PURPOSE: To prevent system down by checking whether a fault is generated in an added exclusive floating-point operating device or not at the initial recognition of the system and, if any fault is detected, executing only by a CPU floating-point operation.

CONSTITUTION: When initialization is started, whether each microinstruction is executed based upon its definition or not is checked. If any fault is detected, whether the fault is generated in a floating-point operating device 20 or not is checked. When the fault is generated in the device 20, operands are exchanged between a main storage device 3 and registers 1611, 1612 in a general purpose operating device 16, and a firmware including a microporgram routine for executing the floating-point operation specified for the 2 operands stored in the registers 1611, 1612 is stored in a control storage 11 while the controlling of a general purpose operating circuit 161 in addition.