Abstract: A portable electronic device in which when data is written in a PROM the start address of the next unwritten area is stored in a RAM incorporated in a CPU of the portable electronic device. A data memory is divided into a plurality of areas, and each area is further divided into a plurality of blocks. The data length of each block is determined for each area, and input data is divided in accordance with the data length as a processing unit for storage. After the final block of data is stored, a CPU stores the address position of the final data of the final block at the beginning of the corresponding area.