Questions and Answers
What is the primary cause of data hazard in standard MIPS?
Which of the following is an example of a name dependence?
What is the main purpose of code reordering in pipeline execution?
What is the term for a situation where an instruction is delayed in the pipeline due to a dependence on a previous instruction?
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Which of the following hazard types is most difficult to detect?
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What is the primary advantage of out-of-order execution in pipelines?
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Which of the following techniques is used to avoid data hazards in pipelines?
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What is the term for a sequence of instructions that have dependencies between them?
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Which type of data hazard occurs when an instruction attempts to read a source before a previous instruction writes to it?
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In which situation do Write after Write (WAW) hazards typically arise?
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What is a primary characteristic of Write after Read (WAR) hazards?
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What condition must be met for an instruction to be considered control dependent on a branch?
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Which of the following statements about RAR (Read after Read) hazards is accurate?
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How do control dependences restrict instruction movement in a pipeline?
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Which pipeline stage is most susceptible to data hazards involving Read after Write operations?
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What happens when an instruction is not control dependent on a branch?
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What is the main effect of anti-dependence in instruction execution?
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Which technique is commonly used to resolve hazards related to output dependencies?
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What is a data hazard in the context of instruction execution?
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In a loop structured as 'Loop: L.D F0,0(R1)', which instruction causes an anti-dependence when followed by 'S.D F4,0(R1)'?
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What is the purpose of using dynamic scheduling in a pipelined processor?
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What can be done to avoid hazards in instruction pipelines?
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Which of the following instructions could potentially lead to an output dependence when executed consecutively?
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Which aspect of instruction execution is affected by the presence of name dependencies?
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Study Notes
Data Hazards
- Data hazards arise from name or data dependence between closely related instructions.
- Types of data hazards include:
- RAW (Read After Write): Occurs when an instruction attempts to read a value before it has been written by another instruction. It represents a true data dependence.
- WAW (Write After Write): Occurs when one instruction tries to write to an operand before another instruction writes to the same operand, leading to out-of-order writes (output dependence).
- WAR (Write After Read): Occurs when an instruction writes to a location before another instruction reads from it, resulting in reading a new value prematurely. Mostly found in anti-dependence scenarios, WAR hazards are rare in static issue pipelines.
Control Dependence
- Control dependence dictates the order of instructions in relation to branch instructions, ensuring correct execution according to program order.
- Two main rules of control dependence:
- An instruction dependent on a branch cannot be moved before the branch; its execution must be governed by the branch.
- An instruction that is not dependent on the branch cannot be executed after it to ensure its execution remains controlled.
Name Dependences
- Arises when two instructions reference the same register or memory without actual data flow:
- Anti-dependence: An instruction writes to a memory location that another instruction reads from.
- Output dependence: Two instructions write to the same location, requiring ordering preservation.
- Not considered a true dependence.
- Solutions include Register Renaming, which allows for changing register references to avoid conflicts.
Dependences in Programming
- Data dependence in programming loops can be illustrated with examples:
- Example loop highlights the impact of data dependence on instruction execution.
- Name dependencies in loops can also lead to inefficiencies if not managed properly. Register renaming is a potential solution to manage dependencies effectively.
Pipeline Hazards
- Hazards in pipelined systems can be categorized into three types:
- Structural Hazards: Occur when hardware resources are insufficient to support all active instructions.
- Data Hazards: Arise from dependencies on data between instructions, preventing simultaneous execution.
- Control Hazards: Related to branch instructions and correct order of execution.
Solutions to Hazards
- Common strategies to handle hazards include:
- Adding hardware resources for forwarding to manage data hazards.
- Introducing stalls (NOPs) into the instruction stream to resolve conflicts.
- Implementing code scheduling or reordering to minimize hazard occurrences.
General Overview of Dependences
- Instruction dependencies manifest in various forms:
- Data Dependences: Direct dependencies requiring sequential execution.
- Name Dependences: Occur without data flow but sharing register or memory references.
- Control Dependences: Govern execution order relative to branching.
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Description
This quiz covers the concept of data hazards, including RAW (Read after Write) and WAW (Write after Write) types, which occur due to name or data dependence between instructions.