Abstract: A method for allocating real pages larger than a conventional size to a plurality of virtual pages of the conventional size in a system including a real storage containing a plurality of real pages and a storage key for holding storage keys used for storage protection purposes of the real pages. The method includes the steps of (A) allocating one of plural split regions having the conventional size obtained by subdividing one of a plurality of real pages having a larger size equal to a value n being a positive integer larger than 1) times larger than the conventional size, to one virtual page having the conventional size (B) repeating step (A) so that split regions having the conventional size within the one real page and within other real pages having the larger size are allocated to virtual pages having the conventional size.