Abstract: During the diagnosis of a data processor, when errors are detected, a logic tracing apparatus permits a hardware history concerning these errors to be obtained. The data processor produces a stop signal when or before a result comparison routine of a test program is started to be executed, a start signal when no errors are detected by the execution of the result comparison routine, and a hold signal when errors are detected. When supplied with this stop signal, the logic tracing apparatus stops the memory from writing the hardware information. Subsequently, when supplied with the hold signal, the logic tracing apparatus holds the state in which the memory stops writing operation until it is reset by the data processor. In addition, under the condition in which the stopped state is released, when the start signal is supplied to the apparatus, the write operation of memory is restarted thereby.