RISC-V basics: The truth about custom extensions - EDN - Voice of the Engineer

Abstract

This article clarifies how the open and modular RISC-V architecture fundamentally facilitates the creation of custom instruction set extensions (ASIs). It details the mechanisms designers use, primarily reserved opcode space, to implement proprietary application-specific instructions without violating the standard ISA structure. This extensibility is crucial for hardware differentiation and optimizing specialized workloads through highly efficient domain-specific accelerators.

Report

Key Highlights

• Native Extensibility: RISC-V is uniquely designed to accommodate custom instruction extensions, providing dedicated, reserved opcode space for proprietary use.
• Differentiation Mechanism: Custom extensions are the primary way hardware vendors differentiate their core implementations and optimize performance for specific application domains (e.g., AI/ML or cryptography).
• Preserving Compatibility: Extensions are designed not to conflict with the standard Base Integer Instruction Set (e.g., RV32I or RV64I) or standard ratified extensions, ensuring base software compatibility.

Technical Details

• Reserved Opcode Fields: The RISC-V ISA specification permanently allocates specific instruction slots (opcodes) for custom instructions, often referred to as 'custom-0', 'custom-1', etc., within the 32-bit instruction word format.
• Application-Specific Instructions (ASIs): Custom extensions allow architects to define new micro-architectural operations, potentially accessing specialized functional units, custom registers, or memory interfaces outside the standard specification.
• Toolchain Integration: Implementing custom extensions requires modifying or extending standard RISC-V toolchains (like GCC and LLVM) to recognize and correctly compile code utilizing the new instructions.

Implications

• Enabling Domain-Specific Architectures (DSAs): The ease of customization makes RISC-V the architecture of choice for specialized accelerators, dramatically increasing performance and power efficiency for dedicated tasks.
• Competitive Advantage: Chip developers gain a significant competitive edge by embedding performance-critical proprietary logic directly into the processor core, rather than relying solely on external co-processors.
• Fostering Innovation: RISC-V lowers the barrier to entry for new architectural features, accelerating innovation in emerging fields that require tightly integrated, high-performance computing elements.