Memory system

A controller for controlling a nonvolatile memory including first to x-th word lines and first to x-th memory groups, a y-th memory group being connected to a y-th word line, each of the memory groups including a plurality of memory cells, each of the memory groups being capable of storing data of m pages, each of the plurality of memory cells being capable of storing m-bit data by belonging to one of first to n-th threshold voltage distributions, a (k+1)-th threshold voltage distribution having a higher voltage level than a k-th threshold voltage distribution, x being a natural number of two or more, y being a natural number of one or more and (n−1) or less, m being a natural number of two or more, n being a natural number of m-th power of two, k being a natural number of one or more and (n−1) or less, the controller being configured to repeat a first processing, wherein

• the first processing includes first to x-th internal processing,
• a y-th internal processing includes reading data from the y-th memory group, executing an error correction operation for the read data, and writing error-corrected data in a certain memory cell of the y-th memory group in a case where a certain condition is satisfied,
• the y-th internal processing includes reading data of m pages from the y-th memory group as a first data set using a read voltage, storing the read first data set in a buffer memory, storing a second data set generated by an error correction on the read first data set in a buffer memory, and executing a second processing,
• the second processing includes
• counting a first number and a second number, the first number being a number of first memory cells corresponding to a first combination of first data in the first data set and second data in the second data set, the second number being a number of second memory cells corresponding to a second combination of third data in the first data set and fourth data in the second data set, the first memory cells being among the one of the plurality of memory groups, the second memory cells being among the y-th memory group, the second data corresponding to the first data, the fourth data corresponding to the third data,
• calculating a ratio of the second number to the first number to calculate a shift amount of the read voltage,
• determining a polarity of the shift amount according to whether the ratio exceeds a reference value or not,
• determining a magnitude of an absolute value of the shift according to a magnitude of an absolute value of the ratio, and
• determining the read voltage based on the calculated shift amount of the read voltage, and
• the controller is, in a next first processing, configured to read data of the m pages from the y-th memory group as the first data set using the determined read voltage and store the read first data set in the buffer memory.