RVCoreP-32IM: An effective architecture to implement mul/div instructions for five stage RISC-V soft processors

Abstract: RISC-V, an open instruction set architecture, is getting the attention of soft processor developers. Implementing only a basic 32-bit integer instruction set of RISC-V, which is defined as RV32I, might be satisfactory for embedded systems. However, multiplication and division instructions are not present in RV32I, rather than defined as M-extension. Several research projects have proposed both RV32I and RV32IM processor. However, there is no indication of how much performance can be improved by adding M-extension to RV32I. In other words, when we should consider adding M-extension into the soft processor and how much hardware resource requirements will increase. In this paper, we propose an extension of the RVCoreP soft processor (which implements RV32I instruction set only) to support RISC-V M-extension instructions. A simple fork-join method is used to expand the execution capability to support M-extension instructions as well as a possible future enhancement. We then perform the benchmark using Dhrystone, Coremark, and Embench programs. We found that RV32IM is 1.87 and 3.13 times better in performance for radix-4 and DSP multiplier, respectively. In addition to that, our RV32IM implementation is 13\% better than the equivalent RISC-V processor.