Alter page map and call

        Entry:
          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 invalid subfunction