Compatibility: EMS 3.2+
Expects: AX    4700H
DX    EMM handle owned by caller
Returns: AH    EMM status (0=successful)
Info: EMM will save the current mapping of physical to logical EMS
pages, so that a TSR program or device driver may page-in its own
memory transparently to the interrupted program.

Use INT 67H 4800H restores the original mapping.

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INT 67H 4700H: Save Mapping Context