In a data holding mode, data storage in a one bit/one cell scheme in a normal
operating mode are rearranged into data storage in a twin-cell mode in which data
are stored in a one bit/two cell scheme. In the twin-cell mode, two sub word lines
are simultaneously driven into a selected state, and storage data of memory cells
are read out on both of bit lines in a pair, to perform a sense operation. Thus,
the read-out voltage can be increased to improve the data retention characteristics
for lengthening a refresh interval, resulting in a reduced power consumption in
the data holding mode.
