can be used to generate parity bits for hamming code 64 bit data needs 7 parity bits to achieve the functions of detecting and correcting one bit incorrect data thus a flag bit can serve as one data bit in a 65 bit data string and the flag bit will expresses whether dm occurs as shown in fig4 nevertheless it is should be understood that the flag bit can also serve as the 8th bit every 8 bits in dram constitute a basic unit in parity bits the flag bit for this bit is free from encoding decoding and correction of ecc there is a need to judge whether decoding and correction are needed according to this data prior to decoding and correction the judgment and decoding certainly can be performed at the same time to improve efficiency for example if u201c 0 u201d represents that there is no dm and u201c 1 u201d represents that there is dm if the flag bit is u201c 0 u201d decoding and correction are performed on the data and if the flag bit is u201c 1 u201d decoding and correction are not performed on the data in the example of fig4 bit width of the flag bit is 1 nevertheless it should be understood that bit width of the flag bit.