INT 31H 0304H: Free Real-mode Callback Address

                                                    Compatibility: DPMI 0.9+ 
 Expects: AX    0304H
          CX:DX segment:offset of real-mode callback to free
 Returns: CF    NC (clear) if successful; else AX = DPMI Error Code
    Info: This frees up the system resources consumed when a real-mode
          callback is allocated and it makes that callback address invalid.

          This is part of the DPMI system that allows a real-mode program
          to access services to otherwise communicate with a protected mode

          See INT 31H 0303H (allocate real-mode callback address) for
          related details.

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