A low density parity check decoder for performing LDPC decoding based on a layered algorithm applied to a parity check matrix, the decoder including a channel memory, a metrics memory, first and second operand supply paths each arranged to provide operands based on channel values and metrics values; a processor block including a plurality processing units in parallel and arranged to receive operands from the first supply path and to determine updated metric values, a buffer arranged to store at least one of the operands from the first supply path; and an adder coupled to an output of the processor block and arranged to generate updated channel values by adding the updated metrics values to operands from a selected one of the buffer and the second supply path.