Pipeline Performance and CPI Calculations Quiz Quiz

Assess your understanding of pipeline performance and cycles per instruction (CPI) calculations with these beginner-friendly questions. Strengthen your grasp of core processor concepts, including hazards, instruction throughput, and how design choices impact pipeline efficiency.

1. Basic CPI Definition

   What does CPI represent in the context of computer pipelines?

   1. Cycles per instruction
   2. Calculations per input
   3. Central processing indicator
   4. Clock per increment

   Explanation: CPI stands for cycles per instruction, which measures the average number of clock cycles each instruction takes to execute. The other options are incorrect: 'Calculations per input' and 'Clock per increment' do not relate to standard CPU pipeline terminology, and 'Central processing indicator' is not a recognized performance metric in this context.

2. Pipeline Stages and Performance

   If a processor's pipeline increases from 5 to 10 stages without changing clock speed, what is the most likely immediate effect on instruction latency?

   1. Instruction latency increases
   2. Pipeline hazards are eliminated
   3. Instruction throughput halves
   4. Instruction latency decreases

   Explanation: Adding more pipeline stages typically increases instruction latency, as each instruction passes through more stages to complete execution. Throughput may improve due to increased parallelism, but instruction latency is about how long one instruction takes. Throughput halving is incorrect, and additional stages do not eliminate pipeline hazards; they often make hazard handling more complex.

3. Pipeline Throughput

   In a perfectly balanced 4-stage pipeline, what is the maximum throughput after the pipeline is filled?

   1. 0.25 instructions per cycle
   2. 1 instruction per cycle
   3. 2 instructions per cycle
   4. 4 instructions per cycle

   Explanation: Once the pipeline is filled, a balanced pipeline can complete one instruction per clock cycle. '4 instructions per cycle' and '2 instructions per cycle' overstate the throughput, while '0.25 instructions per cycle' suggests the pipeline is not full or there are other delays, which is incorrect for an ideal filled pipeline.

4. Stall Impact on CPI

   If a pipeline must stall every fifth cycle due to a hazard, how will this affect the average CPI?

   1. CPI remains at 1
   2. CPI becomes zero
   3. CPI drops below 1
   4. CPI increases above 1

   Explanation: Stalling introduces extra cycles without completing new instructions, increasing the average cycles per instruction (CPI) above 1. The CPI cannot remain at 1 if stalls occur, cannot drop below 1 for a classic pipeline, and cannot become zero, as that would imply no cycles are used.

5. Data Hazard Scenario

   Given two instructions where the second uses the result of the first, what hazard may occur in a pipeline?

   1. Memory overlap
   2. Data hazard
   3. Instruction fetch stall
   4. Code hazard

   Explanation: A data hazard arises when an instruction depends on the result of a previous instruction that has not yet completed. 'Code hazard' is not a standard term, 'memory overlap' refers to a different issue, and 'instruction fetch stall' is possible but is not specific to data dependencies.

6. CPI Calculation Example

   If a processor executes 100 instructions in 120 cycles, what is the average CPI?

   1. 120
   2. 0.83
   3. 100
   4. 1.2

   Explanation: CPI is calculated as total cycles divided by total instructions, so 120 divided by 100 equals 1.2. 0.83 is the inverse and incorrect, 120 is the number of cycles, and 100 is the number of instructions, not the CPI.

7. Effect of Pipeline Hazards

   What is the typical effect of pipeline hazards on overall pipeline performance?

   1. They cause pipeline stalls and reduce throughput
   2. They always increase the clock speed
   3. They eliminate instruction dependencies
   4. They speed up instruction execution

   Explanation: Pipeline hazards such as data or control hazards often lead to pipeline stalls, reducing throughput and efficiency. Hazards cannot speed up execution or clock speed, and rather than eliminating dependencies, they occur because of existing dependencies.

8. Ideal CPI in Fully Clocked Pipeline

   What is the ideal CPI for a fully pipelined architecture with no hazards or stalls?

   1. Negative value
   2. Greater than 2
   3. 0
   4. 1

   Explanation: In an ideal situation with no hazards or stalls, the pipeline can complete one instruction per cycle, giving a CPI of 1. A CPI of 0 or a negative value is not possible, and a CPI greater than 2 would indicate substantial inefficiency, which is not the ideal.

9. Instruction Mix and CPI

   If a program has a mix of fast and slow instructions, how does this affect the overall CPI?

   1. CPI always drops to 1
   2. CPI stays the same regardless of instruction mix
   3. CPI reflects the weighted average based on instruction types
   4. CPI becomes infinite

   Explanation: The overall CPI is a weighted average determined by the frequency and cycle count of each instruction type. It does not become infinite, always drop to 1, or remain unchanged—instead, it varies depending on the program's instruction mix.

10. Pipeline Performance Factors

    Which factor is least likely to directly improve pipeline performance?

    1. Increasing clock speed without reducing stall frequency
    2. Reducing the number of pipeline stalls
    3. Improving hazard detection mechanisms
    4. Balancing pipeline stage durations

    Explanation: While a higher clock speed can quicken cycles, if stall frequencies remain high, performance gains are limited. Reducing pipeline stalls, balancing stage durations, and improving hazard detection all directly boost performance by keeping the pipeline running efficiently. Simply increasing clock speed does not resolve the underlying pipeline inefficiencies caused by frequent stalls.