Don’t Just Choose an ISA, Shape It: The Hardware Engineer’s Guide to RISC-V Customization

For decades, hardware engineers have lived in a world of take-it-or-leave-it instruction sets. Working with x86 or Arm was like being a chef given a fixed, pre-packaged spice kit. You could create amazing things, but you could never add a new, unique flavor of your own. The fundamental recipe was locked away.

Enter RISC-V (pronounced “risk-five”). It’s not just another processor architecture; it’s a paradigm shift. At its heart, RISC-V is an open-standard Instruction Set Architecture (ISA). But its true superpower, the feature that has engineers buzzing with excitement, is something far more profound: the ability to create custom ISA extensions.

This isn’t just about choosing a core. It’s about designing the core itself to fit your exact problem. For hardware engineers, this changes everything.

The “Why”: Breaking Free from the One-Size-Fits-All Trap

Imagine you’re designing a chip for a very specific task—say, processing sensor data for a ultra-low-power IoT device or accelerating a proprietary neural network model. With a traditional ISA, your processor comes with a generic set of instructions. It might have features you don’t need (wasting power and area) and lack the specific, powerful operations that would make your design brilliant.

RISC-V flips this model. It provides a clean, minimalist base ISA—the essential instructions every processor needs. Then, it hands you the tools and the permission to build on top of it. This allows you to:

• Achieve Extreme Efficiency: Add instructions that do exactly what your application needs in a single cycle, replacing dozens of standard instructions.
• Gain a Performance Edge: Hardware tailored to your algorithm will always outperform a general-purpose core struggling to run the same task.
• Protect Your IP: Your secret sauce can be baked directly into the processor’s instruction set, making it harder to reverse-engineer than software running on a standard core.

The “How”: Your Toolkit for Customization

So, how do you actually create a custom extension? RISC-V’s modularity is your guide. The ISA is structured into mandatory base integer instructions (RV32I, RV64I) and a variety of standard extensions (like M for multiply, F for single-precision float, etc.). Your custom work lives in its own namespace.

Here’s the practical workflow for an engineer:

1. Profile Your Application: Before you write a single line of new ISA, you need to be a software detective. Use tools to analyze your code. Where are the bottlenecks? Is there a critical loop that consumes 80% of the execution time? This loop is your prime candidate for hardware acceleration.
2. Identify Custom Instruction Opportunities: Look for sequences of operations that are executed frequently. Can you combine them into a single, custom instruction? For example, if your algorithm constantly performs a “multiply-accumulate-shift” sequence, you can define a custom instruction CUSTOM1 that does it all at once.
3. Define the Extension: You’ll formally define your new instructions—their mnemonics, operands, and behavior. This is like writing a specification for a new mini-processor within your processor.
4. Modify the Hardware: This is where the rubber meets the road. You need to extend your RISC-V core’s pipeline:
   • Add control logic to decode your new custom opcodes.
   • Design the execution unit that will carry out the new operation.
   • Update the compiler so it recognizes your new instructions and can generate code that uses them. This is often done through intrinsic functions or a custom LLVM backend.

It’s a significant effort, but one that pays massive dividends for performance-critical applications.

The “What to Consider”: Navigating the Freedom

This newfound power comes with responsibility and engineering trade-offs.

• The Compiler Toolchain: Your custom instructions are useless if your software tools can’t use them. You must budget for the effort of extending the compiler (like GCC or LLVM).
• Verification Overhead: Every new instruction is new logic that must be rigorously verified. This adds complexity to your validation schedule.
• Portability: Code using your custom extensions will, by definition, not run on a standard RISC-V core. The trade-off is proprietary performance for loss of portability.
• The Ecosystem: One of RISC-V’s greatest strengths is its collaborative ecosystem. While custom extensions are your private IP, ensuring they don’t conflict with future standard extensions is good practice. For a deeper look at the physical implementation of these custom cores, our guide on Understanding Semiconductor Packaging is a great resource.

The Bottom Line for Engineers

RISC-V is more than a royalty-free ISA. It’s an invitation to innovate at the most fundamental level of processor design. It asks hardware engineers not just “What will you build?” but “What will you build with?”

The rise of RISC-V signals a move towards a new era of domain-specific architecture (DSA), where the hardware is perfectly sculpted to its workload. For engineers willing to dive in, it offers an unparalleled opportunity to create truly groundbreaking, efficient, and unique processors.

The question is no longer “Which core will we use?” but “What will we invent?”To see real-world examples of companies leveraging custom RISC-V extensions, you can read about Apple’s early exploration in this external article on The Verge.