Single Error Correcting Code Maximizes Memory System Efficiency

Jf eliable memory systems can be designed either by using highly reliable but expensive components or by employing inexpensive protective redundancy in terms of a single error correcting code that uses redundant check bits. The degree of reliability ean be increased if this protective redundancy matches the failure mode of the memory system, Presently, use of semiconductor memory is increasing because of lower cost, higher speed, higher density, and better long-term reliability compared with core memories. Semiconductor readwrite memory chips are available in n-word x I-bit configurations (where n = 256 through 16,384 words) . For memory systems built with these chips, a single-bit failure in a word is more probable than a multiple-bit failure, Therefore, a single error correcting code is quite effective in increasing system reliability.l,2'3 For memory systems in need of increased reliability, a single error correcting and double error detecting (src-onl) code can be incorporated. This code needs an additional check bit to indicate overall parity.3 Double error information, when detected, can be designed to interrupt the computer which, in turn, can display the failure mode. Inherently, a single error correcting code has the potential of partial double error detection. 'Without using an extra check bit, as necessary in a SEC-DED code, a modified sec code is capable of detecting an appreciable percentage of total double error possibilities. ,Since variation exists in the amount of double error detection, to maximize memory system effieiency, an algorithmcalled a single error correcting and partial double error detecting (snc-eteo) code ,has been evolved that gen. erates a code for correcting all single errors and detecting a maximum number out of the total possible double errors. This spc-pom code requires only as many check bits as are needed in the ssc code, while approaching the reliability of the sEC-DED code. Moreover, extra hardware for the implementation of the sEC-rDED code is minimal.