Method of operating a memory system having an erase control unit

A method of operating a memory system including a nonvolatile memory including a memory block, and a memory controller including an erase control unit, the method comprising:

• generating read data by applying a first read voltage to a word line of the memory block;
• generating an off cell count based on the read data;
• by operation of the erase control unit, determining an erase class among a plurality of erase classes based on the off cell count;
• determining an erase operation condition according to the determined erase class;
• by operation of the nonvolatile memory, erasing the memory block based on the determined erase operation condition.

A method of operating a memory system, comprising:

• determining an erase class from among a plurality of erase classes representative of a degree of shift in a threshold voltage for a plurality of cells in a memory block;
• and
• erasing the memory block based on the determined erase class;
• wherein determining the erase class comprises one of:
• determining the erase class based on a an off cell count determined by performing a pre-reading operation on the plurality of cells in the memory block,
• determining the erase class based on a voltage difference between a program verify voltage level for the plurality of memory cells in the memory block and a read voltage level for the plurality of memory cells in the memory block, and
• determining the erase class based on a retention time associated with the plurality of memory cells in the memory block.