Single-chip memory system including buffer

To operate faster, a memory system includes a central processing unit (CPU) for executing a first instruction, and for outputting first, second and third signals, a memory device for storing first data and the first instruction, a first buffer for storing second data, a controller for writing the second data into the memory device when the controller receives the first signal, for reading the first data from the memory device after writing the second data into the memory device when the controller receives the second signal, and for reading the first instruction from the memory device and sending the first instruction to the CPU before writing the second data into the memory device when the controller receives the third signal.