Instruction Pipelining: Hazards and Solutions Quiz Quiz

Explore the fundamentals of instruction pipelining by identifying different types of hazards and the techniques used to resolve them. This quiz helps learners understand data, control, and structural hazards in pipeline processing, along with common solutions like forwarding, stalling, and branch prediction.

1. Identifying Structural Hazards

   Which of the following best describes a structural hazard in instruction pipelining?

   1. Instructions depend on the results of previous instructions.
   2. Incorrect instruction fetching due to a branch.
   3. Errors caused by a missing control signal.
   4. Multiple instructions compete for the same hardware resource at the same time.

   Explanation: A structural hazard occurs when two or more instructions in the pipeline need the same hardware resource simultaneously, causing a conflict. The other options describe different types of hazards: data hazards are about instruction dependencies, and control hazards relate to branches. A missing control signal can cause an error, but it's not specifically a structural hazard.

2. Data Hazard Example

   If instruction B needs the result generated by instruction A before it can proceed, what type of hazard is present?

   1. Branch hazard
   2. Structural hazard
   3. Execution hazard
   4. Data hazard

   Explanation: A data hazard arises when instructions have data dependencies, such as when one needs the output of another. Branch hazard is not a standard term; the correct terminology is control hazard. Structural hazard involves hardware resource conflicts, not data dependencies. Execution hazard is not typically used in this context.

3. Purpose of Pipeline Stalling

   What is the primary effect of introducing a stall (or bubble) in a pipelined processor?

   1. It increases the clock speed of the pipeline.
   2. It eliminates the need for instruction decoding.
   3. It delays instruction execution to resolve hazards.
   4. It merges two instructions into one stage.

   Explanation: Stalling intentionally delays instruction execution, helping to resolve hazards like data or control hazards. Increasing the clock speed is unrelated to stalling. Merging instructions or skipping decoding are not effects of stalling; they are concepts unrelated to the prevention of pipeline hazards.

4. Forwarding Solution

   How does forwarding help in managing data hazards in instruction pipelines?

   1. It halts the pipeline until hazards are cleared.
   2. It disables unnecessary hardware during data transfers.
   3. It changes the program order of instructions.
   4. It sends the result directly from one pipeline stage to a previous stage as needed.

   Explanation: Forwarding (or bypassing) redirects data from later stages to earlier stages, allowing dependent instructions to proceed without stalling. Halting the pipeline is stalling, not forwarding. Disabling hardware is unrelated, and changing program order is called instruction reordering, not forwarding.

5. Classic Control Hazard Scenario

   When a branch instruction is encountered and its result is unknown, what pipeline hazard occurs?

   1. Memory hazard
   2. Control hazard
   3. Power hazard
   4. Write-back hazard

   Explanation: A control hazard happens when the direction of branch instructions is unclear, potentially leading to incorrect instruction fetches. Write-back hazard and memory hazard are not standard terms for this scenario, and power hazard relates to electrical issues, not pipeline control.

6. Branch Prediction Solution

   Which technique helps reduce control hazards during instruction pipelining by making educated guesses about branch outcomes?

   1. Frequency scaling
   2. Register renaming
   3. Branch prediction
   4. Data transfer

   Explanation: Branch prediction tries to guess the outcome of a branch instruction to minimize stalls. Data transfer doesn't address hazards, and register renaming typically resolves data hazards, not control hazards. Frequency scaling involves changing clock speed, not hazard resolution.

7. Read After Write Hazard Detection

   If instruction C tries to read a register immediately after instruction D writes to it, which hazard type is this?

   1. Output hazard
   2. Write after read (WAR) hazard
   3. Fetch hazard
   4. Read after write (RAW) hazard

   Explanation: A RAW hazard occurs when an instruction depends on a value written by a previous instruction that has not yet completed. WAR hazard is a less common type where writing occurs before a read is done. Output and fetch hazards are not standard terms in this context.

8. Role of Pipeline Interlocks

   What is the function of a pipeline interlock in instruction pipelining?

   1. It automatically pauses the pipeline when a hazard is detected.
   2. It prevents overheating in the processor.
   3. It boosts memory access speed by duplicating registers.
   4. It reallocates functional units for faster execution.

   Explanation: Pipeline interlocks detect hazards and automatically introduce stalls (pauses) to prevent incorrect operation. Duplicating registers does not relate to interlocks. Reallocating functional units is not the primary function, and overheating prevention is unrelated to pipeline hazards.

9. Result of Not Handling Hazards

   What is the likely outcome if hazards are not managed properly in an instruction pipeline?

   1. Instructions may execute incorrectly or out of order.
   2. No effect will occur, as hazards are rare.
   3. The processor speed will always increase.
   4. Only structural hazards will cause issues.

   Explanation: Without hazard management, instructions can use wrong data or execute in the wrong order, causing errors. Processor speed does not always increase with unhandled hazards. Hazards are common in pipelining, so ignoring them has substantial effects. All types of hazards, not just structural, can cause problems.

10. Pipeline Depth Impact

    How does increasing the number of pipeline stages (pipeline depth) generally affect hazard frequency?

    1. It guarantees all hazards are eliminated.
    2. It increases the likelihood of hazards occurring.
    3. It makes pipeline implementation unnecessary.
    4. It reduces hazards to zero.

    Explanation: More pipeline stages increase instruction overlap, making hazards like data and control dependencies more frequent. Deep pipelines do not eliminate hazards; in fact, they can increase their prevalence. Hazards are not reduced to zero, and increased depth does not make pipelining unnecessary.