Energy Efficient Data Encoding in DRAM Channels Exploiting Data Value Similarity

As DRAM data bandwidth increases, tremendous energy is dissipated in the DRAM data bus. To reduce the energy consumed in the data bus, DRAM interfaces with symmetric termination, such as Pseudo Open Drain (POD) and Low Voltage Swing Terminated Logic (LVSTL), have been adopted in modern DRAMs. In interfaces using asymmetric termination, the amount of termination energy is proportional to the hamming weight of the data words. In this work, we propose Bitwise Difference Encoding (BD-Encoding), which decreases the hamming weight of data words, leading to a reduction in energy consumption in the modern DRAM data bus. Since smaller hamming weight of the data words also reduces switching activity, switching energy and power noise are also both reduced. BD-Encoding exploits the similarity in data words in the DRAM data bus. We observed that similar data words (i.e. data words whose hamming distance is small) are highly likely to be sent over at similar times. Based on this observation, BD-coder stores the data recently sent over in both the memory controller and DRAMs. Then, BD-coder transfers the bitwise difference between the current data and the most similar data. In an evaluation using SPEC 2006, BD-Encoding using 64 recent data reduced termination energy by 58.3% and switching energy by 45.3%. In addition, 55% of the LdI/dt noise was decreased with BD-Encoding.