PURPOSE: To execute the diagnosis/initialization of a cache memory and a main storage memory by a memory diagnostic program on the cache memory at high speed in a processor provided with the cache memory between the main memory and the processor.

CONSTITUTION: The memory diagnosis/initialization system diagnoses/initializes the cache memory 4 and a volatile main storage memory 7 at high speed in the processor 1 provided with the cache memory 4 between the processor 1 the main memory 6. The system is composed of the processor 1 provided with the cache memory 4, a cache memory writing circuit 11 and a cache memory selection circuit 5 and the main memory 6 and the system is provided with a means directly reading/writing the cache memory to write a cache memory diagnostic program 10a into the cache memory by the means to diagnose/ initialize the cache memory by the pertinent program.