A data processing system comprises a memory, a processing unit, a MMU operable to process memory transactions for the processing unit; and an error-detecting circuit located between the MMU and the memory. The MMU is configured to determine from respective memory address mappings corresponding memory addresses to which memory transaction applies. The determined memory addresses comprise a first part representing a memory location and a second part representing an error-detecting code generated using the first part of the memory address. The error-detecting circuit can then check using the error-detecting code for the memory address whether the memory address received by the error-detecting circuit is valid.