AH = 4Eh
        AL = 00h if getting mapping registers
             01h if setting mapping registers
             02h if getting and setting mapping registers at once
             03h if getting size of page-mapping array
        DS:SI = pointer to array holding information (AL=01/02)
        ES:DI = pointer to array to receive information (AL=00/02)
Return: AH = status
            00h successful
                AL = bytes in page-mapping array (subfunction 03h only)
                array pointed to by ES:DI receives mapping info (AL=00/02)
            80h internal error
            81h hardware malfunction
            84h undefined function requested
            8Fh undefined subfunction parameter
            A3h contents of source array corrupted (EMS 4.0?)
Note: this function was designed to be used by multitasking operating systems
      and should not ordinarily be used by appplication software.
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