PURPOSE: To economize system constitution by making it possible for CPU to use channels by separating them mutually by providing a partition mode indicating method.

CONSTITUTION: When the partition mode flag in main memory controller MCU4 is not set, this system operates in multiprocessor mode. Namely, one multiprocessor OS (operating system) controls two CPUs and each CPU uses main memory unit MSU5, 6 and channel processor CHP3 in common. On the other hand, when the partition mode flag is set, each CPU is brought under the control of two independent OSs separately and each CPU partitions MSU5, 6 and CHP3 fixedly for use. Through MCU4, accessed CPU changes over memory access areas fixedly. Namely, even when both CPUs make memory access requests to the same address, MCU4 attains access to different memory areas according to CPUs.