Fault Alignment Exclusion for Memory Using Address Permutation

Abstract

A significant improvement in memory fault tolerance, beyond what is already provided by the use of an appropriate error-correcting code (ECC), can be achieved by electronic chip swapping, without any compromise of data integrity as large numbers of faults are allowed to accumulate. Since most large and medium-sized semiconductor memories are organized so that each bit position of the system word (ECC codeword) is fed from a different chip, and quite often from a different array card, or at least from distinct partitions of an array card, the various bit positions have separate address circuitry on the array cards. This fact is important, and can be exploited to provide effective address permutation capability, which allows the realignment of faults which would otherwise have caused an uncorrectable multiple error in an ECC codeword. When faults occur in a codeword to produce an uncorrectable error (UE), the addressing within one of the error bit position array cards can be altered using simple EX-OR circuitry and storage latches. The content of the latches is computed using a fault map of the memory together with an algorithm. These techniques are referred to as Fault Alignment Exclusion (FAE) using address permutation. Practical considerations as to the complexity of the fault map, the number of storage latches per bit position, and the overall effectiveness of the permutation to disperse the expected numbers of errors are presented in this paper.