Computer Architecture: Pipelining Concepts
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Questions and Answers

What is a key benefit of pipelining in execution?

  • It reduces the complexity of the program.
  • It eliminates all execution delays.
  • It always requires fewer resources.
  • It enables overlapping execution. (correct)

    • How does parallelism affect performance based on the content provided?

  • It consistently increases potential speedup. (correct)
  • It can lead to resource contention.
  • It decreases overall execution time.
  • It operates independent of the number of loads.

    • What is the calculated speedup for the pipelined laundry analogy given in the content?

  • 2.3 (correct)
  • 1.8
  • 3.5
  • 4.0

    • According to the analysis provided, what is the speedup formula for non-stop execution?

    <p>2n / 0.5n + 1.5</p> Signup and view all the answers

    What does the term 'number of stages' imply in the context of pipelining?

    <p>It relates to the calculation of speedup.</p> Signup and view all the answers

    What is a structural hazard in the context of pipelining?

    <p>A structural hazard occurs when there is a conflict for a required resource, preventing the pipeline from continuing smoothly.</p> Signup and view all the answers

    Explain the concept of data hazards and give an example.

    <p>Data hazards arise when one instruction depends on data that has not yet been written back by a previous instruction. For example, if an instruction adds registers $t0 and $t1 and a subsequent instruction subtracts from the result before it is available, a data hazard occurs.</p> Signup and view all the answers

    What is the purpose of forwarding in a pipelined processor?

    <p>Forwarding allows a processor to use a computation result immediately instead of waiting for it to be stored in a register, thus minimizing stalls.</p> Signup and view all the answers

    Describe a load-use data hazard and its implications.

    <p>A load-use data hazard occurs when an instruction that needs to use a value from a load instruction is executed before the value is available, causing a stall.</p> Signup and view all the answers

    How does addressing affect the stages of execution in an x86 processor?

    <p>In an x86 processor, load/store addressing can calculate the address in the third stage and access memory in the fourth stage, improving efficiency.</p> Signup and view all the answers

    What kind of stall cycles are needed if a comparison register is the destination of an immediately preceding load instruction?

    <p>2 stall cycles are needed.</p> Signup and view all the answers

    Explain how forwarding can resolve data hazards for branches.

    <p>Forwarding allows the processor to route data from a previous instruction directly to a subsequent instruction, reducing stalls.</p> Signup and view all the answers

    What role does a branch prediction buffer (or branch history table) play in dynamic branch prediction?

    <p>It stores the outcomes of recent branch instructions to predict the result of a branch attempt.</p> Signup and view all the answers

    What happens to the pipeline if a branch prediction is incorrect?

    <p>The pipeline is flushed, and the prediction is flipped.</p> Signup and view all the answers

    Identify the consequence of a comparison register being a destination of two preceding ALU instructions in branch execution.

    <p>It results in the need for one stall cycle.</p> Signup and view all the answers

    How does a 1-bit predictor potentially affect the accuracy of branch predictions in loops?

    <p>It may lead to mispredictions in inner loops due to its simplistic approach.</p> Signup and view all the answers

    What is the impact of deeper and superscalar pipelines on branch penalty?

    <p>The branch penalty becomes more significant in deeper and superscalar pipelines.</p> Signup and view all the answers

    What should the processor do to begin fetching from the appropriate location after a branch decision?

    <p>It must check the branch prediction table to determine the expected outcome.</p> Signup and view all the answers

    What is the primary goal of code scheduling in a pipelined architecture?

    <p>The primary goal is to reorder code to avoid stalls caused by dependencies between instructions.</p> Signup and view all the answers

    What happens during the stall on a branch instruction in pipelining?

    <p>The pipeline must wait until the branch outcome is determined before fetching the next instruction.</p> Signup and view all the answers

    How does branch prediction help reduce the penalty of pipeline stalls?

    <p>Branch prediction allows the pipeline to continue fetching instructions based on predicted outcomes, reducing stalls if predictions are correct.</p> Signup and view all the answers

    What is the difference between static and dynamic branch prediction?

    <p>Static branch prediction relies on predetermined behaviors, while dynamic branch prediction uses actual execution history to inform predictions.</p> Signup and view all the answers

    What are the three types of hazards that pipelines can encounter?

    <p>Pipelines can encounter structural hazards, data hazards, and control hazards.</p> Signup and view all the answers

    What role do pipeline registers play in a pipelined architecture?

    <p>Pipeline registers hold information produced in the previous cycle to facilitate instruction flow through the pipeline.</p> Signup and view all the answers

    In the MIPS pipeline, why is it important to compare registers and compute target early?

    <p>It's important to ensure the pipeline can fetch the correct instruction based on branch outcomes without unnecessary delays.</p> Signup and view all the answers

    What is the impact of instruction set design on pipelined architecture complexity?

    <p>Instruction set design can increase the complexity of pipeline implementation by introducing more dependencies and hazards.</p> Signup and view all the answers

    How does a stalled pipeline affect overall processor performance?

    <p>A stalled pipeline increases the cycle count for completing instructions, lowering overall throughput and efficiency.</p> Signup and view all the answers

    What technique can be used in MIPS pipelines to predict branches not taken?

    <p>MIPS pipelines can fetch the instruction after a branch instruction with the assumption that the branch will not be taken.</p> Signup and view all the answers

    What happens when an imprecise exception occurs in a pipeline?

    <p>The pipeline stops, saves the state including exception causes, and allows the handler to determine which instructions had exceptions.</p> Signup and view all the answers

    How does a deeper pipeline affect instruction-level parallelism (ILP)?

    <p>A deeper pipeline reduces the work per stage, leading to shorter clock cycles and potentially higher instruction throughput.</p> Signup and view all the answers

    What is the difference between static and dynamic multiple issue?

    <p>Static multiple issue relies on the compiler to group instructions together, while dynamic multiple issue allows the CPU to choose instructions to issue based on the instruction stream.</p> Signup and view all the answers

    What role does speculation play in instruction execution?

    <p>Speculation allows operations to start early based on guesses about future instruction outcomes, rolling back if the guesses are incorrect.</p> Signup and view all the answers

    How can compilers aid in speculation?

    <p>Compilers can reorder instructions and insert 'fix-up' instructions to recover from incorrect speculative executions.</p> Signup and view all the answers

    What happens in the case of an exception occurring on a speculatively executed instruction?

    <p>The system may need to roll back and handle the exception to ensure correct execution of instructions following the exception.</p> Signup and view all the answers

    How does a CPU resolve hazards during dynamic multiple issue?

    <p>The CPU uses advanced techniques at runtime to resolve hazards that may arise from issuing instructions out of order.</p> Signup and view all the answers

    What is the purpose of using multiple issue in a pipeline?

    <p>Multiple issue aims to start several instructions per clock cycle, thereby increasing the overall throughput of the processor.</p> Signup and view all the answers

    What is the purpose of loop unrolling in programming?

    <p>Loop unrolling aims to expose more parallelism and reduce loop-control overhead.</p> Signup and view all the answers

    Explain how register renaming helps in loop unrolling.

    <p>Register renaming uses different registers for each unrolled loop iteration, avoiding loop-carried anti-dependencies.</p> Signup and view all the answers

    In the context of dynamic multiple issue, what do superscalar processors do?

    <p>Superscalar processors can issue multiple instructions each cycle, depending on the availability of resources.</p> Signup and view all the answers

    What is dynamic pipeline scheduling and its main advantage?

    <p>Dynamic pipeline scheduling allows out-of-order execution of instructions to avoid stalls while maintaining in-order commit.</p> Signup and view all the answers

    How does a reservation station contribute to register renaming?

    <p>A reservation station holds operands and allows the register associated with those operands to be overwritten.</p> Signup and view all the answers

    What role does speculation play in dynamic scheduling?

    <p>Speculation allows the CPU to predict branch outcomes and load values before their actual confirmation.</p> Signup and view all the answers

    Describe the concept of loop-carried anti-dependencies.

    <p>Loop-carried anti-dependencies occur when a stored value is followed by a load of the same register, creating a conflict.</p> Signup and view all the answers

    Why is it significant to manage data hazards in CPU architectures?

    <p>Managing data hazards is critical to ensure correct instruction execution without stalls or incorrect dependencies.</p> Signup and view all the answers

    How do dynamically scheduled CPUs ensure that code semantics are preserved?

    <p>They commit results to registers in order, ensuring that the program's intended execution sequence is maintained.</p> Signup and view all the answers

    What is the calculation for IPC in the provided loop unrolling example?

    <p>The IPC is calculated as $14/8$, resulting in an IPC of $1.75$.</p> Signup and view all the answers

    Study Notes

    Pipelining Analogy

    • Pipelining is analogous to a laundry process where multiple tasks (like washing, drying, and folding) are performed in parallel on different items.
    • This overlapping execution increases the overall speed of the laundry process.

    Parallelism and Performance

    • Parallelism, like in pipelining, can significantly improve performance.
    • This improvement is due to the ability to perform different tasks simultaneously.

    Overview of Pipelining

    • Pipelining is a technique that allows for overlapping execution of instructions in a processor.
    • This leads to a faster overall execution time by breaking down instructions into smaller stages and processing them concurrently.

    Speedup Calculation

    • Speedup is the ratio of the time taken for non-pipelined execution to the time taken for pipelined execution.
    • In a scenario involving four loads, speedup is calculated as 8/3.5, which is approximately 2.3.
    • This indicates a significant performance improvement using pipelining.

    Non-Stop Pipelining

    • Non-stop pipelining refers to a scenario where the pipeline is always kept busy with instructions.
    • This leads to a maximum speedup that is proportional to the number of stages in the pipeline.
    • With n stages, the speedup is approximately 2n/0.5n + 1.5, which simplifies to almost 4 (approaching the number of stages).

    x86 Architecture

    • x86 instructions range from 1 to 17 bytes
    • Features simple and consistent instruction formats
    • Allows decoding and reading registers in one step
    • Uses load/store addressing
    • Allows address calculation in the third stage and memory access in the fourth stage
    • Memory operands have alignment, allowing memory access to take a single cycle

    Hazards in Pipelined Architecture

    • Hazards are situations that prevent the next instruction from beginning in the next cycle.
    • Structural hazards occur when a necessary resource is unavailable.
    • Data hazards result from needing to wait for a previous instruction to finish reading/writing data.
    • Control hazards happen when determining a control action depends on outcomes from the previous instruction.

    Structural Hazards

    • Arises from conflicts in resource use.
    • For example, MIPS pipeline requires separate instruction/data memories or caches to address the conflict between instruction and data access.

    Data Hazards

    • Occur when an instruction depends on the completion of data access by a previous instruction.
    • Forwarding (aka bypassing) can be used to avoid stalls by utilizing the result as soon as it's computed.

    Load-Use Data Hazards

    • Forwarding might not prevent stalls if the value is not computed when needed.
    • These hazards prevent backward forwarding in time.

    Code Scheduling to Avoid Stalls

    • Instructions can be reordered to avoid using the loaded result in the next instruction.
    • This optimization can significantly reduce the execution cycle count.

    Control Hazards

    • Branch instructions are necessary to determine the control flow of the program.
    • The pipeline might not be able to fetch the correct instruction due to the branch outcome being determined later.
    • Hardware can be added to compute the branch target early in the pipeline.

    Stall on Branch

    • The processor can stall until the branch outcome is determined before fetching the next instruction.

    Branch Prediction

    • Longer pipelines might not be able to determine the branch outcome early enough for efficient execution.
    • Speculatively predicting the branch outcome can mitigate the stalling penalty.
    • MIPS can predict branches as not taken, fetching the instruction after the branch without delays.

    More Realistic Branch Prediction

    • Static branch prediction relies on the usual branch behavior.
    • Dynamic branch prediction uses hardware to measure the actual branch behavior.

    MIPS Pipelined Datapath

    • The MIPS pipelined datapath consists of five stages: IF (Instruction Fetch), ID (Instruction Decode), EX (Execute), MEM (Memory), and WB (Write Back).
    • These stages are connected by pipeline registers to hold information from the preceding cycle.

    Data Hazards for Branches

    • To avoid stalls, forwarding can be used if the comparison register is the destination of a preceding ALU instruction or the second preceding load instruction.
    • For deeper pipelines, branch prediction is used to avoid the large branch penalty.

    Dynamic Branch Prediction

    • Uses a branch prediction buffer to store the branch outcome based on recent branch instruction addresses.
    • It predicts the same outcome in future execution, fetching from the fall-through or target and updating the prediction if wrong.

    Imprecise Exceptions

    • The pipeline can be stalled and the state saved, including the exception cause.
    • This simplifies the hardware and allows the exception handler to determine which instructions had exceptions, which need to be completed or flushed.

    Instruction-Level Parallelism (ILP)

    • Achieved through pipelining, multiple issue, and speculation.
    • Deeper pipelines can reduce the work per stage.
    • Multiple-issue architecture replicates pipeline stages, allowing for multiple instructions per clock cycle, leading to a CPI < 1 and peak IPC > 1.
    • Dependencies reduce this in practice.

    Multiple Issue

    • Static multiple issue utilizes the compiler to group instructions into issue slots and avoid hazards.
    • Dynamic multiple issue allows the CPU to examine the instructions and issue multiple instructions each cycle at runtime.

    Speculation

    • The processor "guesses" the outcome of an instruction before completing its execution.
    • It starts the operation immediately and only rolls back if the guess was wrong.
    • This is commonly applied to branch outcome and load operations.

    Loop Unrolling

    • Replicates the loop body to improve parallelism and reduce loop control overhead.
    • Register renaming is used to allocate different registers for each replication, avoiding anti-dependencies.

    Dynamic Multiple Issue

    • Super-scalar processors dynamically decide the number of instructions to issue each cycle, avoiding hazards dynamically.

    Dynamic Pipeline Scheduling

    • Allows the CPU to execute instructions out of order, avoiding stalls.
    • Results are committed to registers in order though.

    Dynamically Scheduled CPU

    • Reservation stations buffer instructions and operands until all dependencies are satisfied.
    • This allows for dynamic scheduling and avoids stalls due to data dependencies.

    Register Renaming

    • Reservation stations and the reorder buffer provide register renaming.
    • This allows for the register to be overwritten when the operand is copied to the reservation station, avoiding anti-dependencies.

    Speculation

    • Predicts branch outcomes and loads, allowing for operations to start before completing dependencies and improving performance.

    Dynamic scheduling benefits:

    • Eliminates the need for compiler scheduling.
    • Increases instruction throughput and reduces stall cycles.
    • Enables efficient dynamic branch prediction and load speculation, boosting performance.

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    Description

    This quiz explores the essential concepts of pipelining in computer architecture. It covers the principles of parallelism, performance improvement through pipelining, and speedup calculations. Perfect for students looking to deepen their understanding of how processors execute instructions efficiently.

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