According to one embodiment, a computer system comprises a first memory that stores a first program, a second memory that stores a second program or data, a processor, a first and a second power control circuits. The first power control circuit causes the first memory to operate at a first power consumption when detecting change of an input signal to the processor, and causes the first memory to operate at a second power consumption smaller than the first power consumption and transmits a temporary halt instruction to the processor when the execution of the first program or the second program by the processor is completed. The second power control circuit causes the second memory to operate at a third power consumption before the processor executes the second program, reads or writes the data. The second memory accepts read and write operations while operating at the third power consumption.