RISC-V3: A RISC-V Compatible CPU With a Data Path Based on Redundant Number Systems

Abstract

Redundant number systems (RNS) are a well-known technique to speed up arithmetic circuits. However, in a complete CPU, arithmetic circuits using RNS were only included on subcircuit level e.g. inside the Arithmetic Logic Unit (ALU) for realization of the division. Still, extending this approach to create a CPU with a complete data path based on RNS can be beneficial for speeding up data processing, due to avoiding conversions in the ALU between RNS and binary number representations. Therefore, with this paper we present a new CPU architecture called RISC-V3 which is compatible to the RISC-V instruction set, but uses an RNS number representation internally to speed up instruction execution times and therefore increase the system performance. RISC-V is very suitable for RNS because it does not have a flags register which is expensive to calculate when using an RNS. To present reliable performance numbers, arithmetic circuits using RNS were realized in different semiconductor technologies. Moreover, an instruction set simulator was used to estimate system performance for a benchmark suite (Embench). Our results show, that we are up to 81% faster with the RISC-V3 architecture compared to a binary one, depending on the executed benchmark and CMOS technology.