Instruction Extension of a RISC-V Processor Modeled with IP-XACT

Abstract

Short time-to-market and cost consideration of hardware design promotes reuse of ever more complex intellectual property even up to processors. In processor design, the instruction set architecture (ISA) selection is a major design decision driven largely by application requirements. Extendable ISAs enable application-specific adjustments and improved performance at the cost of more complex design. Adding a custom instruction introduces a choice of either utilizing existing hardware or adding new dedicated hardware. This work presents an instruction extension flow for a RISC-V processor core modeled in IP-XACT. We demonstrate the workflow by adding three bit manipulation instructions "popcnt", "parity" and "bswap" in the instruction set that executes on an extended processor platform and evaluate their performance in simulation. The simulated instruction count and performance are used to evaluate the benefit of adding dedicated hardware. The effort analysis of the design flow shows approximately 110 minutes work for adding a new instruction to the RISC-V core. This suggests a straightforward and easy to follow approach that can be extended to other instructions as well. In addition, we propose the workflow to cover adding dedicated hardware in IP-XACT for improved re-usability and design consistency.