RISC-V: Open Source Instruction Set Architecture

Author: Prakash Mohapatra

The new kid on the block


The emerging IoT industry is an aggregation of products and services, complementing each other to enable efficiency and cost optimization in multiple industries. It does not have a vertically oriented value chain. IoT end nodes will be scattered in billions in various industries.

As mentioned in my earlier post ARM vs Intel: The new war frontiers, COTS processors will not be ideal for building these end nodes, as the latter are application specific. Companies would be inclined to adopt custom processors as they offer flexibility to assemble only required parts. These parts can include analogue sensor, DSP, proprietary IP, etc. Further, custom processors substantially reduce BoM cost and die size, which will minimize power dissipation. It also helps companies to differentiate their product from those of their competitors. In view of failing Moore’s Law, customization is the answer as it can reduce the BoM cost significantly.

Enter RISC-V

Open Source Software (OSS) has played a vital role in democratization of the software industry. One of the most popular examples of OSS is Linux operating system. OSS enables innovation and differentiation at a low cost of adoption. This promotes small firms and start-ups to build products based on OSS such as Linux. A large community of developers support the software development, so there is no risk of vendor-locking or obsolescence of proprietary technology. The collective efforts of the community ensure a large ecosystem, and this benefits all the users.

RISC-V extends the open source movement into CPU ISA. It is an open source ISA that is license free and royalty free. As RISC-V is void of any licensing the ISA can be used for building custom processors with zero licensing cost. RISC-V is gradually building an ecosystem.

A few ARM customers have already started using RISC-V for designing custom processors. Now, SoC design companies can develop custom processors at a low cost without the paying licensing rents. With some NRE investment, these firms can develop the SoC and get it manufactured in fabs. Thus, the price of the processor will be lesser than those based on ARM IPs. At the face value, it looks like we have found an ideal candidate that has the potential to become the dominant ISA for IoT industry. With customization and zero licensing cost, RISC-V looks like a winner. Is it really that simple?

Are there free lunches?

Linux is a quite successful with billions of deployments in diverse products. Although it needs considerable effort and expertise in using Linux for commercial products, the benefits outweigh the person hours. Linux offers unmatched flexibility. The enormous community provides a good ecosystem around the OS, with extensive support for peripherals, third-party software, etc.

However, extending the concept of open source to chip design is a different ball game. Let ‘s consider the case of open source RISC-V. In a SoC, CPU IP is just one part. There are many other physical IPs and peripherals needed. So, an extensive IP and EDA ecosystem is needed around the CPU IP. You can only get the CPU IP without licensing rents; however, the surrounding ecosystem is going to cost you. IP vendors should see a viable business case to add support for RISC-V in their portfolio. Let’s assume that a strong community backs the RISC-V, and it offers all the IPs and tools needed for building SoCs. However, the question remains whether companies building custom SoCs will take the risk of using a community backed ISA. Bugs can lead to multiple tape-outs, which add huge cost. Finally, designing a SoC is complex and needs good expertise in multiple areas such as implementation, physical design, packaging, etc.

With ARM ISA, most of the issues mentioned above are alleviated. You get access to proven IPs, robust ecosystem (software, cloud services, security solutions, silicon vendors, fabs), and committed support, instead of community support offered by open source ISA. Design complexity is reduced; though some expertise in SoC design is needed for building custom processors. With DesignStart license for Cortex-M0, ARM has enabled custom processor design at low cost with less risk. This program will be really useful for start-ups and small companies.

Who will build RISC-V based SoCs?

The idea of open source is disruptive, as it enables a level playing field to companies with limited budget to compete against big players. Although the concept of open source ISA is revolutionary, it may not have a disruptive effect in democratizing chip design.

In my view, it is unlikely that small companies and start-ups addressing some niche application in the IoT space will invest time, effort and money in building custom processors based on a community-backed ISA, as they have to validate whether the entire system meets their specifications. It would be a safer bet for them to use licensed ISA as they get a proven system, complemented with a robust ecosystem. Multiple tape-outs of the SoC can add substantial cost. A matured ISA with some initial cost would be a good starting point, instead of a free and fledgling ISA. SoC design is not their core activity. So hiring a diverse team for chip design, may not be a pragmatic decision. ARM is used widely across the industry. So the design part can be outsourced to some small companies specializing in ARM-based SoC design. EDA tools and fabs are costly. EDA vendors and fabs already support ARM-based IPs; they should see economic benefits for adding support for RISC-V. Until RISC-V reaches a critical mass adoption, it’s like a chicken-and-egg situation. Multi-homing adds cost for any company be it a fab, EDA vendor, design firm, or an app developer. Low volume business will attract higher rents. All these cost overheads have to be factored while building RISC-V based SoCs.

Market leaders in the SoC design will definitely develop RISC-V-based SoCs, as it increases their buying power by having some alternative to ARM. However, I believe that these companies will not be interested in engaging with low-volume customers, who needs custom processors. Owing to their large overhead, it makes business sense to sell millions of standardized SoCs.

Summing from above, in my view RISC-V, in its current state, cannot significantly disrupt the semiconductor market structure. One of the key virtues of open source movement is to minimize entry barriers into a market by offering a good enough base in comparison to licensed entities. Although RISC-V will offer flexibility for building custom SoCs at low cost, the ecosystem is not yet ready to accept it. The whole semiconductor industry need to work in sync to make RISC-V successful.


It’s too early to pronounce the verdict on RISC-V. Any new idea takes time to flourish. I’m sure the people at RISC-V are smart enough that they would have foreseen the issues that I have mentioned above and many of the issues would have been already mitigated internally.

Low cost and customization are often mutually exclusive. Any ISA addressing both these ends will play a dominant role in the IoT industry.

Author: Prakash Mohapatra

Prakash Mohapatra

Prakash has been associated with the technology industry for around a decade. He is keenly interested in exploring avenues to bridge the gap between customer utility and technology. He is an MBA graduate and also holds a Bachelors in Engineering.

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