JPS6325380B

PURPOSE:To confirm easily an error correcting and checking function, and to execute a patrol effectively, by rewriting a diagnostic signal together with a data signal in a main storage device, instead of an error correcting and checking signal of a read-out data from the main storage device. CONSTITUTION:At the time of a patrol, a part of a write data from a CPU 1 is used for controlling the patrol, a controlling circuit 22 is controlled by its controlling bit, a data read out of a main storage device 3 and an error correcting and checking ECC signal are checked by an ECC circuit 21, and the corrected read-out data and ECC code are rewritten in the device 3 as a write data ECC code. On the other hand, at the time of diagnosis, when a diagnostic signal CHW is applied to the circuit 22 from the CPU 1, the circuit 22 uses the signal CHW instead of ECC from the circuit 21, and sends an artificial error signal to the device 3. Subsequently, the circuit 21 detects exactly an error in a read-out signal from the device 3, and the CPU 1 checks whether the automatic correction can be executed or not.