Embedding Read-Only Memory in Spin-Transfer Torque MRAM-Based On-Chip Caches

We propose a design technique for embedding read-only memory (ROM) in spin-transfer torque MRAM (STT-MRAM) arrays by adding an extra bit-line in every column of the array. RAM and ROM data, which can be different, are stored in the same bitcell and the ROM capacity may be as large as the RAM capacity. Furthermore, our proposed ROM-embedding technique is applicable to any resistive memory technology in which the bit-cell topology is identical to that of the STT-MRAM bit-cell. An additional sense amplifier is required in the peripheral circuitry, hence we propose an area-optimized peripheral circuitry to minimize the total area penalty of embedding ROM. Our analysis reveals that the ROM may be embedded in the STT-MRAM array without area overhead and without any penalty in the performance of the memory as RAM. Furthermore, our simulations show that the embedded ROM may be used to accelerate applications that use lookup tables with as much as 30% improvement in instructions per cycle of a processor using ROM-embedded STT-MRAM for its L2 cache.