1. A computer system comprising: a system controller including a central processing unit and a memory bus controller and configured to operate in a first interface mode; a system memory connected with the system controller through the system bus; a NAND flash memory configured to store a system driving code, an operating system program, and user data for the computer system; and an interface unit configured to communicate with the system controller through the system bus in the first interface mode and configured to communicate with the NAND flash memory in a second interface mode, the interface unit being synchronized with a clock signal generated in response to predetermined command information.