Nonvolatile memory device and memory system including nonvolatile memory device

A nonvolatile memory device performs a compare and write operation. The compare and write operation includes reading read data from memory cells, inverting first write data to generate second write data, adding a first flag bit to the first write data to generate third write data and adding a second flag bit to the second write data to generate fourth write data, performing a reinforcement operation on each of the third write data and the fourth write data to generate fifth write data and sixth write data, and comparing the read data with each of the fifth write data and the sixth write data and writing one of the fifth and sixth write data in the memory cells based on a result of the comparison.