Nonvolatile memory device, controller for controlling the same, storage device having the same, and method of operating the same

A nonvolatile memory device comprising:

• a plurality of memory blocks including at least two strings between each of bitlines and a common source line, wherein each of the at least two strings includes at least one string select transistor connected between one of the bitlines and the common source line in series, a plurality of memory cells, and at least one ground transistor, wherein the at least one string select transistor has a gate connected to a string selection line, wherein each of the plurality of memory cells receives a wordline voltage from a corresponding wordline, and wherein the at least one ground transistor has a gate connected to a ground selection line;
• and
• a control logic configured to receive a specific command and an address and to perform a cell count-based dynamic read (CDR) operation on the plurality of memory cells connected to one of wordlines of a selected block among the plurality of memory blocks in response to the address, wherein the CDR operation is configured to provide a first cell count value for a highest state among a plurality of states and a second cell count value for an erase state among the plurality of states, wherein the control logic includes:
• a cell count comparator circuit configured to compare the first cell count value with at least one reference value according to the CDR operation and to compare the second cell count value with the at least one reference value;
• and
• a read level selector configured to select a read level according to a result of a comparison of the cell count comparator circuit.