Data Hazards in Computer Architecture

Questions and Answers

What is the primary cause of data hazard in standard MIPS?

Control flow instruction

Load-use data hazard (correct)

Branch prediction failure

Resource contention

Which of the following is an example of a name dependence?

instruction j uses the result of instruction i

instruction i is a control flow instruction that affects instruction j

instructions i and j access the same memory location

instructions i and j use the same register, but there is no flow of data between them (correct)

What is the main purpose of code reordering in pipeline execution?

To increase the number of instructions in the pipeline

To reduce the number of stalls in the pipeline (correct)

To reduce the latency of memory access

To improve the accuracy of branch prediction

What is the term for a situation where an instruction is delayed in the pipeline due to a dependence on a previous instruction?

Stall Signup and view all the answers

Which of the following hazard types is most difficult to detect?

Data hazard Signup and view all the answers

What is the primary advantage of out-of-order execution in pipelines?

Increased instruction-level parallelism Signup and view all the answers

Which of the following techniques is used to avoid data hazards in pipelines?

Code reordering Signup and view all the answers

What is the term for a sequence of instructions that have dependencies between them?

Dependency chain Signup and view all the answers

Which type of data hazard occurs when an instruction attempts to read a source before a previous instruction writes to it?

Read after Write Signup and view all the answers

In which situation do Write after Write (WAW) hazards typically arise?

When two instructions write to the same operand in reverse order Signup and view all the answers

What is a primary characteristic of Write after Read (WAR) hazards?

They arise when a subsequent instruction writes before a prior instruction reads Signup and view all the answers

What condition must be met for an instruction to be considered control dependent on a branch?

It cannot be moved before the branch instruction Signup and view all the answers

Which of the following statements about RAR (Read after Read) hazards is accurate?

RAR hazards are not considered a hazard at all Signup and view all the answers

How do control dependences restrict instruction movement in a pipeline?

They limit instructions that depend on branches from moving before them Signup and view all the answers

Which pipeline stage is most susceptible to data hazards involving Read after Write operations?

Decode stage Signup and view all the answers

What happens when an instruction is not control dependent on a branch?

It can be moved before the branch safely Signup and view all the answers

What is the main effect of anti-dependence in instruction execution?

It prevents an instruction from executing immediately after a preceding one. Signup and view all the answers

Which technique is commonly used to resolve hazards related to output dependencies?

Register renaming. Signup and view all the answers

What is a data hazard in the context of instruction execution?

It prevents the next instruction in the sequence from executing during its designated clock cycle. Signup and view all the answers

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)'?

L.D F0,0(R1) Signup and view all the answers

What is the purpose of using dynamic scheduling in a pipelined processor?

To improve instruction throughput by allowing out-of-order execution. Signup and view all the answers

What can be done to avoid hazards in instruction pipelines?

Employing techniques such as register renaming. Signup and view all the answers

Which of the following instructions could potentially lead to an output dependence when executed consecutively?

S.D F4,0(R1) Signup and view all the answers

Which aspect of instruction execution is affected by the presence of name dependencies?

The ordering of instruction execution. Signup and view all the answers

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.