Multistate Register Based on Resistive RAM

In recent years, memristive technologies, such as resistive random access memory (RRAM), have emerged. These technologies are usually considered as alternates for static RAM, dynamic RAM, and Flash. In this paper, a novel digital circuit, the multistate register, is proposed. The multistate register is different from conventional types of memory, and is used to store multiple data bits, where only a single bit is active and the remaining data bits are idle. The active bit is stored within a CMOS flip flop, while the idle bits are stored in an RRAM crossbar co-located with the flip flop. It is demonstrated that additional states require an area overhead of 1.4% per state for a 64-state register. The use of multistate registers as pipeline registers is demonstrated for a novel multithreading architecture-continuous flow multithreading (CFMT), where the total area overhead in the CPU pipeline is only 2.5% for 16 threads compared with a single thread CMOS pipeline. The use of multistate registers in the CFMT microarchitecture enables higher performance processors (40% average performance improvement) with relatively low energy (6.5% average energy reduction) and area overhead.