Abstract: Disclosed is an exclusive control method in an I/O subsystem having an exclusive controller which is provided with an exclusive control table and which permits a host interface to use the I/O device when the I/O device is not used by any other host interface while prohibiting the use when another host interface is using the I/O device.

Abstract: An input/output control device controlling data exchange between computers and memory units includes transfer units connected to the computers and accessing the memory units so as to effect the data exchange, an exclusive-control unit for preventing more than one of the transfer units from having access to the same one of the memory units by denying all but one of the transfer units the access, and an expropriation unit for taking control of one of the memory units when one of the transfer units is denied access to the one of the memory units a predetermined number of times, so that the one of the memory units can be set aside for use by the one of the transfer units.

Abstract: A semiconductor memory unit is operated by supplying an electric power from a system power source (200 v) used so as to be cut off at a predetermined time. The semiconductor memory unit includes a volatile memory for storing data that can be accessed by a host unit coupled to the semiconductor memory unit, a battery used as a back-up power source for the volatile memory, and a power supply controller, coupled to the system power source, the battery and an all-day service power source (100 v) having a capacity less than that of the system power source, for switching a power source by which electric power is supplied to the volatile memory from the system power source to the all-day service power source when the system power source is cut off, and for switching it from the all-day service power source to the battery when a service interruption of the all-day service power source occurs.