Memory controller and memory system

A memory system comprising:

• a nonvolatile semiconductor memory including a plurality of blocks, the block being erase unit of the nonvolatile semiconductor memory;
• and
• a memory controller configured to control the nonvolatile semiconductor memory,
• wherein the memory controller includes:
• a block group managing unit configured to manage a plurality of block groups, each block group including active blocks of a nonvolatile semiconductor memory;
• a retention time managing unit configured to manage a retention time of each block group that is an indicator of a block allocation period of each block group;
• a degree of wear managing unit configured to manage a degree of wear of each block;
• and
• a block selecting unit configured to select a free block to allocate to each block group based on the retention time of each block group and the degree of wear of each block.