Apparatus for processing data under control of data processing instructions specifying data processing operations, said apparatus comprising: a first execution mechanism operable to execute a first set of data processing instructions; a second execution mechanism operable to execute a second set of data processing instructions, said first set of data processing instructions overlapping with said second set of data processing instructions such that one or more data processing instructions are executable by either said first execution mechanism or said second execution mechanism; and an execution mechanism selector operable to pseudo randomly selected either said first execution mechanism or said second execution mechanism to execute one or more data processing instructions that are executable by either said first execution mechanism or said second execution mechanism.