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⚠️

Write After Write (WAW) Hazard

Understand how writes to the same destination create WAW hazards in pipelines.

Computer Science • Computer Architecture and Pipelining🔥 Intermediate

This module explores Write After Write (WAW) hazards, a data hazard type that occurs when two instructions write to the same destination in close sequence. Learners will distinguish WAW from RAW and WAR hazards, identify WAW scenarios in code and diagrams, and review mitigation strategies such as register renaming, scheduling, and out-of-order execution with practical examples.

⏱️
30 minTypical time
📋
2Prerequisites
🎯
3Learning outcomes

📋Prerequisites

  • Intro to computer architecture
  • Basics of data hazards (RAW, WAR, WAW)

🎯What You'll Learn

  • Explain what a Write After Write (WAW) hazard is and why it can occur in pipelined CPUs
  • Differentiate WAW from RAW and WAR hazards
  • Identify WAW hazards in code sequences and diagrams, and describe mitigation strategies

Core Concepts

  • Pipelining basics
  • Data hazards taxonomy
  • WAW vs RAW vs WAR
  • Visualization & Practice

  • Timeline diagrams
  • Pipelined execution traces
  • Interactive simulations
  • Mitigation Techniques

  • Register renaming
  • Scoreboarding
  • Out-of-order execution
  • Instruction scheduling
  • Assessment & Resources

  • Practice problems
  • Quizzes
  • Further reading
  • Reference slides
  • ⚠️

    Introduction to Write After Write (WAW) Hazard

    A primer on WAW hazard in instruction scheduling, memory ordering, and pipeline design.

    ⏱️8 min

    Overview of Write After Write (WAW) Hazard: A WAW hazard occurs when two instructions write to the same destination in close proximity and the second write reaches the destination before the first write has been committed. As a result, the first instruction's value can be overwritten by the second, producing an incorrect result. WAW hazards are a concern in pipelined or out-of-order processors and are mitigated by techniques such as register renaming, in-order commit, and careful scheduling to preserve the intended write order.

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    Diagram showing a Write-After-Write (WAW) hazard in a write-back path

    Diagram: how WAW occurs in a write-back path

    Quiz
    1/6
    🔍

    WAW Hazard in Practice: Examples

    Scenarios showing WAW hazard with memory writes

    ⏱️10 min

    Real-world Scenario: Two stores to the same variable. In many applications, two parts of the system may update the same shared variable without coordination. This can cause race conditions, inconsistent reads, and hard-to-trace bugs. For example, two modules updating a counter or configuration object simultaneously can overwrite each other's changes. To manage this, design with a single source of truth, apply immutability, and synchronize updates through a central store or message bus. Use patterns like locks, atomic operations, or update queues, and prefer event-driven or state management libraries that serialize changes.

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    Pseudo-code: two consecutive writes
    # Pseudo-code: two consecutive writes
    # Demonstrates two write calls appending to the same stream
    def write_two_consecutive(path, s1, s2):
        with open(path, "w") as f:
            f.write(s1)
            f.write(s2)
    
    # Example usage
    write_two_consecutive("out.txt", "Hello, ", "World!")
    Pipeline writeback path diagram

    Pipeline diagram illustrating the writeback path and its delay.

    Checklist: Avoid WAW in code paths

    Multiple-Choice Quiz
    1/4
    1

    Which statement about WAW is true in the context of this module?

    A
    WAW stands for 'World At War' in this module
    B
    WAW stands for 'World At Work' in this module
    C
    WAW stands for 'Wave After Wave' in this module
    D
    WAW has no defined meaning in this module

    Which statement about WAW is true in the context of this module?

    Your Answer
    Correct Answer
    WAW stands for 'World At War' in this module
    Incorrect
    Explanation

    The module defines WAW as 'World At War' for demonstration purposes.

    Score: 0/4
    2

    Which statement about WAW is true in this course?

    A
    WAW is defined as an acronym used to illustrate case studies in this module
    B
    WAW refers to a programming language in this context
    C
    WAW is unrelated to the case studies
    D
    WAW is a deprecated term with no current use

    Which statement about WAW is true in this course?

    Your Answer
    Correct Answer
    WAW is defined as an acronym used to illustrate case studies in this module
    Incorrect
    Explanation

    The content uses WAW as an acronym to frame scenarios.

    Score: 0/4
    3

    Which option best describes the relationship between WAW and the case studies?

    A
    WAW is the primary subject of each case study
    B
    WAW is a contextual label used to frame scenarios
    C
    WAW is a numerical metric used to score outcomes
    D
    WAW is unrelated to the case studies

    Which option best describes the relationship between WAW and the case studies?

    Your Answer
    Correct Answer
    WAW is a contextual label used to frame scenarios
    Incorrect
    Explanation

    WAW is used as a contextual label in the scenarios.

    Score: 0/4
    4

    True or False: WAW can have multiple meanings in different contexts.

    A
    True, WAW can have multiple meanings in different contexts
    B
    False, WAW always means 'World At War'
    C
    False, WAW is never used outside this module
    D
    True, but only in highly technical contexts

    True or False: WAW can have multiple meanings in different contexts.

    Your Answer
    Correct Answer
    True, WAW can have multiple meanings in different contexts
    Incorrect
    Explanation

    Acronyms often have context-dependent meanings.

    Score: 0/4
    🛡️

    Techniques to Mitigate WAW Hazards

    Strategies to prevent WAW hazards in systems

    ⏱️12 min

    Mitigation techniques involve preventing, detecting, and recovering from security threats. Key approaches include defense in depth, regular patching and configuration management, strong access controls, network segmentation, secure baseline configurations, backups and disaster recovery planning, user education, and prepared incident response. Together, these measures reduce exposure, limit impact, and enable rapid recovery when an incident occurs.

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    Checklist for Implementation

    Code Snippet: Correct write ordering example

    def write_ordered(data): # Output keys in a stable, sorted order for key in sorted(data.keys()): print(f"{key}={data[key]}")

    Quiz
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    ⚙️

    Real-World Implications of WAW Hazards

    Consequences in modern CPUs, compilers, and software

    ⏱️8 min

    Out-of-order execution speeds up processors by letting independent instructions run ahead of those that are waiting. Correctness is preserved because the CPU enforces dependencies and commits results in program order. Speculative or out-of-order results can be rolled back when a misprediction or exception occurs, and architectural state is only updated when instructions retire. Memory operations must respect the memory model, with fences or atomic operations used when needed to order effects visible to the software. In short, performance gains come with careful handling of data and control dependencies to ensure the observable behavior matches the original program.

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    Processor pipeline with WAW hazard

    Illustration of a Write-After-Write hazard in a processor pipeline

    Multiple-Choice Quiz
    1/1
    1

    Which scenario best illustrates a WAW hazard?

    A
    A worker uses a grounded extension cord with a visible fray while standing on a wet concrete surface without footwear protection
    B
    A neatly labeled toolbox stored in a designated cabinet with proper PPE
    C
    A supervisor reviews a safety plan before starting a task with all required permits
    D
    A machine guarded with a fixed barrier and warning signs during maintenance

    Which scenario best illustrates a WAW hazard?

    Your Answer
    Correct Answer
    A worker uses a grounded extension cord with a visible fray while standing on a wet concrete surface without footwear protection
    Incorrect
    Explanation

    WAW hazard refers to hazards arising from working on wet surfaces or with electrical equipment in damp conditions; using a damaged extension cord on a wet surface creates shock and slip risks.

    Score: 0/1