out-of-order execution

Out-of-Order Execution

Concept

Out-of-order execution is a processor execution strategy in which instructions are dispatched and completed in an order different from the program order. The evidence presents it as a common feature of pipelined RISC processors and high-performance CPU cores, and discusses it primarily from a verification standpoint: it enlarges the state space, creates corner cases that demand constrained-random and formal techniques, and is one of the complex performance enhancements that companies such as TVS target with specialized CPU verification tooling.

First seen 5/26/2026

Last seen 7/5/2026

Evidence 16 chunks

Wiki v3

WIKI

Overview

Out-of-order execution is described in the provided evidence as a processor execution principle or execution strategy used in pipelined and high-performance cores. One cited case study states that the Verified Architecture MicroProcessor (VAMP) is "a pipelined reduced instruction set (RISC) processor based on the out-of-order execution principle." [C1] A second source groups it among "complex performance enhancements" of CPU cores alongside pipelines, multiple instruction issue, and branch prediction. [C2] A third source lists "in-order or out-of-order execution strategies" among the features that make microprocessor verification challenging. [C3]

Use in pipelined RISC processors

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NEIGHBORHOOD

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RELATIONSHIPS

15 connections

microarchitectural state space exploration ← mentions 85% 2e

Microarchitectural state space exploration is needed to find bugs related to out-of-order execution.

Spectre mentions → 90% 1e

Out-of-order execution exposes potential security vulnerabilities such as Spectre.

Meltdown mentions → 90% 1e

Out-of-order execution exposes potential security vulnerabilities such as Meltdown.

RISC-V ← mentions 88% 1e

RISC-V implementations using out-of-order execution increase complexity and security risks.

Microprocessor Verification ← mentions 95% 1e

Out-of-order execution is listed as one of the challenges in microprocessor verification.

Transient Execution Attack ← uses 100% 1e

Transient execution attacks exploit CPU out-of-order execution.

openC910 RTL mentions → 85% 1e

The openC910 features out-of-order execution which increases performance but complicates simulation.

XiangShan ← implements 100% 1e

XiangShan is an out-of-order superscalar CPU.

data-flow non-determinism ← mentions 85% 1e

Data-flow non-determinism is related to out-of-order execution and load reordering.

BOOM ← implements 100% 1e

BOOM is an out-of-order superscalar CPU.

Toooba ← implements 100% 1e

Toooba is an out-of-order superscalar CPU.

NaxRiscv ← implements 100% 1e

NaxRiscv is an out-of-order superscalar CPU.

SNAP ← uses 90% 1e

SNAP is designed and implemented based on the out-of-order BOOM core.

VAMP (Verified Architecture Microprocessor) ← implements 100% 1e

VAMP is based on the out-of-order execution principle.

RISC-V BOOM ← implements 100% 1e

RISC-V BOOM is an out-of-order processor core with modern performance optimizations.

LINKED ENTITIES

1 links

CPU Verification MENTIONS Extracted graph relationship

CITATIONS

9 sources

9 citations — click to expand

[1] VAMP is a pipelined RISC processor based on the out-of-order execution principle, with VAMPasm as its instruction set at the assembly level of the Verisoft hardware layer. VAMP case study (Verisoft)

[2] CPU cores implement complex performance enhancements including pipelines, multiple instruction issue, out-of-order execution, and branch prediction, which can break functional correctness and require specialist verification techniques such as constrained-random instruction-stream execution. TVS extends CPU Verification Capability - Design And Reuse

[3] In-order or out-of-order execution strategies are listed among the features that make microprocessor verification challenging. EETimes constrained-random verification article

[4] VAMPasm contains 56 instructions grouped into memory data transfer, constant data transfer, register data transfer, arithmetic and logical operations, test operations, shift operations, control operations, and interrupt handling. VAMP case study (Verisoft)

[5] A formal test-generation study reports that switching from in-order to out-of-order execution substantially increases reachable global states, transitions, and required test programs across both scalar and 2-way superscalar configurations (e.g., 13,232 test programs for in-order scalar vs. 84,401 for out-of-order scalar, and up to 116,260 for out-of-order 2-way superscalar). Generating Test Programs to Cover Pipeline Interactions (DAC09)

[6] The DAC09 test-generation framework builds executable assembly-level programs by traversing a global FSM composed of operation-level and hardware-level communicating EFSMs, with the ISA in MADL and the micro-architecture in XML. Generating Test Programs to Cover Pipeline Interactions (DAC09)

[7] In the VAMP case study, tests are generated from a formal model of the instruction set, testing conformance of the gate-level implementation against the assembly-level model. VAMP case study (Verisoft)

[8] A RISC-V microarchitecture verification source identifies out-of-order execution as a common high-performance technique, alongside speculative execution, that increases microarchitectural complexity and can expose security vulnerabilities such as Spectre and Meltdown. RISC-V microarchitecture verification source

[9] The same RISC-V verification source notes that harder verification challenges lie in the microarchitecture and pipeline rather than instruction-level correctness, and that simulation-based verification alone can be inadequate, motivating formal verification. RISC-V microarchitecture verification source