320 of a second tier an entry 310 of a first tier is provided with a pointer 311 to the entry of the second tier and a representative bit 313 and the entry 320 of the second tier is made unnecessary when all values of the corresponding bits 321 are equal to a value of the representative bit 313 application_number US 19719405 A description hereinafter embodiments of the present invention are explained by using accompanied drawings but it is needless to say that the present invention is not limited to the embodiments explained hereinafter first a first embodiment of an information processing system to which the present invention is applied is explained by using fig1 through fig4 fig1 is a diagram showing a general configuration of a storage apparatus of the first embodiment this system is provided with at least one set of host processor 12 at least one set of storage apparatus 13 and a san storage area network link 17 the host processor 12 is a computer that uses data stored on the storage apparatus 13 the host processor 12 has a built in san interface san i f 171 the san i f 171 is an adaptor for the host processor 12 to input and output data to from the storage apparatus 13 through the san link 17 and more specifically.