SAFETY PROGRAM VALIDATION BY MEANS OF CONTROL CHECKING

Abstract Recent progress in the field of computer hardware makes reasonable the search for improvements in software reliability by using larger and faster computers, which permit the usage of redundant programs or the application of redundancy during the execution. The basic idea is to memorize what has been tested during the debugging, licensing or burn-in phase of software and to switch to the safe side if any program status appears during on line operation that had not been tested before. Concerning control sequence checking, it is supervised during program execution whether the program path used has already been tested. Three alternatives are considered: The first stores the tested paths as a tree, which describes the connections between the consecutive arcs. The second one associates an identification number to each program arc and stores the string of numbers, that identify each tested path. The third maps each program path on a number and stores it. In the same way also three checking methods for addressing arrays are feasible: The first memorizes the tested sequences of addressing as a tree structure, the second memorizes the tested sequences of addressing as a string of numbers and the third maps the sequence of addressing, used during a program run, on a single number. The size of the overhead in program execution time and memory space required is a function of the paths and mappings memorized. The application of one or more of these methods seems reasonable primarily for safety relevant programs which have to meet nearly identical requirements for long periods of operation time, and where safety actions are normally appropriate if input conditions change to unanticipated constellations.