execution the execution is as follows at step 1900 it is determined if an input output request that has been received requires access to the disk unit 204 to specifically define the type of an input output request which requires access to the disk unit 204 is not directly related to the present invention and therefore is omitted from the detailed description if the input output request received does not require access to the disk unit 204 the processing proceeds to the step 1918 if the answer to the determination in step 1900 is yes step 1901 starts executing the input output request by first determining if the requested input output track resides in the cache specifically a track 800 which the input output request wishes to gain access to is checked to determine whether or not a segment 1000 is assigned thereto if an assignment is made processing proceeds to step 1903 if no assignment is made as determined by step 1901 step 1902 assigns segment management information 1100 to the track 800 to which the input output request wishes to gain access and links it to the corresponding area of the track table 1101 also the store completion flag 1204 for the assigned segment management information 1100 is turned off and the active flag 1205 is turned on at this time the segment management information 1100 of the assignment is one selected from the segment management information 39 s 1100 that are in an inactive state starting at the empty segment head que 1102 if.