a magnetic disk sub - system provides for sudden cutting off of a main power supply such as a power failure so that the backup by battery is effected to supply electric power to the cache memory from a battery upon cutting off of the main power supply in order to protect data in the cache memory and when the main power supply is cut off by operation of a main power supply switch the backup by battery is stopped to cut off supply of electric power to the cache memory from the battery after data in the cache memory is written into a magnetic disk unit and supply of electric power from the main power supply is then cut off . further , the magnetic disk sub - system divides the cache memory into several portions so that the backup by battery can be controlled independently to thereby extend the backup period .