Method of operating memory device

A memory system comprising:

• a nonvolatile memory device comprising a plurality of memory cells into which data is programmed;
• and
• a controller configured to control the nonvolatile memory device, the controller comprising:
• a microprocessor configured to change a first read voltage, for determining whether the data stored in the memory cells is a first voltage state or a second voltage state, to a voltage within a first range and to determine the voltage within the first range as a first select read voltage, and the microprocessor to change a second read voltage, for determining whether the data stored in the memory cells is a third voltage state or a fourth voltage state, to a voltage within a second range and to determine the voltage within the second range as a second select read voltage;
• an error correction code (ECC) encoder to create data added with parity bits by performing ECC encoding on data which is to be provided to the nonvolatile memory device;
• and
• an ECC decoder to correct error bits of the data added with the parity bits, wherein the nonvolatile memory device comprises:
• control logic to control operation of the nonvolatile memory device in accordance with received commands;
• a voltage generator to generate the first and second read voltages, n candidate voltages comprising the first select read voltage, and m candidate voltages comprising the second select read voltage, where n and m are natural numbers;
• an X decoder to receive the first and second read voltages and the first and second select read voltages generated by the voltage generator and to drive the memory cells using the first and second read voltages and the first and second select read voltages;
• and
• a register to store the determined first and second select read voltages.