1. A method of changing the number of pages assigned to each memory block in a memory cell array, the method comprising: selecting one of a plurality of memory blocks to be changed, each memory block including a plurality of memory strings, wherein each memory string includes a plurality of memory cells respectively configured to store one or more bits of data, the plurality of memory cells being connected in series between a string selecting transistor (SST) and a ground selecting transistor (GST); and modifying the number of pages assigned to the selected memory block into a power of two or not a power of two in accordance with a requirement.