1. A decoder comprising: a message memory to store layers of blocks of updated extrinsic reliabilities; a column-sum memory to store soft bits comprising column-sum data; a subtraction element to subtract a layer of the updated extrinsic reliabilities from corresponding soft bits of the column-sum data; a layer processor to generate the updated reliabilities from soft bits provided by the subtraction element; and a sum element to add the updated extrinsic reliabilities generated by the layer processor with soft bits of prior subiterations provided by the subtraction element to generate the column-sum data.