1. A method of operating a memory system, the memory system including a nonvolatile memory device and a memory controller for the nonvolatile memory device, the method comprising: the memory controller receiving information data corresponding to a second program unit that is a part of a first program unit and a write request for the information data from a host; the memory controller generating a codeword by performing error correction code (ECC) encoding on the information data such that a partial parity bit corresponding to the information data among all parity bits of the codeword is updated; the memory controller providing the nonvolatile memory device with the codeword and a write command regarding the codeword; and the nonvolatile memory device performing a write operation with respect to the codeword on cells corresponding to the updated partial parity bit and cells corresponding to the information data.