Storage device, memory controller and memory control method

A memory controller which performs reading and writing data from and into a nonvolatile memory having multiple memory cells that can store data by having a set threshold voltage thereof included in one of threshold areas divided according to multiple read voltages, the memory controller comprising:

• an assignment control unit configured to perform first writing based on first data value assignment, which sets a power of two of data values to correspond respectively to a power of two of threshold areas, in first-time writing into a first memory cell of the nonvolatile memory and performs second writing on the first memory cell on which writing was performed by the first writing without erasing data written in the first writing based on second data value assignment, which sets a power of two of data values to correspond respectively to a power of two of threshold areas including threshold areas not used in the first data value assignment,
• wherein a number, not being a power of two, of the threshold areas can be set in the memory cells, and
• wherein the control unit, taking the power of two of threshold areas used in the first data value assignment as a first combined area, in the second data value assignment, the first combined area and the threshold areas not used in the first data value assignment are set to correspond to the power of two of data values.