Abstract: In a data processing system having a host processor, a cache store for storing segments of data, a bulk memory and a storage control unit for controlling transfers between the processor, cache store and bulk memory, the storage control unit normally responds to a read or write command from the host processor to control the transfer of data. If a copy of the data transferred is not resident in the cache store then a copy is written therein by the storage control unit. If the length of a data transfer exceeds a first threshold length then the storage control unit does not write a copy of the data into the cache store. If the length of a data transfer exceeds a second threshold length, and the transfer begins on a segment boundary and comprises an integral number of segments, then the storage control unit does not write a copy of the data into the cache store. The writing into the cache store is transparent to the host processor.