PURPOSE: To make it possible to use both microinstruction and macroinstructions mixed freely by making it possible to discriminate micro and macro instructions by their operation code parts and by executing a microinstruction as it is while making a jump to a fixed microinstruction in case of a macroinstruction by consulting a table.

CONSTITUTION: System storage 1 consists of resident area RS, control memory area CS, and main memory area MS. An instruction read out to instruction register 3 is generally composed of an operation code part indicating the kind of the instruction and a discrimination between micro and macro instructions and other register assignment parts, and the operation code part is supplied to decoder 6. As a result, when the instruction is a microinstruction, an arithmetic part, etc., are controlled by a decoding result as it is and when a macroinstruction, gate 7 is opened to index table 8. An obtained address is inputted to address register 2 and access to a system storage is attained. Here, 4 and 5 represent arithmetic registers, and 9 an arithmetic method.