INT 67 - LIM EMS 4.0 - ALTER PAGE MAP AND CALL
        AH = 56h
        AL = subfunction
            00h physical page numbers provided by caller
                DX = handle
                DS:SI -> structure containing page map and call address
            01h segment addresses provided by caller
                DX = handle
                DS:SI -> structure containing page map and call address
            02h get page map stack space required
                Return: BX = stack space required
Return: (if successful, the target address is called.  Use a RETF to return and
         restore mapping context)
        AH = status (see AH=55h)