Semiconductor memory with single cell and twin cell refreshing

Abstract

Flags are formed to respectively correspond to memory cell groups each including volatile memory cells. Each flag indicates as a set state that the memory cells store data in a second memory mode. In a changing operation of changing from a first memory mode in which data is independently retained by each memory cell to a second memory mode in which same data are retained in the memory cells of each memory cell group, each flag is reset in response to the first access to the corresponding memory cell group. Therefore, only the first access is made in the second memory mode in each memory cell group. The memory cells are accessed in a mode according to the flag in the changing operation, thereby allowing a system managing the semiconductor memory to freely access the memory cells even during the changing operation. Consequently, a practical changing time can be eliminated.