US Patent 9594627 Controller and control method

According to one embodiment, a controller controls a nonvolatile memory stores data page by page. The controller is configured to extract, from a first data sequence shorter than the data length of a page, a second data sequence shorter than the first data sequence, to refer to the difference between threshold voltages corresponding to two data included in the second data sequence, to convert the second data sequence into a third data sequence longer than the second data sequence, and to control the percentage of the length of an error correction code added to the third data sequence.