PURPOSE: To utilize effectively a control designation section by designating a part of address data of a micro instruction to be executed next at a bit location not in use in the micro instruction stored sequentially in a prescribed address of a control storage section.

CONSTITUTION: The entire micro instruction constitutes the control designating section (a), X and Y are parts to which the address data of the micro instruction to be executed next is to be positioned and Z is a bit location designating either the address data is located in the part X or the part Y. When an instruction of an address A of a control storage section 1 is stored in a resister 2, a computer system conducts processing according to the control designating information of the instruction and when the processing is finished, a logical 1 of the Z part is inputted to a selector 3. Thus, the part Y of the micro instruction is outputted to the control storage section 1 as an address designating data and the instruction stored in an address B from the storage section 1 is stored in the register 2.