PURPOSE:To register a new data without minding an intermediate dead area by using a data map of a first face for a retrieval and a deletion and using a data map of a second face for registering. CONSTITUTION:The data No. is stored is order of its registering, a retrieval and a deletion of a data are executed by a data map of the first face, and registering of the data is executed by a data map of the second face. In such a state, at the time of retrieving a data, an intermediate dead area of the data map of the first face is justified and the data No. existing in the data map of the second face is registered successively in the next of the last data of the data map of the first face. After the data is retrieved, the data No. of the data which is registered before the next data is retrieved is written again from the head of the data map of the second face. Also, as for the maximum number of store pieces, the total number of pieces of the data No. registered in the data map of the first face and the data No. registered in the data map of the second face are managed.
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