duplex system form in which one module 10 serves as the online system and the other module 20 serves as the stand by system in such system configuration the bus controller 14 within the online system is activated via the cpu bus 11 and the circuit 4 if a machine failure should occur in the online systems the module 20 designated to the stand by system operates instead of the online system when the module 10 has been recovered from the failure to its normal state the module 20 is capable of operating to activate the bus controller 14 via the high rank bus 1 and the circuit 2 it is now assumed that the disk storage 17 and the disk control unit 16 are connected to only one module 10 and it is also assumed now that all of other modules such as the module 20 serve as the online system but the disk storage 27 and the disk control unit 26 are not connected to those modules in this case the bus controller 14 of the module 10 is activated via the cpu bus 11 and the circuit 4 in other modules however the bus controller 24 and the like can be activated via the high rank bus 1 and the circuit 2 in all of the above described system structures it is a matter of course that bus controllers in all modules may have identical structure in accordance with the present invention the memory in which the microprogram for controlling the bus controller is stored is constituted by the ram as described above in.