AH = 56h
        AL = subfunction
            00h physical page numbers provided by caller
               DX = handle
               DS:SI = pointer to structure containing page map and call address
            01h segment addresses provided by caller
               DX = handle
               DS:SI = pointer to structure containing page map and call address
            02h get page map stack space required
Return: (if successful, the target address is called.  Use a RETF to return and
         restore mapping context)
        BX = stack space required (subfunction 02h)
        AH = status
            00h successful
            80h internal error
            81h hardware failure
            83h invalid handle
            84h undefined function requested
            8Ah invalid logical page number encountered
            8Bh invalid physical page number encountered
            8Fh undefined subfunction
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