a redundant column is mapped to a defective or over - erased column in the memory array . when an erase command occurs , both the redundant column and the defective or over - erased column are preconditioned by preprogramming them . the defective or over - erased column is preprogrammed and verified a predetermined number of times or until the verification passes . the redundant and defective or over - erased columns are then erased with the rest of the memory block .