Algirdas Avizienis

The N-Version Approach to Fault-Tolerant Software

Evolution of the N-version software approach to the tolerance of design faults is reviewed. Principal requirements for the implementation of N-version software are summarized and the DEDIX distributed supervisor and testbed for the execution of N-version software is described. Goals of current research are presented and some potential benefits of the N-version approach are identified.

The STAR (Self-Testing And Repairing) Computer: An Investigation of the Theory and Practice of Fault-Tolerant Computer Design

This paper presents the results obtained in a continuing investigation of fault-tolerant computing which is being conducted at the Jet Propulsion Laboratory. Initial studies led to the decision to design and construct an experimental computer with dynamic (standby) redundancy, including replaceable subsystems and a program rollback provision to eliminate transient errors. This system, called the STAR computer, began operation in 1969. The following aspects of the STAR system are described: architecture, reliability analysis, software, automatic maintenance of peripheral systems, and adaptation to serve as the central computer of an outerplanet exploration spacecraft.

Dependable computing: From concepts to design diversity

This paper is composed of two sections. The first provides a conceptual framework for expressing the attributes of what constitutes dependable and reliable computing: a) the impairments to dependability (faults, errors, and failures), b) the means for dependability (fault avoidance, tolerance, removal, and forecasting), and c) the measures of dependability (reliability, availability, safety). The second section focuses on one of the most challenging problems for dependable computing: coping with design faults.

Toward systematic design of fault-tolerant systems

After 30 years of study and practice in fault tolerance, high-confidence computing remains a costly privilege of several critical applications. It is time to explore ways to deliver high-confidence computing to all users. The speed of computing will ultimately be limited by the laws of physics, but the demand for affordable high-confidence computing will continue as long as people use computers to enhance the quality of their lives. Eventually, one enterprising chip builder will deliver the first fault-tolerant microprocessor at a competitive price, and soon thereafter fault tolerance will be considered as indispensable to computers as immunity is to humans. The remaining manufacturers will follow suit or go the way of the dinosaurs. Once again, Darwin will be proven right.

Arithmetic Error Codes: Cost and Effectiveness Studies for Application in Digital System Design

The application of error-detecting or error-correcting codes in digital computer design requires studies of cost and effectiveness trade-offs to supplement the knowledge of their theoretical properties. General criteria for cost and effectiveness studies of error codes are developed, and results are presented for arithmetic error codes with the low-cost check modulus 2a-1. Both separate (residue) and nonseparate (AN) codes are considered. The class of multiple arithmetic error codes is developed as an extension of low-cost single codes.

In search of effective diversity: a six-language study of fault-tolerant flight control software

Multiversion software systems achieve fault tolerance through software redundancy and diversity. The authors investigated multiversion software systems using six different programming languages to create six versions of software for an automatic landing program. The rationale, preparation, execution, and evaluation of this investigation are reported.<<ETX>>

A fault tolerance approach to computer viruses

Extensions of program flow monitors and n-version programming can be combined to provide a solution to the detection and containment of computer viruses. The consequence is that a computer can tolerate both deliberate faults and random physical faults by one common mechanism. Specifically, the technique detects control flow errors due to physical faults as well as the presence of viruses.<<ETX>>

ARLES 81: A RELIABILITY AND LIFE--CYCLE EVALUATTON TOOL FOR FAULT-TOLERANT SYSTEMS

?his paper desdes a program catled ARIES 81 (Automated Reliability htsrcrctive Btimatian %stem) which is designed to provide students, researchers, and &signers with an environment in which they may defrne and analyze the reliability models for fault-tolrrrant computers. ARIES 81 is an eztended and improved zwSia of a reliability esfimation program called ARIES 76. ?he user of ARIES 81 is presented with a mathematical framework 01 annlysis -- stochastic homogeneous Markov process, in which to model the system he is designing. ARIES 81 has the capability to conshct, process, and waiuate the predejinad speciflc models tor closed. nspairabie, and pwbd.ica((y renewed ciosed faulttdwant systems. A can also process and evaluate any newly defIned and user constructed models for me complex systems ad can model both psrmanent and transient fault tscovea(. Wth reliability and lite-cycle analy& can be psrfrnmed with the aid of ARIES 81.

Binary-compatible signed-digit arithmetic

Binary compatible signed digit number system characterized by variable length and significant arithmetic operations

N-VERSION PROGRAMMINC: A FAULT-TOLERANCE APPROACH TO RELlABlLlTY OF SOFTWARE OPERATlON

N-version programing is defined as the independent generation of N2 2 functionally e quivalent prograns fran the sme initial specification. A methodology of N-version programing has been devised and three types of special mechanisms have been identified t hat are needed to coordinate the execution of an N-version software unit and to canpare the correspondent results generated by each version. Two experiments have been conducted to test the feasibility of N-version programing. The results o f these experiments are discussed. In addition, constraints are identified that must be met for effective application of N-version programing.