A memory device, a memory system, and a method of operating the same. The memory device includes a memory cell array including a plurality of memory cells and a write command determination unit (WCDU) that determines whether a write command input to the memory device is (to be) accompanied a masking signal. The WCDU produces a first control signal if the input write command is (to be) accompanied by a masking signal. A data masking unit combines a portion of read data read from the memory cell array with a corresponding portion of input write data corresponding to the write command and generates modulation data in response to the first control signal. An error correction code (ECC) engine generates parity of the modulation data.