI= selector:offset of 32H-byte buffer for real
           mode register data structure to be used when
           calling callback routine.

Returns:

   if function successful
   Carry flag= clear
   CX:DX  = segment:offset of real mode callback

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8015H   callback unavailable

Notes:

o  DPMI hosts must provide a minimum of 16 callback
   addresses per client.
o  A descriptor may be allocated for each callback to hold
   the real mode SS descriptor. Real mode callbacks are a
   limited system resource.  A client should use the Free
   Real Mode Callback Address function (Int 31H Function
   0304H) to release a callback that it is no longer using.
o  For further information on writing real mode callback
   procedures, see page 34.
o  The contents of the real mode register data structure is
   not valid after the function call, but only at the time
   of the actual callback.