PURPOSE: To simplify the main memory control of an operating system by securing a test main memory area as a main memory area for a normal one execution part.

CONSTITUTION: Receiving a test memory securing request, the operating system activates a memory securing program and allocates the main memory area used by the memory securing program. The operating system transmits a test memory securing completion report including a main memory address allocated to a test auxiliary job to a test controller 14 through a test control interface 15, and causes the memory securing program in an execution pending state. The test controller 14 writes a test program and data in the main memory area allocated to the memory securing program through a test control interface 17, sets a test a test main memory area to an arithmetic processor 12 through a test control interface 16 and conducts a test on the arithmetic processor 12. Thus the main memory control can be simplified.