A case for asymmetric-cell cache memories

In this paper, we make the case for building high-performance asymmetric-cell caches (ACCs) that employ recently-proposed asymmetric SRAMs to reduce leakage proportionally to the number of resident zero bits. Because ACCs target memory value content (independent of cell activity and access patterns), they complement prior proposals for reducing cache leakage that target memory access characteristics. Through detailed simulation and leakage estimation using a commercial 0.13-/spl mu/m CMOS process model, we show that: 1) on average 75% of resident data cache bits and 64% of resident instruction cache bits are zero; 2) while prior research carefully evaluated the fraction of accessed zero bytes, we show that a high fraction of accessed zero bytes is neither a necessary nor a sufficient condition for a high fraction of resident zero bits; 3) the zero-bit program behavior persists even when we restrict our attention to live data, thereby complementing prior leakage-saving techniques that target inactive cells; and 4) ACCs can reduce leakage on the average by 4.3/spl times/ compared to a conventional data cache without any performance loss, and by 9/spl times/ at the cost of a 5% increase in overall cache access latency.