Abstract: A manipulated error detection matrix by which check bits are generated and decoded so that errors that are detected are next to each other and occur within a nibble. The invention consists of a check bit generator 14 which generates check bits in accordance with a nibble protect matrix according to a Modified Hamming Code which is configured to provide detection of triple and quadruple bit errors occurring within a nibble. The check bits are compared to those in memory 12 by a syndrome generator 16. An error corrector 18 and an error detector 20 decode syndrome bits.