There are provided a multisystem network, and a device and method for access to a data storage unit in order to allow processors in a plurality of devices to access the data storage unit. The data storage unit has a data storage area divided in a first area for a first device and a second area for a second device. The first and second devices are connected to the data storage unit via a connection circuit. A first processor in the first device can access directly the data storage unit. The connection circuit has provided therein an emulation register corresponding to a register which stores a command or control data used for the first processor in the first device to access the data storage unit. A second processor in the second device accesses the data storage unit via the emulation register and with intervention of the first processor.