According to the portable electronic apparatus of the invention, when a data string is to be written in a data memory, information indicating whether the data string is valid or not is appended to the data string, and the data string is stored. When a read instruction is supplied from a portable electronic...http://www.google.ca/patents/US4839792?utm_source=gb-gplus-sharePatent US4839792 - Portable electronic apparatus with a device for determining data validity
