PURPOSE: To generate an error simply by providing a substitutive instruction register storing a microinstruction for generating an error, and switching an instruction in executing to the substitutive instruction.

CONSTITUTION: The microinstruction for generation an error is set up in the substitutive instruction register 4 by a control line from a service processor. A microinstruction stored in a control storage 1 is successively executed by an instruction counter 2, and when the value of the counter 2 coincides with the value of an address comparing register 6, the output of a comparator 7 is turned to '1'. Thereby, the output of an AND gate 10 is turned '1' and a switching circuit 8 selects the output of a substitutive instruction register 4. Consequently, the microinstruction in the register 4 is executed by a microinstruction control circuit 11, an error is generated and an error processing routine is driven.