Some embodiments provide a method to reduce the refresh power consumption by effectively extending the memory cell retention time. Conversion from 1 cell/bit to 2N cells/bit reduces the variation in the retention time among memory cells. Although active power increases by a factor of 2N, the refresh time increases by more than 2N as a consequence of the fact that the majority decision does better than averaging for the tail distribution of retention time. The conversion can be realized very simply from the structure of the DRAM array circuit, and it reduces the frequency of disturbance and power consumption by two orders of magnitude. On the basis of this conversion method, some embodiments provide a partial access mode to reduce power consumption dynamically when the full memory capacity is not required.