RISC-V Architecture Explorer
Understanding the open standard instruction set architecture
What is RISC-V?
RISC-V (pronounced "risk-five") is an open standard instruction set architecture (ISA) based on established reduced instruction set computer (RISC) principles. Unlike most other ISA designs, RISC-V is provided under open-source licenses that do not require fees to use.
RISC-V is defined by the RISC-V Foundation, a non-profit corporation founded to build an open, collaborative community of software and hardware innovators. The architecture is designed to be simple, modular, and extensible, making it suitable for a wide range of applications from embedded systems to supercomputers.
Open Standard
RISC-V is an open standard ISA that anyone can implement without licensing fees. This openness encourages innovation and reduces barriers to entry for hardware development.
Modular Design
The RISC-V ISA is organized as a small base integer ISA with optional standard extensions. This modularity allows implementations to customize the instruction set for specific applications.
Modern Architecture
RISC-V incorporates lessons learned from decades of computer architecture research. It avoids the legacy包袱 of older ISAs while maintaining simplicity and efficiency.
RISC-V vs Other Architectures
| Feature | RISC-V | x86 | ARM |
|---|---|---|---|
| Licensing | Open source, royalty-free | Proprietary | Proprietary (various licensing models) |
| Instruction Set | Modular, extensible | Complex, legacy-heavy | Multiple versions, somewhat modular |
| Simplicity | Designed for simplicity | Very complex | Moderately complex |
| Customization | Highly customizable | Not customizable | Limited customization |
| Community | Growing open community | Controlled by Intel/AMD | Controlled by ARM Ltd. |
RISC-V History
2010 - Project Begins
The RISC-V project started at the University of California, Berkeley, led by Krste Asanović, David Patterson, and Andrew Waterman. The goal was to create an open ISA for research and education.
2011 - First Implementation
The first RISC-V chip was fabricated, demonstrating the practicality of the architecture. This early implementation already showed the benefits of the clean-slate design.
2015 - RISC-V Foundation
The RISC-V Foundation was established to promote and maintain the RISC-V ISA. Major technology companies joined as founding members.
2017 - Commercial Adoption
First commercial RISC-V chips began appearing in embedded systems and specialized applications. Western Digital announced plans to transition to RISC-V cores.
2019 - Linux Support
Mainline Linux kernel added official support for RISC-V architecture, enabling a full software ecosystem to develop around the ISA.
2020 - High-Performance CPUs
Companies began announcing high-performance RISC-V CPU designs targeting data centers and high-performance computing applications.
2023 - Widespread Adoption
RISC-V has seen adoption across various segments from microcontrollers to application processors, with a growing ecosystem of software and development tools.
RISC-V Architecture
The RISC-V architecture follows RISC (Reduced Instruction Set Computer) principles with a focus on simplicity, regularity, and performance. The architecture is designed to support a wide range of implementations while maintaining binary compatibility.
RISC-V Processor Diagram
Click on any component to learn more
Architecture Components
CPU Core
The central processing unit that executes RISC-V instructions
Memory System
Stores instructions and data for processing
Cache Hierarchy
Fast memory that reduces access to main memory
I/O System
Handles input and output operations
Pipeline
Instruction execution pipeline for performance
Register File
RISC-V has 32 integer registers (x0-x31) with x0 hardwired to zero. The architecture also supports floating-point registers when the F extension is implemented.
Memory Model
RISC-V uses a load-store architecture with a flat memory address space. Memory accesses must be aligned to their natural boundaries for better performance.
Exceptions & Interrupts
RISC-V has a precise exception model and supports multiple privilege levels (User, Supervisor, Machine) for system security and virtualization.
RISC-V Instruction Set Architecture
The RISC-V ISA is organized as a small base integer ISA with optional standard extensions. This modular approach allows implementations to include only the instructions needed for their specific application.
ISA Base and Extensions
Base Integer ISA
The RISC-V base integer ISA is minimal yet complete. It includes instructions for basic arithmetic, logical operations, control flow, and memory access. There are two primary base variants:
- RV32I: 32-bit base integer instruction set
- RV64I: 64-bit base integer instruction set
- RV128I: 128-bit base integer instruction set (defined but not yet widely implemented)
add x5, x6, x7 # x5 = x6 + x7
lw x10, 0(x11) # x10 = memory[x11 + 0]
beq x5, x0, label # if x5 == 0, jump to label
Base Instruction Formats
| Format | Usage | Example Instructions |
|---|---|---|
| R-type | Register-register operations | add, sub, and, or, xor |
| I-type | Immediate operations, loads | addi, lw, jalr |
| S-type | Store operations | sw, sb, sh |
| B-type | Conditional branches | beq, bne, blt |
| U-type | Upper immediate operations | lui, auipc |
| J-type | Unconditional jumps | jal |
Integer Extensions
RISC-V defines several standard extensions to the base integer ISA that add specialized instructions for improved performance in specific domains.
| Extension | Description | Key Instructions |
|---|---|---|
| M Extension | Integer multiplication and division | mul, div, rem |
| A Extension | Atomic operations | lr.w, sc.w, amoswap |
| C Extension | Compressed instructions (16-bit) | c.addi, c.lw, c.j |
| B Extension | Bit manipulation operations | clz, ctz, bswap |
mul x5, x6, x7 # x5 = x6 * x7
div x8, x9, x10 # x8 = x9 / x10
# Example with C extension (compressed)
c.addi x1, 5 # x1 = x1 + 5 (16-bit instruction)
c.lw x2, 4(x3) # x2 = memory[x3 + 4] (16-bit)
Floating-Point Extensions
RISC-V provides separate extensions for single-precision and double-precision floating-point operations, following the IEEE 754 standard.
| Extension | Description | Precision | Key Instructions |
|---|---|---|---|
| F Extension | Single-precision floating-point | 32-bit | fadd.s, fmul.s, fdiv.s |
| D Extension | Double-precision floating-point | 64-bit | fadd.d, fmul.d, fdiv.d |
| Q Extension | Quad-precision floating-point | 128-bit | fadd.q, fmul.q, fdiv.q |
fadd.s ft0, ft1, ft2 # ft0 = ft1 + ft2 (single)
fmul.d ft3, ft4, ft5 # ft3 = ft4 * ft5 (double)
fcvt.s.d ft0, ft1 # Convert double to single
Vector Extensions
The RISC-V Vector extension (V) provides support for data-level parallelism, making it suitable for scientific computing, machine learning, and multimedia applications.
| Feature | Description | Benefits |
|---|---|---|
| Vector Registers | Configurable vector register length | Adapts to available hardware |
| Masked Operations | Conditional execution of vector elements | Efficient conditional processing |
| Strided & Indexed | Flexible memory access patterns | Optimized for various data layouts |
| Mixed-Precision | Support for different element widths | Efficient for various data types |
vadd.vv v1, v2, v3 # Vector-vector addition
vle32.v v1, (a0) # Load vector of 32-bit elements
vse64.v v2, (a1) # Store vector of 64-bit elements
RISC-V Simulator
Try out RISC-V assembly programming with this interactive simulator. Write code, run it, and see how the registers and memory change.
RISC-V Assembly Simulator
Assembly Code
Simulation Output
x0: 0 (zero)
x1: 0
x2: 0
x3: 0
x4: 0
x5: 0
x6: 0
x7: 0
...
0x0000: 00 00 00 00
0x0004: 00 00 00 00
0x0008: 00 00 00 00
0x000C: 00 00 00 00
...
RISC-V Learning Tools
Assembly Explorer
Interactive tool to learn RISC-V assembly programming with visual execution tracing and step-by-step debugging.
CPU Designer
Design your own RISC-V CPU with customizable extensions and pipeline stages. See how design choices affect performance.
ISA Customizer
Create custom RISC-V ISA extensions for specialized applications and test them in simulation.
Performance Analyzer
Analyze RISC-V code performance with detailed metrics on instruction mix, pipeline efficiency, and cache behavior.
Learning Path
Structured learning path with tutorials, exercises, and projects to master RISC-V architecture and programming.
SoC Builder
Build complete RISC-V System-on-Chip designs with peripherals, memory controllers, and custom accelerators.
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