According to one embodiment, a computer system includes a first memory unit, a second memory unit having a data transfer rate lower than that of the first memory unit and a controller. The controller controls transfer of unit data. The unit data includes an indicating portion indicating whether the unit data is to be retained in the second memory unit. When the unit data is transferred from the second memory unit to the first memory unit and the unit data is to be retained in the second memory unit, the controller sets a first state to the indicating portion of the respective unit data. When the unit data is transferred from the first memory unit to the second memory unit, the controller writes the respective unit data in which the indicating portion is set to the first state, to the second memory unit.