1. A microprocessor comprising: at least one-element arithmetic means for performing arithmetic processing of data; at least two-element memory means for storing said data processed by said arithmetic means; control means for performing control to select one of said at least two-element memory means; save means for saving data on a second task held by the other memory means while a first task is executed by using said memory means selected by said control means; and restore means for restoring data on a third task saved by said save means in said other memory means while said first task is executed by using said memory means selected by said control means.