A CPU includes a bus interface, a control unit, an instruction cache, a data cache, a secondary cache, an instruction decoder, an arithmetic unit, and registers. When operations can be performed only with the cache, the CPU inhibits access to external memory and stops power supply to the external memory. With this arrangement, by performing operations in the CPU without using the external memory, it is possible to realize a speedy processing and to stop power supply to the external memory, thus allowing for reduction in power consumption.