Patent ID: 8266396

Claim:
A memory system comprising: a nonvolatile semiconductor memory including a plurality of blocks as a unit of data erasing; and a controller including a block selecting unit that selects, in writing data in the nonvolatile semiconductor memory, a required number of free blocks not including valid data from the nonvolatile semiconductor memory and a writing control unit that writes data in the selected free blocks, wherein the block selecting unit includes: a list unit in which free blocks included in the nonvolatile semiconductor memory are classified into a plurality of free block management lists using a set unit of blocks operable in parallel; a first free-block selecting unit that operates to acquire, in acquiring a first free block at normal priority, the first free block from a free block management list in which a number of free blocks is larger than a first threshold; and a second free-block selecting unit that operates to acquire, in acquiring a second free block at high priority, the second free block from the free block management list in which a number of free blocks is larger than the first threshold and a free block management list in which a number of free blocks is not larger than the first threshold.