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custom instruction verification

Technique WIKI v1 · 5/27/2026

Custom instruction verification is the verification work needed when processor teams add application-specific instructions, such as in RISC-V designs. The evidence describes it as an expansion of processor verification scope: teams must re-verify affected functionality and check that new instructions do not negatively affect pipeline control, ALU interactions, cache behavior, load-store paths, SoC behavior, workloads, power, or performance.

Overview

Custom instruction verification is the verification activity required after adding application-specific instructions to a processor design. In the cited RISC-V context, the ability to modify a processor for specific applications is described as appealing, but each added feature increases verification effort and complexity. Custom instructions expand verification scope because teams must re-verify impacted functionality and ensure that the additions do not negatively affect the rest of the design, especially when they touch pipeline control, ALU conflicts, cache behavior, or load-store paths.

Role in processor verification

Custom instruction verification is a specialized case of processor verification. The cited source describes processor verification as a hybrid activity that can include formal verification, simulation, UVM-style test platforms, test software, reference-model comparison, hardware-assisted validation, and operational software testing. For custom instructions, the same source emphasizes that understanding the microarchitecture and the effects of changes on the SoC and workloads is essential.

Verification concerns

Custom instructions can interact with multiple parts of the processor and SoC. The cited concerns include:

  • Pipeline control: additions may affect control decisions and dynamic events inside the pipeline.
  • ALU conflicts: instruction additions may introduce conflicts in arithmetic or execution resources.
  • Cache behavior: changes can affect cache-related behavior.
  • Load-store paths: changes can affect memory access paths.
  • SoC and workload effects: custom-instruction changes should be evaluated in terms of their effect on the wider SoC and on real workloads.
  • Power and performance tradeoffs: hardware-assisted validation can help identify unintended microarchitectural tradeoffs.

Methods and flow

The cited source supports a hybrid verification strategy rather than reliance on a single method:

  1. Formal verification: Formal methods can exhaustively explore input combinations against ISA-specified behavior, often expressed as SystemVerilog assertions.
  2. Simulation: Simulation remains necessary to validate modules of a large processor, check SoC integration, and run software on the device under test.
  3. Reference-model comparison: Teams commonly compare implemented behavior against a reference model. If the reference and RTL differ, engineers analyze whether the RTL behavior is acceptable, especially where specifications do not define every scenario.
  4. Coverage analysis: Simulation can produce coverage reports showing which design portions have been exercised, but coverage alone is described as insufficient for processors.
  5. Hardware-assisted validation: Virtual prototypes, simulation acceleration, and hardware prototyping are described as critical parts of the overall verification flow.
  6. Operational software testing: Running real software workloads for extended periods is recommended as a practical heuristic when exhaustive verification is impossible.

Completion criteria

The cited source states that verification is never truly complete. A practical criterion is that verification is sufficient when the residual risk is manageable. For custom instructions, this implies that teams should not only check the new instruction behavior, but also verify affected functionality and assess whether the addition introduces unintended effects elsewhere in the design.

LINKED ENTITIES

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Processor Verification USES

CITATIONS

7 sources
7 citations
[1] Custom instructions increase verification scope and require re-verification of impacted functionality. RISC-V Microarchitecture Verification Approaches
[2] Custom instruction additions must be checked for negative effects on the rest of the design, particularly pipeline control, ALU conflicts, cache behavior, and load-store paths. RISC-V Microarchitecture Verification Approaches
[3] Processor verification can use a hybrid strategy including formal verification, simulation, UVM test platforms, test software, and reference-model comparison. RISC-V Microarchitecture Verification Approaches
[4] Coverage alone is insufficient for processor verification because instruction sequences and dynamic pipeline events must also be considered. RISC-V Microarchitecture Verification Approaches
[5] For RISC-V custom instructions, understanding the microarchitecture and how changes affect the SoC and workloads is essential. RISC-V Microarchitecture Verification Approaches
[6] Virtual prototypes, simulation acceleration, and hardware prototyping are described as critical hardware-assisted validation techniques in the processor verification flow. RISC-V Microarchitecture Verification Approaches
[7] Verification is never truly complete; a practical view is that it is sufficient when residual risk is manageable. RISC-V Microarchitecture Verification Approaches