a storage hierarchy has a caching buffer and a backing store ; the backing store preferably having disk - type data - storage apparatus . a directory indicates data stored in the caching buffer . upon a data - storage access , read or write , within a series of such accesses , resulting in a cache miss , all subsequent data storage accesses in the series are made to the backing store to the exclusion of the caching buffer even though the caching buffer has storage space allocated for such a data transfer . selected limits are placed on the series to the backing store , such as receiving on end of series indication from a using unit , crossing dasd cylinder boundaries , receiving an out of bounds address or receiving certain device oriented commands .