A nonvolatile RAM for reading and writing data in a random manner includes a memory area configured by a plurality of memory cells suited to a nonvolatile-mode write operation, in which the stored content thereof is not lost irrespective of a power-off event, and a volatile-mode write operation, in which the stored content thereof is lost in the power-off event. A register designates a first portion of the memory area adapted to the nonvolatile-mode write operation regarding fixed data such as program codes and a second portion of the memory area serving as a work area adapted to the volatile-mode write operation. A control circuit performs the nonvolatile-mode write operation on the first portion of the memory area while performing the volatile-mode write operation on the second portion of the memory area.