Understanding RISC-V: The Open Standard Instruction Set Architecture

Introduction to RISC-V

RISC-V (pronounced as risk five) is an open standard Instruction Set Architecture (ISA) based on Reduced Instruction Set Computing (RISC) computer architecture. Unlike proprietary ISAs, RISC-V is freely available to the public and is being used for many purposes, including designing, manufacturing, and selling customized RISC-V chips and software. This open standard approach has the potential to drive innovation and competition in the tech industry, as it allows for a greater diversity of products and solutions.

The significance of RISC-V in the context of modern computing cannot be overstated. As the tech industry is evolving, the need for efficient, scalable, and customizable computing solutions is more important than ever. RISC-V, with its simple and modular design, is well-positioned to meet these needs. Furthermore, as an open ISA, RISC-V has the potential to democratize access to high-performance computing, making it a key player in the future of the tech industry.

The Evolution of Instruction Set Architectures (ISAs)

Instruction Set Architectures (ISAs) have been a fundamental part of computing systems since their inception. They serve as the interface between the hardware and software of a computer, defining the machine language instructions that a processor can execute. Over the years, ISAs have evolved significantly, driven by the need for increased performance, efficiency, and flexibility.

In the early days of computing, Complex Instruction Set Computing (CISC) was the dominant architecture. CISC architectures are characterized by a large number of complex machine language instructions, which can perform multiple operations in a single instruction. This approach was initially beneficial, as it allowed for more efficient use of memory at a time when memory was a significant cost factor.

However, as technology advanced and memory costs decreased, the limitations of CISC became apparent. The complexity of CISC instructions led to inefficiencies in execution speed and power consumption. Furthermore, the large number of instructions made it difficult to optimize the performance of CISC processors.

The Emergence of RISC Architecture

In response to the limitations of CISC, the concept of Reduced Instruction Set Computing (RISC) was introduced in the late 1970s and early 1980s. RISC architectures are characterized by a small set of simple instructions, each performing a single operation. This simplicity allows for faster execution of instructions and more efficient use of processor resources.

Following are the numerous benefits of the RISC processing unit over CISC [1] : 

1. Efficient Pipelining: The simplicity of RISC instructions allows for more efficient pipelining, a technique where multiple instructions are overlapped in execution. This leads to increased instruction throughput, improving the processor’s overall performance.
2. Optimized Performance: The reduced instruction set makes it easier to optimize the processor's performance. With fewer instructions to implement, designers can focus on optimizing the execution of each instruction, leading to more efficient processors.
3. Increased Scalability: RISC architectures are more scalable than CISC architectures. The simplicity of the instruction set allows for easier implementation of parallel processing techniques, such as superscalar execution and multithreading, which can significantly increase the processor performance.

Despite these benefits, the transition from CISC to RISC was not immediate. Many existing software applications were written for CISC CPUs, and rewriting these applications for RISC CPUs was a significant undertaking. However, as the benefits of RISC became more apparent, the tech industry gradually shifted towards RISC architectures. Today, RISC architectures are the dominant choice for many applications, from embedded systems to high-performance servers.

The Limitations of Proprietary RISC Architectures

Proprietary RISC (Reduced Instruction Set Computing) architectures, such as those developed by Intel, AMD, and ARM, are systems designed to execute a small number of instructions at high speed. While they offer impressive processing performance, these architectures have their limitations. A few of these are mentioned below:

1. Incompatibility: While RISC architectures offered significant improvements over CISC, the early RISC architectures were proprietary, meaning they were owned and controlled by individual companies. This led to a fragmented landscape where different RISC architectures were not compatible with each other. Software written for one RISC architecture could not be run on a processor using a different RISC architecture without significant modifications. This lack of interoperability was a major barrier to the widespread adoption of RISC.
2. Limited Access: Another limitation of proprietary RISC architectures was the lack of transparency. Because the architecture details were owned by a single company, other companies and developers did not have access to the full specifications of the architecture. This made software optimisation difficult for these architectures and limited the ability of the broader tech community to contribute to the development and improvement of the architecture.
3. Vendor lock-in: Furthermore, proprietary RISC architectures often led to vendor lock-in. Once a company had invested in a particular RISC architecture, it was costly and time-consuming to switch to a different architecture. This reduced competition and innovation in the market, as companies were less likely to invest in developing new and improved architectures.

These limitations of proprietary RISC architectures highlighted the need for a new approach. An approach that would combine the benefits of RISC with the openness and interoperability of an open standard. This led to the development of RISC-V, an open standard RISC architecture that aims to serve as an alternative microprocessor technology to proprietary RISC architectures.

The Birth of RISC-V

The RISC-V Processors. Credits: IEEE XploreThe development of what is known as RISC-V International began in 2010 at the University of California, Berkeley [2]. The project was initiated by a group of researchers led by David Patterson, Krste Asanović, and a team of other graduates. Their original goal was to conduct computer architecture research that would overcome the limitations of proprietary RISC architectures [3].

RISC-V was designed from the ground up to be simple, efficient, and scalable. A few of the following advantages that it had over the proprietary RISC, were:

1. Modular: It was designed to be modular, allowing for the addition of new instructions to meet specific needs without disrupting the existing instruction set. 
2. Customizable: Its modularity makes RISC-V highly customizable, enabling it to be tailored to a wide range of applications, from tiny embedded systems to massive data centres.
3. Broadly Available: The development of RISC-V was not just a technical endeavor, but also a social one. The creators of RISC-V recognized that for the architecture to succeed, it would need to be adopted by a broad community of users and developers. To facilitate this, they made the RISC-V specifications freely available and established the RISC-V Foundation to promote the use of RISC-V.

Suggested Reading: RISC-V vs ARM: A Comprehensive Comparison of Processor Architectures

Suggested Reading: RISC-V Architecture: A Comprehensive Guide to the Open-Source ISA

The Principles of RISC-V

RISC-V is based on key principles that differentiate it from other ISAs. These principles reflect the lessons learned from decades of ISA design and aim to address the limitations of previous architectures.

1. Simplicity: One of the key principles of RISC-V is simplicity. The core instruction set of RISC-V is intentionally kept small and simple, making it easier to understand, implement, and optimize. This simplicity also reduces the complexity of the hardware, leading to a more efficient and reliable CPU.
2. Modular & Customizable: Another key principle of RISC-V is modularity. The RISC-V ISA is designed as a base instruction set with optional extensions. This allows for a high degree of customization, as users can implement only the instructions they need. This modularity also allows for future expansion, as new extensions can be added to meet emerging needs [4].
3. Open: A third key principle of RISC-V is openness. As an open standard, RISC-V is freely available to anyone, without licensing fees or non-disclosure agreements. This openness encourages collaboration and innovation, as anyone can contribute to the development of RISC-V.

These principles of simplicity, modularity, and openness are not just abstract ideals but are deeply embedded in the design of RISC-V. They guide the architecture development and shape the surrounding community. Together, they make RISC-V a powerful future computing tool.

The RISC-V Foundation & RISC-V International

RISC-V International, initially known as The RISC-V Foundation was established in 2015 to promote the RISC-V ISA adoption and development. The Foundation is a non-profit organization that brings together varied stakeholders, including academic institutions, technology companies, and individual developers. Its mission is to foster collaboration and innovation within the RISC-V ecosystem, ensuring that the architecture continues to evolve and meet the needs of its users. In a strategic move to ensure global neutrality and foster wider international collaboration, the RISC-V Foundation transitioned to RISC-V International and relocated its base to Switzerland in early 2020. 

One of the primary roles of RISC-V International is to maintain and develop the RISC-V specifications. It oversees creating new extensions and updating existing ones, ensuring that the ISA remains consistent and coherent. This process is open and transparent, with input from the broader RISC-V community.

In addition to its work on the specifications, the RISC-V International also plays a key role in promoting the use of RISC-V. The community organises events, such as workshops and conferences, to unite RISC-V users and developers. These events provide a forum for sharing knowledge, discussing challenges, and showcasing new developments in the RISC-V ecosystem.

The RISC-V International also supports the development of open-source tools and resources for RISC-V. This includes software development tools, such as compilers and debuggers, as well as hardware design tools and reference implementations. By providing these resources, the community helps to lower the barriers to entry for RISC-V and encourages ecosystem growth.

Overall, the RISC-V International plays a crucial role in the success of the RISC-V architecture. By fostering collaboration, promoting adoption, and ensuring the continued development of the ISA, the Foundation is helping to shape the future of computing.

The Technical Aspects of RISC-V

RISC-V is designed with a focus on simplicity, efficiency, and modularity. These characteristics are reflected in various technical aspects of the architecture, including the instruction set, the register file, and the memory model. By understanding these technical aspects, it becomes clear how RISC-V achieves its goals and stands out from other ISAs.

The RISC-V Instruction Set

The RISC-V instruction set [5][6] is the core of the architecture, defining the set of operations that a RISC-V processor can perform. The instruction set is designed to be simple and efficient, with a few basic instructions that can be combined to perform complex operations.

RISC-V instructions are 32 bits long, with a fixed format that makes decoding and execution straightforward. The instructions are divided into several categories, including arithmetic, logical, memory, control flow, and system instructions. Each category contains a few instructions that perform the most common operations required by software.

One of the key features of the RISC-V instruction set is its support for variable-length instruction encoding. While the base instruction set uses 32-bit instructions, RISC-V also supports 16-bit compressed instructions, which can be used to reduce code size and improve energy efficiency. This feature is particularly useful for embedded systems and other applications where memory and power consumption are critical factors.

The RISC-V instruction set is also designed to be extensible, allowing for the addition of custom instructions to meet specific application requirements. This extensibility is achieved through instruction set extensions, which can be added to the base instruction set without affecting the existing instructions. This modular approach enables RISC-V to be tailored to a variety of applications, from low-power embedded systems to high-performance computing platforms.

The Modularity of RISC-V

Modularity is a defining characteristic of RISC-V that sets it apart from other ISAs. The architecture is designed as a small base instruction set with a series of optional extensions. This modular design allows for a high degree of customization, enabling users to implement only the instructions they need for their specific application [7].

The base instruction set of RISC-V, known as RV32I, includes a minimal set of 32-bit integer instructions. This base set is sufficient to run a simple operating system and perform basic computational tasks. However, for more complex applications, additional functionality may be required.

To accommodate this, RISC-V includes a range of optional extensions that can be added to the base instruction set. These extensions provide additional functionality, such as floating-point arithmetic, atomic operations, and vector processing. Each extension is designed to be orthogonal to the base instruction set and to other extensions, meaning that they can be added or removed independently without affecting the operation of the rest of the ISA.

This modularity extends beyond the instruction set to other aspects of the architecture. For example, RISC-V supports multiple privilege levels, with separate instruction sets for each level. This allows for the implementation of secure systems, where different tasks can be run at different privilege levels to protect sensitive data and system resources.

The modularity of RISC-V also facilitates scalability. The architecture supports a range of address sizes, from 32-bit to 128-bit, allowing it to scale from small embedded systems to large supercomputers. This scalability is achieved using different base instruction sets for different address sizes, with the same set of extensions applicable to all base sets.

In summary, the modularity of RISC-V is a key factor in its flexibility and adaptability. This modularity enables RISC-V to meet a wide range of application requirements and adapt to evolving technology trends by allowing for customization, scalability, and future expansion,.

The Impact of RISC-V on the Tech Industry

The emergence of RISC-V has had a profound impact on the tech industry. Its open, modular, and scalable design has made it an attractive option for various of applications, from embedded systems to high-performance computing. The impact of RISC-V can be seen in several key areas, including the democratization of processor design, the acceleration of innovation, and the shift toward hardware customization.

The Adoption of RISC-V

The adoption of RISC-V has been growing steadily since its introduction. A diverse range of organizations, from startups to tech giants, have embraced RISC-V for its flexibility, efficiency, and openness. These organizations are using RISC-V in a variety of applications, including IoT devices, data centres, and supercomputers.

One of the key drivers of RISC-V adoption is its open nature. As an open standard, RISC-V eliminates the licensing fees and restrictions associated with proprietary ISAs. This has made it an attractive option for companies looking to reduce costs and increase control over their hardware.

Another factor driving the adoption of RISC-V is its modularity. The ability to add or remove instruction set extensions allows companies to tailor the architecture to their specific needs. This flexibility enables companies to optimize their hardware for performance, power efficiency, or other factors, depending on their application requirements.

The adoption of RISC-V has also been facilitated by the support of the RISC-V International and the broader RISC-V community. The availability of open-source tools and resources, along with the collaborative development of the ISA, has made it easier for companies to adopt and implement RISC-V. Let’s touch on a few cases below:

• SiFive: For example, SiFive, a pioneering force in the RISC-V revolution, is renowned for its extensive portfolio of RISC-V Core IP. The company provides a broad range of highly customizable processor IPs, including microcontrollers, which are tailored to meet exact application-specific requirements. One of SiFive's notable offerings in the microcontroller sector is the HiFive1 Rev B, a board that features the FE310-G002 microcontroller. Operating at voltages of 3.3 V and 1.8 V, it is designed for a variety of applications – from IoT to wearables. Additionally, SiFive has introduced the E2 Core IP Series, which is considered the smallest and lowest-power RISC-V design available. It is designed for the microcontroller, embedded, IoT, and wearable applications, featuring SiFive's E31 CPU Coreplex, a high-performance, 32-bit RV32IMAC core
• Linux: One of the most compelling aspects of RISC-V is its compatibility with Linux. The Linux kernel has been officially supporting RISC-V since its 4.15 release. This means developers can run full-fledged Linux operating systems on RISC-V-based hardware, opening up a myriad of opportunities for software development, system customization, and hardware innovation. With Linux's robustness and versatility combined with RISC-V's open-source nature, this alliance is set to drive the next wave of computing technology [8].
• SoC: RISC-V's open standard instruction set architecture is essential for the development of SoCs. SoC integrates all necessary electronic circuits and parts onto a single, integrated circuit, acting as the 'brain' of a system. With RISC-V, custom processors can be designed and manufactured for diverse applications, from embedded designs to data-intensive tasks like AI and machine learning. The flexibility and adaptability of RISC-V make it an appealing choice for SoC designers, especially when combined with the versatility of FPGAs.
• FPGAs: FPGAs, or field-programmable gate arrays, are versatile integrated circuits that offer post-manufacturing programmability,  a key component in the chip design process. They consist of configurable logic blocks (CLBs) connected through programmable interconnects, allowing customization for specific application needs. FPGAs play a crucial role in creating system-on-chips (SoCs) by enabling high levels of customization and adaptability, especially when utilized alongside RISC-V. This combination empowers companies to achieve optimal design solutions for their SoCs.
• Automotive Industry: RISC-V has emerged as a remarkable game-changer in the automotive industry. With the increasing technological sophistication of vehicles, the demand for customized, efficient, and secure computing systems has become paramount. RISC-V offers a unique solution to these requirements through its open-source instruction set architecture. It empowers automotive engineers to design processors that precisely match the demands of modern vehicular systems, ranging from infotainment and advanced driver-assistance systems (ADAS) to fully autonomous driving technologies. Therefore, RISC-V not only influences the present state of the automotive industry but also plays a significant role in shaping its future trajectory.

Lastly, RISC-V is making a significant impact on major tech companies such as Nvidia, Apple, and Qualcomm, as well as countries like China. Nvidia, known for its advanced GPU, is exploring the use of RISC-V to develop chips for embedded applications that comply with U.S. export restrictions. Qualcomm sees potential in RISC-V's open-source nature for customizing their SoCs. Apple is also considering RISC-V for future developments. With China's semiconductor industry in mind, RISC-V is becoming an attractive option amid a tech war with the U.S.

 Qualcomm and Google to release RISC-V-based Wearable (as of 13th November 2023). Image Credits: Android CentralOverall, RISC-V's influence is expanding globally, revolutionizing processor design and shaping the future of the tech industry. The growing adoption of RISC-V is a testament to its potential to transform the tech industry. By providing an open, flexible, and efficient architecture, RISC-V enables a new era of processor design and innovation.

The Future of RISC-V

The future of RISC-V looks promising, with increasing adoption and ongoing development of the architecture. The open nature of RISC-V, coupled with its modularity and scalability, positions it well to adapt to future technology trends and meet evolving application requirements.

One of the key trends shaping the future of RISC-V is the growing demand for customized hardware. As applications become more diverse and specialized, the need for hardware that can be tailored to specific requirements is increasing. The modularity of RISC-V, with its support for custom instruction set extensions, makes it well-suited to meet this demand.

Another trend that bodes well for the future of RISC-V is the shift towards edge computing. As more processing is done at the edge of the network, closer to the source of data, there is a growing need for efficient, low-power CPUs. The simplicity and efficiency of RISC-V make it an attractive option for edge computing applications.

The future of RISC-V is also being shaped by advances in semiconductor technology. As process nodes shrink and transistor densities increase, the ability to design efficient, high-performance processors becomes more critical. The simplicity and modularity of RISC-V can help to address these challenges, enabling the design of processors that make optimal use of advanced semiconductor technologies.

In addition to these trends, the future of RISC-V will also be influenced by the ongoing development of the architecture. The RISC-V community continues to work on new extensions and improvements to the ISA, ensuring that it remains relevant and capable of meeting future needs.

Overall, the future of RISC-V looks bright. With its open, modular, and efficient design, along with the support of a vibrant and collaborative community, RISC-V is well-positioned to play a significant role in the future of computing.

Conclusion

RISC-V has emerged as a powerful and flexible open standard ISA that addresses the limitations of proprietary RISC architectures. Its simplicity, modularity, and scalability make it well-suited for a wide range of applications, from embedded systems to high-performance computing. The ongoing development of RISC-V, along with the support of the RISC-V Inrwenational and the broader community, ensures that the architecture will continue to evolve and adapt to future technology trends. As a result, RISC-V is poised to play a significant role in shaping the future of computing.

FAQs

What is RISC-V?

RISC-V is an open standard instruction set architecture (ISA) based on reduced instruction set computing (RISC) principles. It is designed to be simple, efficient, and scalable, making it suitable for a wide range of applications.

How does RISC-V differ from other ISAs?

RISC-V is an open standard, meaning it is freely available for anyone to use and implement. It is also modular, allowing for customization through the addition of instruction set extensions. These features set it apart from proprietary ISAs, which are often restricted and less flexible.

What are the benefits of using RISC-V?

RISC-V offers several benefits, including simplicity, efficiency, modularity, and scalability. Its open nature encourages collaboration and innovation, while its modular design allows for customization to meet specific application requirements.

Who is behind the development of RISC-V?

RISC-V was initially developed at UC Berkeley and is now supported by RISC-V International, a non-profit organization that promotes the adoption and development of the RISC-V ISA.

What are some applications of RISC-V?

RISC-V can be used in a wide range of applications, from low-power embedded systems to high-performance computing platforms. Its flexibility and efficiency make it an attractive option for IoT devices, data centres, and supercomputers, among other applications.

References

[1] Stanford. Analyzing the advantages and disadvantages of RISC by comparing it with CISC. Link

[2] RISCV. History of RISC-V. Link

[3] Digitaltrends. Closed-source vs Open-source. Link 

[4] IEEE. RISC-V Instruction Set Architecture Extensions: A Survey. Link

[5] Devopedia. RISC-V Instruction Set. Link

[6] RISCV. The RISC-V Instruction Set Manual Volume I: User-Level ISA Document Version 2.2. Link 

[7] RISCV. RISC-V Offers Simple, Modular ISA. Link 

[8] RISCV. RISC-V And Linux Foundations Will Partner To Promote Open Source CPU. Link