PURPOSE: To recognize operational conditions in the case of evaluating a microprogram and to surely guarantee quality by exactly and precisely measuring the comprehension ratio of the microprogram.

CONSTITUTION: This circuit is provided with an address register 10, operation display memory 30 and write/read control circuit, and the address register 10 reads the microprogram to be executed from a control storage part. The display memory 30 enables writes, has words as many as that of the control storage part 20 and defines the output of the address register 10 as an address input. According to an instruction from a diagnostic device, the write/read control circuit controls the write of used display data to the entry of the display memory 30 selected by the address and the read of those data to the display memory 30 in the diagnostic device in the case of initialization or operation to the display memory 30. This microprogram comprehension ratio measuring circuit measures this operating state by storing and reading the used words of the control storage part 20 in the case of operating the microprogram.