Multibyte error correcting system involving a two-level code structure

Claim 1

A multibyte error correcting system operable to correct up to t 1 errors in each subblock of a block of data and t 2 errors in said block, where t 2 and t 1 are positive integers and t 2 >t 1 , said block having: (a) at least one subblock, each of which has at least one codeword containing up to 2 b -1 b-bit character positions, where b is any positive integer, each character position consisting of a unique combination of binary bits representing data and 2t 1 check bytes; and (b) 2t 2 b-bit block check bytes, said system comprising: first syndrome generating means for generating two 2t 1 syndrome bytes each containing b bits by combining 2t 1 stored check bytes associates with each stored codeword with newly developed 2t 1 check bytes during the reading of each said stored codeword, said check bytes corresponding to the modulo 2 sum of bit positions of said codeword selected systematically in accordance with a predetermined parity check matrix; first syndrome processing circuitry operable to correct up to t 1 errors in each said codeword by processing said 2t 1 syndrome bytes; second syndrome generating means for generating 2t 2 block syndrome bytes each containing b bits by combining said 2t 2 stored block check bytes with 2t 2 block check bytes developed after all said codewords of said block have been read and corrected, each of said t 2 block check bytes corresponding to the modulo 2 sum of bit positions of corrected said codewords that are selected systematically in accordance with said parity check matrix; logic means for developing a new set of block syndrome bytes by modifying said original set of block syndrome bytes to reflect a miscorrection in one of said subblocks; and second syndrome processing circuitry for correcting up to t 2 errors in said one subblock by processing said modified set of 2t 2 block syndrome bytes.