A data processing system is provided with at least one processor 4, 6, a main memory 18 and a cache memory 14. Cache data within the cache memory 14 has validity data V and control data associated therewith. The control data controls access to the cached data. Program instructions executed by the processors 4, 6 control a cache controller 26 to modify the control data associated with the cached data while it remains stored within the cache memory 14 and remains valid. The control data may, for example, specify a security flags indicating whether access is restricted to secure processes or processors.