A data processing apparatus comprises at least one memory configured to store data; processing circuitry to access data in the at least one memory. Memory built-in self-test (MBIST) circuitry has an interface to access the at least one memory and is configured to perform a test procedure for testing at least one target memory location of the at least one memory. The test procedure involves at least writing test data to the target memory location. Diagnostic circuitry executes a diagnostic procedure to generate diagnostic data in response to processing operations carried out by the processing circuitry. The MBIST circuitry is configured to control writing of the diagnostic data generated by the diagnostic circuitry to memory locations in a temporarily reserved memory region comprising at least a portion of the at least one memory.