Ternary cache: Three-valued MLC STT-RAM caches

Spin-transfer torque random access memory (STT-RAM) has become a promising non-volatile memory technology for cache memories. Recently, 2-bit multi-level cell (MLC) STT-RAM has been proposed to enhance data density, but it suffers from low reliability of its read and write operations. In this paper, we propose a novel cache design called Ternary cache. In Ternary cache, a memory cell can store three values (i.e., 0,1,2) while MLC STT-RAM can store four values. In this way, Ternary cache achieves much higher read stability than MLC STT-RAM-based caches. To enhance writability, a write operation is performed with high current and terminated as soon as the data is written. Evaluation results show that Ternary cache achieves the data density benefit of MLC STT-RAM and the reliability benefit of SLC STT-RAM.