1. Apparatus for processing data comprising: a memory; at least one a processor coupled to said memory and responsive to program instructions to perform data processing operations; and a cache memory coupled to said memory and to said processor; wherein said cache memory has validity data and control data associated therewith, said validity data specifying if cached data is valid data and said control data specifying how access. to cached data is controlled, and said cache memory is responsive to one or more program instructions executed by one of said at least one processors to modify said control data.