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custom instruction verification

Concept WIKI v1 · 5/27/2026

Custom instruction verification is the verification effort required when adding application-specific instructions to a processor, especially in RISC-V designs. Evidence indicates that custom instructions increase verification scope because teams must re-verify impacted functionality and check for unintended effects on pipeline control, ALU conflicts, cache behavior, load-store paths, SoC integration, workloads, security, power, and performance.

Overview

Custom instruction verification is the process of validating processor behavior after adding custom instructions or other application-specific features. In RISC-V contexts, custom instructions are attractive because the architecture can be modified for specific applications, but each added feature increases verification effort and complexity. Teams must re-verify affected behavior and ensure the addition does not negatively affect the rest of the design, especially when the change touches pipeline control, ALU conflicts, cache behavior, or load-store paths.

Verification strategy

A hybrid verification strategy is needed for processor designs. Formal verification can exhaustively explore input combinations against ISA-specified behavior, commonly expressed as SystemVerilog assertions. Simulation remains necessary to validate all modules of a large processor, check SoC integration, and run software on the device under test.

For custom instructions, this means verification should not be limited to the instruction implementation alone. The affected microarchitecture and system context must also be examined, because instruction additions can interact with pipeline behavior, memory paths, caches, and workload execution.

Reference-model comparison

A common processor verification approach is to compare implemented behavior against a reference model. However, specifications may not define every scenario precisely. When the reference model and RTL differ, engineers must analyze whether the RTL behavior is acceptable. This is relevant to custom instruction verification because custom behavior can introduce new cases that require careful interpretation and review.

Coverage and completion criteria

Verification is never truly complete. A practical completion criterion is that residual risk is manageable. Simulation-based verification can provide coverage reports showing exercised design areas and classes of bugs found, but coverage alone is insufficient for processors.

Processor coverage must account for more than instruction-level coverage or decoder coupling. It should include instruction sequences and dynamic pipeline events. For RISC-V custom instructions, understanding the microarchitecture and how changes affect the SoC and workloads is essential.

Hardware-assisted validation

Hardware-assisted validation techniques such as virtual prototypes, simulation acceleration, and hardware prototyping are important parts of the verification flow. These techniques help check that microarchitectural decisions introduced or affected by custom instructions do not create unintended power or performance tradeoffs.

Security and safety considerations

Security-sensitive designs require stricter verification. The evidence notes that products requiring specific certification levels may need fault injection and diagnostic coverage analysis, such as approaches defined by ISO 26262 for functional safety. If faults are injected into critical functions, the design must include mechanisms to detect and handle them.

The open nature of RISC-V also increases the need for strong security verification, because transparency enables community review while also giving adversaries access to the same information.

Operational software testing

When exhaustive verification is impossible, heuristic methods become important. A practical recommendation is to use the chip from the user's perspective by running real software workloads for extended periods. Booting and running real software can expose issues not found by other checks or formal proofs, including asynchronous timing anomalies.

CITATIONS

8 sources
8 citations
[1] Custom instructions increase verification scope and require re-verification of impacted functionality, especially around pipeline control, ALU conflicts, cache behavior, and load-store paths. RISC-V Microarchitecture Verification Approaches
[2] Formal verification is useful for exhaustively exploring input combinations against ISA-specified behavior, while simulation is needed for full processor modules, SoC integration, and software execution on the device under test. RISC-V Microarchitecture Verification Approaches
[3] Processor teams commonly compare implemented behavior against a reference model, and differences between RTL and the reference require engineering analysis when specifications are not precise. RISC-V Microarchitecture Verification Approaches
[4] Verification is never truly complete; a practical standard is that residual risk is manageable, and coverage alone is insufficient for processor verification. RISC-V Microarchitecture Verification Approaches
[5] Processor verification must consider instruction sequences and dynamic pipeline events, and RISC-V custom instruction work requires understanding microarchitecture, SoC effects, and workloads. RISC-V Microarchitecture Verification Approaches
[6] Virtual prototypes, simulation acceleration, and hardware prototyping are critical validation techniques that can help detect unintended power or performance tradeoffs. RISC-V Microarchitecture Verification Approaches
[7] Security- and safety-sensitive products may require stricter verification, including fault injection and diagnostic coverage analysis such as ISO 26262 functional-safety approaches. RISC-V Microarchitecture Verification Approaches
[8] Running real software workloads for extended periods is a useful heuristic when exhaustive verification is impossible, and real software can expose issues not found by other EDA checks or formal proofs. RISC-V Microarchitecture Verification Approaches