A memory system includes a nonvolatile memory electrically connected to a data bus, a DRAM electrically connected to the data bus, and a memory controller configured to drive the DRAM as a cache memory and the nonvolatile memory as a main memory and to synchronize data of a cache line with data of the nonvolatile memory in units of cache units based on a dirty flag. The DRAM is configured to load data of the cache line that caches data stored in the nonvolatile memory and to store the dirty flag, which indicates whether a cache unit is dirty, in units of cache units, where a size of each cache unit is smaller than a size of the cache line.