PURPOSE: To shorten dispatch time by saving and recovering the context of respective tasks in corresponding save areas by changeover of register banks and protecting it.

CONSTITUTION: The register banks 11 and 12 provided in a central processing unit are fixedly allocated to the tasks A and B in one-to-one correspondence and the register bank 13 is allocated to the tasks C and D in common. When a processing is shifted to the tasks C and D, the contents are saved in the corresponding save areas 2C and 2D and the saved contents are recovered from the save area 2D or 2C corresponding to the task to the remaining register bank 13. Thus, the tasks C and D using the register bank 13 are operated similarly to the tasks A and B allocated to the register banks 11 and 12 in one- to-one correspondence. Thus, the dispatch time in a real time operating system is shortened and the preparation of applications is not limited.