Performance and Energy Enhancement through an Online Single/Multi Level Mode Switching Cache Architecture

STT-RAM cells can be considered as an alternative or a hybrid addition to today's SRAM-based cache memories. This is mostly because of their scalability and low leakage power. Moreover, their data storing mechanism (storing the value as resistance) makes them very suitable and applicable for multivalue cache architectures. This feature results in system performance enhancement without any area overhead. On the other hand, the required two-step read/write procedure in multilevel cells results in a non-uniform time access and energy and power overhead on the system. In this paper, we propose a new architecture to dynamically swap data between soft (fast read access) and hard (slow read access) bits in ML cell. Moreover, by reconfiguring cache block size, the proposed architecture can switch between ML and SL modes at runtime. In other words, the swapping method places the hot part of each cache block into soft-bits and the less accessed part into the hard-bits. The SL/ML switching method benefits from the low latency and energy of SL mode and the high storing capacity of ML mode at the same time. Although experimental results show that our proposed method slightly increases the miss rate compared with the conventional ML caches, the performance and energy are improved by 4.9% and 6.5%, respectively. Also, the storage overhead of our method is about 1% that is negligible.