A data processing system provided with a main storage unit comprising: a main storage including a plurality of configuration units and a main storage controller for selecting one of the configuration units of the main storage and for controlling reading and writing of information with respect to the selected configuration unit; and a central processor unit. Configuration data from a configuration unit. Configuration data from a configuration data generation unit is written in a configuration data storage means and one of the configuration units of the main storage is selected in accordance with the configuration data read from the configuration data storage means and in accordance with an address from the central processor unit so that the selected configuration unit is accessed.