A computer system, including a data storage unit which stores first operating system data of the computer system therein; a main memory to which the first operating system data is copied; a data transmission unit which transmits data between the data storage unit and the main memory; and a controller which performs a power-on self test (POST) of the computer system, and controls the data transmission unit to copy the first operating system data stored in the data storage unit to the main memory during the POST.