Hazards in Pipelined Processing

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Questions and Answers

What are the three types of hazards in pipelining and briefly describe each?

The three types of hazards are structural hazards, data hazards, and control hazards. Structural hazards occur when a required resource is busy, data hazards arise when an instruction needs to wait for previous data operations, and control hazards relate to decisions based on the outcome of previous instructions.

What are structural hazards and how can they be mitigated in a MIPS pipeline?

Structural hazards occur due to conflicts for resource usage, such as when load/store operations require memory access at the same time as instruction fetch. They can be mitigated by using separate instruction and data memories or caches.

Explain the concept of forwarding in pipelining.

Forwarding, also known as bypassing, allows the use of an instruction's result as soon as it is computed instead of waiting for it to be written to a register. This requires additional connections in the datapath to enable quicker data access.

What is a load-use data hazard and why can't it always be resolved by forwarding?

<p>A load-use data hazard occurs when an instruction tries to use data that has not yet been computed due to a previous load instruction. It can't always be resolved by forwarding because the value may not be ready in time for the dependent instruction.</p> Signup and view all the answers

What role does alignment of memory operands play in memory access times?

<p>Alignment of memory operands ensures that memory accesses can be completed in a single cycle, optimizing performance. This alignment is crucial for maintaining efficient data processing within the processor.</p> Signup and view all the answers

What is the primary purpose of the multi-clock-cycle diagram?

<p>To illustrate the resource usage of operations over time in a processor.</p> Signup and view all the answers

Explain the function of the WB stage in the load operation.

<p>The WB stage writes the data from memory back to the register file.</p> Signup and view all the answers

How does the EX stage differ for load and store operations?

<p>In the EX stage for load, the effective memory address is computed, while for store, the address is calculated to store data.</p> Signup and view all the answers

What is pipelined control and how is it simplified?

<p>Pipelined control generates control signals based on instructions, and simplification reduces complexity for easier implementation.</p> Signup and view all the answers

What are data hazards, and how can they be resolved?

<p>Data hazards occur when instructions depend on data from previous instructions, and they can be resolved using forwarding.</p> Signup and view all the answers

Describe the role of register numbers in detecting the need to forward.

<p>Register numbers are passed along the pipeline to identify dependencies and when forwarding should occur.</p> Signup and view all the answers

What happens during the ID stage for both load and store operations?

<p>The ID stage decodes the instruction and reads the necessary register values for load or store operations.</p> Signup and view all the answers

How does a single-cycle pipeline diagram differ from a multi-cycle pipeline diagram?

<p>A single-cycle diagram shows the state of the pipeline in one clock cycle, while a multi-cycle diagram emphasizes resource usage over several cycles.</p> Signup and view all the answers

What are the main types of hazards discussed in the context of pipeline processing?

<p>The main types of hazards are data hazards and control hazards.</p> Signup and view all the answers

Explain what a Load-Use data hazard is.

<p>A Load-Use data hazard occurs when a load instruction is immediately followed by a use of the data loaded, requiring a stall to prevent using incorrect data.</p> Signup and view all the answers

How does the revised forwarding condition help manage data hazards?

<p>The revised forwarding condition allows for data to be forwarded in the pipeline only when specific EX hazard conditions are not true, thus reducing incorrect data usage.</p> Signup and view all the answers

What actions are taken to stall a pipeline during a Load-Use hazard?

<p>To stall the pipeline, control values in the ID/EX register are set to 0, preventing updates to the PC and forcing nop operations in EX, MEM, and WB stages.</p> Signup and view all the answers

What is a branch hazard, and how should instructions be handled during this situation?

<p>A branch hazard occurs when the outcome of a branch instruction is not known in time, requiring the flushing of subsequent instructions by setting control values to 0.</p> Signup and view all the answers

Describe the purpose of moving hardware to determine the branch outcome to the ID stage.

<p>Moving hardware to determine the branch outcome to the ID stage reduces branch delay by enabling the pipeline to make decisions about instruction execution sooner.</p> Signup and view all the answers

Why are stalls considered detrimental to performance in pipeline processing?

<p>Stalls reduce performance because they introduce idle cycles in the pipeline, slowing down the overall instruction throughput.</p> Signup and view all the answers

What mechanism is used to detect Load-Use hazards during instruction execution?

<p>Load-Use hazards are detected by checking the values in the ID stage to see if a load instruction's destination register is used in subsequent instructions.</p> Signup and view all the answers

What is the significance of forcing control values in the ID/EX register to 0 during a pipeline stall?

<p>Forcing control values to 0 prevents further instruction execution and updates, ensuring the current instruction is decoded again properly.</p> Signup and view all the answers

What does the instruction lw $4, 50($7) do?

<p>It loads the word from the memory address calculated by adding 50 to the content of register $7 into register $4.</p> Signup and view all the answers

How can a compiler assist in reducing hazards and stalls in pipeline execution?

<p>A compiler can arrange code and manage instruction scheduling to avoid hazards and stalls by being aware of the pipeline structure.</p> Signup and view all the answers

What is the effect of a data hazard if a comparison register is the destination of the preceding ALU instruction?

<p>It may require a stall cycle in the execution of the following branch instruction.</p> Signup and view all the answers

How many stall cycles are required if the comparison register is a destination of an immediately preceding load instruction?

<p>Two stall cycles are required.</p> Signup and view all the answers

What is dynamic branch prediction and how does it improve pipeline performance?

<p>Dynamic branch prediction uses a branch prediction buffer to anticipate the outcome of branches, reducing pipeline stalls.</p> Signup and view all the answers

What is a branch prediction buffer, and how is it utilized?

<p>A branch prediction buffer, or branch history table, stores outcomes of recent branches indexed by their addresses.</p> Signup and view all the answers

What is one limitation of using a 1-bit branch predictor?

<p>A 1-bit predictor can lead to multiple mispredictions in inner loops.</p> Signup and view all the answers

In the context of the examples provided, what happens if the branch prediction is incorrect?

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

Explain what a stall cycle is and its significance in branch instruction execution.

<p>A stall cycle is a delay inserted into the instruction pipeline to resolve data hazards.</p> Signup and view all the answers

What is loop unrolling and why is it beneficial?

<p>Loop unrolling is the technique of replicating the loop body to expose more parallelism and reduce overhead. This allows for better performance by maximizing instruction throughput.</p> Signup and view all the answers

Explain the concept of register renaming in dynamic scheduling.

<p>Register renaming is a method that prevents name dependencies by allowing the CPU to use different physical registers for the same logical register. This reduces hazards and enables more parallel instruction execution.</p> Signup and view all the answers

How does dynamic multiple issue improve CPU performance?

<p>Dynamic multiple issue allows the CPU to issue multiple instructions simultaneously based on available resources and data dependencies. This flexibility minimizes stalls and maximizes resource utilization.</p> Signup and view all the answers

What is the role of the reorder buffer in a dynamically scheduled CPU?

<p>The reorder buffer holds results of instructions that have executed but not yet committed, ensuring correct execution order while allowing out-of-order processing. It helps manage dependencies until final results are ready.</p> Signup and view all the answers

Define load speculation and its purpose in dynamic scheduling.

<p>Load speculation involves predicting the effectiveness of loads to avoid delays caused by cache misses. It enables the CPU to execute loads before confirming earlier operations, enhancing instruction flow.</p> Signup and view all the answers

What does 'dynamic pipeline scheduling' achieve in a CPU?

<p>Dynamic pipeline scheduling allows instructions to execute out of order to reduce stalls while ensuring results are committed in order. This improves throughput and maintains correct program execution.</p> Signup and view all the answers

Describe the impact of loop-carried dependencies in instruction execution.

<p>Loop-carried dependencies can hinder parallel execution by creating delays whenever a subsequent instruction depends on the result of a previous iteration. This can reduce CPU efficiency.</p> Signup and view all the answers

Why is reducing the number of registers used a challenge in loop unrolling?

<p>Reducing register usage in loop unrolling is challenging because unrolling increases the number of instructions needing unique registers, which may exceed available register resources. This can lead to performance trade-offs.</p> Signup and view all the answers

What are structural hazards, and how does dynamic scheduling help mitigate them?

<p>Structural hazards occur when hardware resources are insufficient to support all concurrent operations. Dynamic scheduling mitigates these hazards by intelligently issuing instructions based on resource availability.</p> Signup and view all the answers

In the context of dynamic scheduling, what is the significance of the 'reservation station'?

<p>The reservation station temporarily holds instructions ready to execute while waiting for available operands, allowing for out-of-order execution. This helps optimize instruction throughput.</p> Signup and view all the answers

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Study Notes

Hazards in Pipelined Processing

  • Hazards are situations occurring in pipelined execution that prevent an instruction from starting in the next cycle.
  • There are three main types of hazards: structural, data and control.

Structural Hazards

  • Structural hazards occur when a needed resource is busy for the next instruction.
  • Pipelined CPUs typically have separate instruction and data memory units to avoid structural hazards.

Data Hazards

  • Data hazards occur when one instruction depends on the result of a previous instruction that has not yet completed.
  • Forwarding (or Bypassing): This technique allows results to be used by subsequent instructions before they are written back to the register file.

Load-Use Hazards

  • Load-Use hazards occur when an instruction uses the result of a load instruction that hasn't completed its memory access.
  • Load-Use hazards cannot be resolved by forwarding alone, requiring a stall cycle, where the pipeline is paused for a cycle to allow memory access to complete.
  • A stall cycle forces control signals to be set to 0 in the ID/EX register.

Control Hazards

  • Control hazards occur when a branch instruction's outcome is not known until the MEM stage.
  • To handle branch hazards, pipelines employ techniques to reduce branch delay.

Dynamic Branch Prediction

  • To further minimize the impact of control hazards, dynamic branch prediction is employed.
  • Dynamic branch prediction uses a branch prediction buffer (aka branch history table).

Loop Unrolling

  • Loop unrolling can improve performance by replicating the loop body to increase parallelism and reduce loop control overhead.

Dynamic Multiple Issues

  • Superscalar processors can execute multiple instructions in a single cycle, dynamically deciding how many instructions to issue.
  • Dynamic multiple issue avoids the need for compiler scheduling, allowing the CPU to handle code semantics independently.

Dynamic Pipeline Scheduling

  • Dynamic pipeline scheduling allows the CPU to execute instructions out of order, but results are committed to the registers in their original program order.

Register Renaming

  • Dynamically scheduled CPUs use reservation stations and reorder buffers to effectively rename registers.
  • Register renaming prevents name dependencies by making sure the same register is not rewritten before its result is used.

Speculation

  • Speculative execution allows the CPU to predict the outcome of branches and loads, continuing execution without waiting for confirmation.
  • If speculation is wrong, instructions are flushed, and the pipeline is restarted.

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