1. A data processing apparatus comprising: a primary processor; a secondary processor configured to perform secure data processing operations and non-secure data processing operations; and a memory configured to store secure data used by said secondary processor when performing said secure data processing operations and configured to store non-secure data used by said secondary processor when performing said non-secure data processing operations, wherein said secure data cannot be accessed by said non-secure data processing operations, wherein said secondary processor comprises a memory management unit configured to administer accesses to said memory from said secondary processor, said memory management unit configured to perform translations between virtual memory addresses used by said secondary processor and physical memory addresses used by said memory, wherein said translations are configured in dependence on a page table base address, said page table base address identifying a storage location in said memory of a set of descriptors defining said translations, wherein said page table base address is defined by said primary processor and cannot be amended by said secondary processor.