To provide a memory device with a function to perform an operation and a method of performing an operation and storage which can save space and cost and which can start, immediately after the power source is recovered, the processing which was being performed at the time of power failure. A memory cell MC can store two independent data sets; DRAM data (volatile data) and FeRAM data (non-volatile data). Thus, the number of memory cells can be reduced to a half. Also, the DRAM data to be used in the next operation of the two data sets which have been read out for the previous operation are temporarily held in a hold circuit 21 of an operation unit OU and then written back into the memory cell MC in a non-volatile manner as new FeRAM data for preparation of the next operation. Thus, even when the power source is shut down by an unexpected trouble, the data necessary for the next operation are not lost.