A memory system includes a memory controller, a first memory, and a second memory. The memory controller has a command address port, a chip select port, a first data port, and a second data port. The first memory is coupled to the command address port, the chip select port, and the first data port, and the second memory is coupled to the command address port, the chip select port, and the second data port. The capacity of the second memory is greater than the capacity of the first memory. The memory controller controls the first memory and the second memory simultaneously through the command address port and the chip select port.