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