MEMORY DATABOOK - Al Kossow's Bitsavers

-'-"-----"1-----------------------.APPLICATIONS.6. MEMORY SYSTEM RELIABILITY6.1 Reliability O_mination FactoRThe memory system reliability depends upon the fourfactors shown in the left column of the following table.These factors are determined as shown in the rightcolumn of this table.Memory systemreliability factorSystem·requ iredreliabilityUnit capacityParts reliabilityCost6.2 Hard Error and Soft ErrorFactor determinationDetermined by the user-requiredspecifications (MTB F).'" [MB] = [word depth] x[bit count]Logic element :::::::z: Hard errorMemory element Soft error(I) Hard errorA hard error is a permanent error which occurs eachlime a certain address is accessed.(I il Soft errorA soft error is a transient error that does not repeat. Thefollowing are the three causes for soft errors:(1) I nsufficient power margin(2) Insufficient system noise margin(3) Particle failure(4) I nsufficient power margin(5) Insufficient system noise margin(6) particle failureItems (1) through (5) are largely influenced by thesystem design. For item (6), it is required to considerwhether a remedy such as ECC should be taken or notto satisfy the system-required reliability based on theperts reliability (pertaining to hard errors and softerrors). See 1.3 and 1.4 for details.6.3 Measures for Reliability EnhancementThe following are the two typical means for the enhancementof system reliability.(1) Parity..(2) ECC.. Error detection only (makes nocontribution to the MTBF enhancement).The SEC-OED" is used in general• Jiingle.s.rror .correct - .Qouble ~rror Qetect(one bit error correction and two bit error detection)6.4 Reliability Calculation MethodMT B F for a hard error and a soft error(1) Memory element reliabilityHard errorSoft errorrH = e -XH·t (XH: Hard error rate)rS = e -XS·t (XS: Soft error rate)Aeliabilityn n-l)A = (rH) + nC, • (rH"rs) ·(1 - rHCD ~