INT 31H 0303H: Allocate Real-mode Callback Address

                                                    Compatibility: DPMI 0.9+ 
 Expects: AX    0303H
          DS:SI (or DS:ESI) sel:off of protected-mode procedure to call
          ES:DI (or DS:ESI) sel:off of buffer to hold a DpmiRegRec structure
 Returns: CF    NC (clear) if successful; else AX = DPMI Error Code
          CX:DX segment:offset of real-mode callback
    Info: This obtains a segment:offset address that may be used by real-
          mode programs to execute a protected-mode procedure.

          This provides a means for real-mode programs, such as device
          drivers, interrupt handlers, or TSRs, to access services supplied
          by a protected-mode program or to notify that program of events
          which occur in real mode.

          The DPMI client uses this fn to obtain an address, then uses some
          other mechanism to pass that address to real-mode programs.
          Then, when the real-mode program call that address, the DPMI host
          fills the DpmiRegRec with the current registers, switches to
          protected mode, and then calls the protected mode procedure.

    DS:SI (or DS:ESI, when in 32-bit mode) is the address of a protected-
          mode procedure.  It must return via an IRET, with the data in the
          DpmiRegRec at ES:(E)DI containing any return information and the
          correct CS:IP to resume control must be in the structure.  That
          means you've got to dig through the stack in order to prepare for
          the return.

    ES:DI (or ES:EDI, when in 32-bit mode) points to a DpmiRegRec
          structure.  It may be empty when you use this fn.  The DPMI host
          fills it with data (as passed from the real-mode app) when it
          performs the callback.

          It is important to note that the segment values that will appear
          in the structure are real-mode segments, not protected mode

    Note: There is a limited number of these callbacks available per client
          (but at least 16).  Be sure to use INT 31H 0304H (free real-mode
          callback address) when you no longer need it.

See Also: DPMI API
          INT 2fH: Multiplex Interrupt
          DOS Functions