A method of operating a memory device includes: generating an internal read command in response to a received masked write command, the internal read command being generated one of (i) during a write latency associated with the received masked write command, (ii) after receipt of a first bit of masked write data among a plurality of bits of masked write data, and (iii) in synchronization with a rising or falling edge of a clock signal received with an address signal corresponding to the masked write command; reading, in response to the internal read command, a plurality of bits of data stored in a plurality of memory cells, the plurality of memory cells corresponding to the address signal; and storing, in response to an internal write command, the plurality of bits of masked write data in the plurality of memory cells.