1.  Field of the Invention
This invention relates to a diagnosis system for data-processing systems. 
2.  Description of the Prior Art
Present-day data-processing systems are becoming increasingly complex and this greatly complicates the task of the technicians responsible for maintaining these machines when they are called upon to locate faulty circuits.  The majority of known data-processing systems incorporate internal checking means which enable them to report the existence of faults.  In the event of a serious fault occurring in a system which is processing data, means are generally provided to prevent incorrect results from being obtained.  These means operate to terminate the carrying out of the processing operation which is under way by stopping the clock circuits of the system.  In certain systems, means are also provided to report minor faults without stopping the carrying out of the processing operations which are under way.  Thus diagnosis of a faulty system may be carried out either immediately, when the fault concerned is a serious one, or after the system has completed the processing operations which are under way when the fault is a minor one.  Diagnostic devices exist which are connected to the systems that need to be serviced so as to feed into them instructions and data which test the various circuits by making them carry out test programs designed to locate the faulty circuits.  Generally, these devices, which may be data-processing systems, operate asychronously in relation to the faulty systems to which they are connected and matching means are required.  Among the matching devices used in carrying out diagnoses, certain ones have the advantage that, with a minimum of connections, they make it possible to carry out tests using systems which may be situated either locally or remotely from the systems being serviced.  However, such matching devices operate from a console which is connected to the system to be tested and do so when the latter is shut down, which involves a considerable waste of time in fault-finding.  To overcome this drawback, certain data processing systems contain means which enable the data-processing units which they contain to test themselves.  Such means are described in commonly-assigned U.S. Pat.  No.  4,025,767--Bottard, entitled "Testing System for a Data Processing Unit," which issued from Application Ser.  No.  655,244 filed Feb. 4, 1976, which was a continuation of abandoned Application Ser.  No.  450,936, filed Mar. 13, 1974, based on French priority Application Ser.  No.  73.09558, filed in France on Mar. 16, 1973.  This test system enables the unit concerned to be tested, after error detection, both when the unit is being initialized and also while data is being processed.  Relatively small special circuits are used for this purpose.  The object of the unit is to locate faults at the level represented by the smallest interchangeable component in the unit.  Although this test system has many advantages, in cases where the number of circuits contained in the unit to be tested is considerable, it may be a disadvantage to store test programs permanently in a memory in the unit (as described in the above-identified U.S. Pat.  No.  4,025,767), inasmuch as this detracts from the processing capacity of the unit.