A system is provided in which a first circuit is protected by security features provided by a second circuit. The first circuit comprises a processor which retrieves content from a memory. Initially, the contents of the memory are authenticated using security features of the second circuit to check that the processor is accessing authenticated content. To maintain security during use, the second circuit checks that the processor is accessing content from valid regions of the memory, being those that have been authenticated, and re-checks the authenticity of the content of the valid regions of memory. The combination of checking that the processor is accessing from valid regions of the memory and authenticating the content stored in the valid regions maximizes the security of the system. If any of the checking or authentication steps fail then operation of the system is impaired. The first circuit is thus protected by security features provided by the second circuit.