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

Posted:10:03 AM January 04, 2024 writer: The Engineering Knowledge


The ISA, or instruction set architecture, defines how certain computer processor cores operate. The ISA explains the register and each machine-level instruction. It accurately explains the workings of each instruction, and it is encoded in bits. RISC V is an instruction set made by California University. It was made based on mostly processor instruction not used by computer programs. There is unnecessary decoding logic applied in processor designs, using the larger power. To reduce the instruction set and make the register work better, the RISC V process was used. Different technology-based companies start working due to its open-source capabilities. Some needed a license to get access, but this process helps people make designs. Here we will cover details of the RISC-V architecture.

History and Evolution of RISC-V

ISAs were completely regulated by certain types of companies, gave limited access to internal details and architecture, and applied license fees. This type of restriction reduces innovation and makes it difficult for new companies to work on them and make new processors. So it resulted in the use of RISC-V. Before RISC-V, there were many RISC processors. Such as PowerPC, SPARC, etc. This design was considered effective and used in different applications, but it had license fees and limited working capabilities.

At the University of California, the work started on RISC-V, which was initially made as a research project in 2010. The purpose of this project was to make new open-source ISAs that can manage the restrictions of older ISAs and offer features to make new processors. This project was headed by computer experts Krste Asanović, Yunsup Lee, and Andrew Waterman, who got ideas from the success of open-source software and worked to make new advantages for hardware. In 2011, the first version of the RISC-V ISA was released, called RV31, based on the instruction set. Its initial introduction, based on efficiency and simplicity, follows the workings of RISC. With time, RISC VISA made new developments by using new extensions and features to increase its capabilities and cover large-area applications. The RISC-V Foundation was created in 2015 to promote and use the RISC-V ISA. This foundation was made through collaboration among industries, universities, and individual learners who want to take part in the development of RISC-V technology.

RISC-V Design Principles

The RISC-V architecture is based on different design rules that promote its efficiency, adaptation, and working operation. The features include modularity, extensibility, and instruction sets. With the use of these instructions, RISC-V helps to develop the processors that can be used for certain applications and provides optimization and customization.

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Reduced Instruction Set Computing (RISC)

The RISC-V-Vrchitecture is based on reduced instruction set computing, or RISC. RISC is a process design that focuses on efficacy and simplicity with the use of a small set of general-purpose instructions. It is contrary to completed instruction set computing, which uses a larger set of complicated instructions that can perform many operations in one instruction. RISC is based on the ease and execution of one instruction per clock cycle and provides an effective and simple coding design. CISC is a complicated application of instruction that can do many operations but requires many clock cycles for execution. But both architecture objectives provide better CPU operations.





Instruction Set Complexity

is a simple design that comes with a small set of instructions.

It is complicated and has a larger set of instructions.

Instruction Execution

One cycle is used for the execution of many instructions.

Many cycles are used to apply the instructions.

Pipeline Length

Normally, longer pipelines for larger clock rates

Shorter pipelines have lower clock rates.

Hardware Utilization

It focuses on optimizing the instructions used.

General-purpose instructions to handle larger operations

Memory Access

Memory access is based on a load-store architecture.

It supported memory-to-memory operations.

Code Density

Codes are compact.

Different instructions are used to make the code verbose.

Compiler Complexity

In optimization, the compiler also takes part.

Less optimization is needed due to the complicated instructions.

Power Consumption

It uses less power.

Due to its completed operation, it uses a lot of power.


Highly modular and extensible.

Complicated instruction and making less modular


Modularity and extensibility

There are important aspects of modularity and extensibility in the design of RISC-V architectures. Modularity defines the ISA organization as distinct and separate components that are linked to find different configurations to make new processor designs. But extensibility defines the features needed to add new instruction features and extensions in ISA without affecting existing functions. The RISC-V ISA is categorized in the base integer instruction set and set out of optional extension. The base integer instruction set is used for core functions needed for general-purpose computing, and the extension adds specialized functions used for certain uses. This thinking helps designers choose features that are needed for certain uses and, as a result, make a processor that has a good circuit, an option for power use, and other design requirements.

For RISC-V extensibility, there is a well-structured extension system that helps with the use of new instructions and features without affecting existing software features. That helps ISA make changes to new features and use new techniques and applications. There are many standard extensions made by the RISC-V community, like floating point, vector processing, and cryptographic operations, that are used to make design simple and easy.

RISC-V instruction set

The RISC-V instruction set is a group of instructions that explain the working RISC-V process. These instructions are simple, effective, and easy to use for a high degree of optimization and customization. The instruction set is configured as a basic integer instruction set and an optional extension set that offers certain functions for specific use.

Base Integer Instruction Set

The base integer instruction set, also called the RV321 or RV641 instruction set, based on address space size, offers the core functions required for general-purpose computing. It comes with instructions, logic, arithmetic, and control operations for accessing the memory. The base integer instruction set is made to last and effectively follows the rules of RISC. RISC-V instructions are encoded with a fixed-length 32-bit format, which helps to easily simplify the decoding and execution. The instruction formats have different categories, and each has a certain working and a different coding layout. These categories are explained here.

  • R instructions: It is used for register-to-register operations, for example, logical operations and arithmetic operations. It comes with two register operands, two source registers, and two used or destination registers.
  • I instructions: It is used to perform immediate operations like arithmetic and logical operations in immediate value. That comes with two register operands and a 12-bit, 12-bit immediate value.
  • Instructions: It is used for storing the operation, like storing data from the register to memory. It also has two register operands and 12 bits of immediate value for memory address offset.
  • B instructions: is employed for conditional branch operation that transfers control to different instructions based on condition. They have two register operands and 12 12-bit immediate values for the target branch address.
  • Instructions: It is used for operations with a twenty-bit immediate value, for example, loading a 20-bit constant in the register or setting the upper 20 bits of the register.
  • J instructions: It is employed for unconditional jump operations that transfer control to different instructions. They have one register operand and 20 bits of immediate value for the jump target address.

RISC-V Register File

The RISC-V register file is the main component of the RISC-V architecture and offers a set of storage locations for data holding when instructions are executed. The register file is arranged in an integer resistor and floating point register, based on the extension used in the processor. Registers are important in the RISC-V architecture since they provide fast access to data and increase operation and processor efficiency.

Integer Registers:

in the RISC-V design integer register is used for string and manipulation of integer data. The number of integer registers and their size can be different based on implementation. These registers are important for performing logical, arithmetic, and control flow operations.

Floating-Point Registers:

Floating registers are made to carry floating-point arithmetic. These registers are different from the integer register and are important for an application that uses real numbers and mathematical computation in decimal points.

RISC-V Memory Model:

This module defines how memory is configured and accessed. It comes with features such as byte addressing, byte ordering, and the addressing mode employed for loading and operation storage. RISC-V uses a load-store layout that manages only loads and stores instructions directly in memory.

RISC-V Privilege Levels:

RISC-V defines different levels to configure different modes of execution and access the resources of the system. These levels come with user mode, supervisor mode, and machine mode. These levels separate user applications from operating systems and offer processes for securing and controlling access to different resources.

RISC-V Implementations:

RISC-V has open standard features, and there are many implementations, such as simple controllers and high-performance processors. Different companies make their own RISC-V implementation, which takes part in architecture adaptability.

Hard-Core Implementations:

Some types of RISC-V implantation are called hard cors, which means they are pre-designed and normally integrated into certain chips. Hard cores are ready-made implementations that work as building blocks for many applications and minimize production time.

RISC-V Ecosystem and Community:

The RISC-V ecosystem comes with an active and vibrant developer community, industrial community, and researcher community. The open nature of RISC-V promotes collaboration and innovation. The ecosystem comes with software tools, simulators, development boards, and different hardware implementations.

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Advantages and disadvantages:


  1. The main advantage of RISC-V is its open and royalty-free features. Contrary to other ISAs, RISC-V is not regulated by a single company, and anyone can use it without a license. This open structure helps to increase different ideas, implementation, and innovation for small companies and students.
  2. It is a highly customizable design. It helps engineers make application-specified instruction set extensions according to certain needs and optimize operation and power efficiency for targeted workloads.
  3. The community base of RISC-V is based on collaboration, and it is very broad and comes with different types of companies, individuals, and universities. It helps to innovate new ideas, share the latest information, and create a broader ecosystem.
  4. RISC-V is versatile and used in different projects, such as embedded systems and IoT devices, to increase their operation and data center. Its modular architecture is used for scalability.
  5. The RISC-V features are openly based and also have a public review, making it easy to understand and verify architecture. It increases trust and security.
  6. This architecture is used in academic research and teaching. It helps to find new ideas and new engines in the RISC-V ecosystem.


  1. In some files, RISC-V does not get the same level of marker adoption as made ISAs such as x86 and ARM. It was getting traction, but its uses in consumer devices and mainstream computing were restricted.
  2. As the RISC-V software ecosystem is developing very fast, it still lags behind the creation of ISAs in the form of software availability, compilers, and different tools. Porting existing software to RISC-V can be difficult.
  3. Creating a mature hardware ecosystem will take longer and more time. As there are RISC-V cores and development boards, the ecosystem cannot have the same features as other ISAs.
  4. This structure is not compatible with different implementations and extensions. There are works in progress for different features, but getting complete compatibility is difficult.
  5. RISC-V does not have compatibility with legacy software designed for other ISAs, such as x86. Transitioning to RISC-V can require recompiling.

RISC-V Applications:

  • It is used in IoT devices and embedded systems.
  • It is used for high-performance computing.
  • It is used in automotive systems.
  • It is part of custom accelerators for machine learning.
  • It is also employed in networking institutions.


RISC-V is an open and modular design that comes with the compiling of a thriving community and is used as a compelling alternative in a universe of processor designs. Its design flexibility and implantation of different ranges make it best to use in different applications, such as resource-constrained embedded systems and high-operation commuting conditions. It will take part actively in the fire of commuting with the growth of the RISC-V ecosystem.


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