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Implementation of the RISC-V Bit Manipulation (RVB) extensions for our RI5CY core

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RISC-V is an open-source Instruction Set Architecture (ISA) governed by the no-profit organization RISC-V ( Thanks to its simplicity, efficiency and free to use nature, in the last years it has been heavily adopted by industries for products, evaluation board as well as by university for vehicle to research project. Interesting examples are products coming from companies like NXP with the Vega board (, GAP8 from GreenWaves (, SiFive core IPs ( or Dolphin Integration with the Tornado board ( and many more. ETH and in particular our Digital Circuits and Systems group at IIS contributed to the open-source RISC-V community by providing three cores RI5CY, zero-riscy and Ariane that have been also used in aforementioned products. Moreover, they have been recently graduated from academic-level IP quality to industry with the support of big companies like Google, SiLabs, NXP, etc. One of the key success point of RISC-V along its free and open architecture is its extendability. Among the official instruction extensions, some of the ISA encoding space is left to implement custom instructions that face custom optimization that vary from context to context. Under the PULP project here in our group (, the RI5CY has been extended with custom instructions to support signal processing, bit manipulation tasks, hardware-loops and so on. Some of the extensions are also organized inside RISC-V to be discussed in task-groups among members of the foundation. For instance, there are task groups to specify Vector extensions (RVV), Packed SIMD extensions (RVP), Bit Manipulation (RVB) etc. Recently, the RISC-V community has proposed a quite stable proposal for the RVB instructions Some of those are indeed very similar to the one developed for the PULP project and some are not. Some are missing and some are not present part of the PULP extensions.

Project description

We propose a semester thesis to implement the proposed RVB extensions to the RI5CY core. Such tasks requires to:

    • Replace the ones that are similar/equal
    • Implement the missing ones
    • Evaluate the impact in AREA and Timing with a detailed report
    • Evaluate the impact in performance/execution time for a given set of benchmarks

In case of a master thesis, this work will be further investigated for the 64 bit RISC-V core Ariane, plus enhancement on the RI5CY verification strategy and execution trace.