A memory unit and memory module using the same. After packets enter said router, a packet retransmission processing unit acquires a characteristic values and messages calculated from each characteristic value of a packet for obtaining the order, size, header, and check bits for assisting in identifying packets, and stores the characteristic values with a link list for saving memory utility. The density level of a developed patch to be formed on a photosensitive drum is set to a different value on a toner color-by-toner color basis. This reduces the amount of space taken up and current drawn, increases the clock rate and simplifies signal processing for signals with high levels.