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CODING TECHNIQUES Many errors in a computer system are committed at the bit or byte level when information is either transmitted along communication lines from one computer to another or else within a computer from the memory to the microprocessor or from microprocessor to input/ output device. Such transfers are generally made over high-speed internal buses or sometimes over networks. The simplest technique to protect against such errors is the use of error-detecting and errorcorrecting codes. | Reliability of Computer Systems and Networks Fault Tolerance Analysis and Design Martin L. Shooman Copyright 2002 John Wiley Sons Inc. ISBNs 0-471-29342-3 Hardback 0-471-22460-X Electronic 2 CODING TECHNIQUES INTRODUCTION Many errors in a computer system are committed at the bit or byte level when information is either transmitted along communication lines from one computer to another or else within a computer from the memory to the microprocessor or from microprocessor to input output device. Such transfers are generally made over high-speed internal buses or sometimes over networks. The simplest technique to protect against such errors is the use of error-detecting and errorcorrecting codes. These codes are discussed in this chapter in this context. In Section we see that error-correcting codes are also used in some versions of RAID memory storage devices. The reader should be familiar with the material in Appendix A and Sections B1-B4 before studying the material of this chapter. It is suggested that this material be reviewed briefly or studied along with this chapter depending on the reader s background. The word code has many meanings. Messages are commonly coded and decoded to provide secret communication Clark 1977 Kahn 1967 a practice that technically is known as cryptography. The municipal rules governing the construction of buildings are called building codes. Computer scientists refer to individual programs and collections of programs as software but many physicists and engineers refer to them as computer codes. When information in one system numbers alphabet etc. is represented by another system we call that other system a code for the first. Examples are the use of binary numbers to represent numbers or the use of the ASCII code to represent the letters numerals punctuation and various control keys on a computer keyboard see 30 INTRODUCTION 31 Table in Appendix C for more information . The types of codes that we discuss in this chapter are