RISC Processor Pipeline Design Quiz and Flashcards

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What is the purpose of performing the register write in the first half of the clock cycle?

To handle reads and writes to the same register simultaneously

Why is it essential to increment and store the PC every clock cycle during the IF stage?

To ensure a new instruction starts every clock cycle

Why is it necessary to have an adder to compute the potential branch target address during ID?

To evaluate the branch condition during ALU stage

What is the function of introducing pipeline registers between successive stages of the pipeline?

To separate instructions in different stages to prevent interference Signup and view all the answers

Why do instructions in the pipeline need to avoid using hardware resources simultaneously?

To prevent interference and conflicts between instructions Signup and view all the answers

Why must we have enough of the ALU function in a pipestage even if a full ALU is not needed for a specific task?

To evaluate comparisons between two registers effectively Signup and view all the answers

What is the primary cause of structural hazards in modern processors?

Resource conflicts in the hardware Signup and view all the answers

Why are structural hazards not a major performance factor according to the text?

Programmers and compiler writers are aware of the lower throughput of these instructions Signup and view all the answers

What is the primary difference between data hazards and control hazards?

Data hazards arise when an instruction depends on a previous instruction, while control hazards arise from pipelining of branches Signup and view all the answers

When a pipeline is stalled due to a hazard, what happens to the instructions that are issued after the stalled instruction?

They are also stalled, since no new instructions are fetched during the stall Signup and view all the answers

What is the reason for allowing instructions issued earlier than the stalled instruction to continue executing?

To ensure that the hazard is cleared as soon as possible Signup and view all the answers

Which type of hazard is more frequent and requires more attention?

Control hazards Signup and view all the answers

What is the consequence of a taken branch in a pipeline?

An additional stall cycle is introduced. Signup and view all the answers

Why does the repetition of the IF stage become unnecessary for an untaken branch?

The instruction is fetched correctly the first time. Signup and view all the answers

How does a one-cycle stall impact the performance of a pipeline?

It leads to a 10-30% performance loss. Signup and view all the answers

Which stage of the pipeline is affected by the one-cycle stall caused by a branch?

IF Signup and view all the answers

What is the purpose of redoing the fetch of the instruction following a branch in a pipeline?

To enhance the fetch accuracy post a branch. Signup and view all the answers

How do branches impact the efficiency of a five-stage pipeline according to Figure C.9?

They introduce unnecessary work cycles. Signup and view all the answers

Which of the following is NOT a reason why the initial implementation is not optimal?

It has a complex control logic Signup and view all the answers

What is the key advantage of pipelining the execution described in the previous section?

It allows the processor to initiate a new instruction on each clock cycle Signup and view all the answers

Which of the following is a challenge that must be addressed when making the RISC pipeline "real"?

Ensuring that the same data path resource is not used for two different operations in the same clock cycle Signup and view all the answers

What is the typical way a pipeline structure is drawn, according to the text?

A timeline with each instruction's execution stages depicted horizontally Signup and view all the answers

What is the key benefit of pipelining mentioned in the text?

It increases the throughput of the processor Signup and view all the answers

What is the purpose of forwarding in the pipeline architecture described in the text?

To reduce the number of stalls in the pipeline by passing results directly to the functional unit that requires them Signup and view all the answers

How many cycles earlier can a result be forwarded from in the pipeline?

Two cycles Signup and view all the answers

What is the purpose of the dashed lines on the registers in Figure C.5?

To highlight the timing of the register reads and writes Signup and view all the answers

Which of the following scenarios is possible with the forwarding mechanism described?

Both (a) and (b) are possible Signup and view all the answers

What is the key advantage of the forwarding mechanism described in the text?

It allows the code sequence to be executed without stalls Signup and view all the answers

What is the main purpose of the forwarding paths shown in Figure C.5?

To allow results to be passed directly to the functional units that require them Signup and view all the answers

Explain the primary goal of the pipeline designer and how it relates to the concept of an assembly line.

The pipeline designer's goal is to balance the length of each pipeline stage, just as the designer of the assembly line tries to balance the time for each step in the process. If the stages are perfectly balanced, then the time per instruction on the pipelined processor is equal to the time per instruction on an unpipelined machine divided by the number of pipe stages. This is similar to how an assembly line with n stages can ideally produce cars n times as fast. Signup and view all the answers

Explain the primary advantage of pipelining as an implementation technique, and how it differs from other speedup techniques.

Pipelining is an implementation technique that exploits parallelism among the instructions in a sequential instruction stream. Unlike some other speedup techniques, pipelining is not visible to the programmer, which is a substantial advantage. Signup and view all the answers

Describe the relationship between pipelining and the reduction in the average execution time per instruction, particularly in the context of a processor that takes multiple clock cycles per instruction.

Pipelining yields a reduction in the average execution time per instruction. If the starting point is a processor that takes multiple clock cycles per instruction, then pipelining reduces the CPI (cycles per instruction), which is the primary view taken in the text. Signup and view all the answers

Explain the role of the adder used to compute the potential branch target address during the ID (Instruction Decode) stage of the pipeline, and why it is necessary to have this adder.

$ ext{The adder is necessary to compute the potential branch target address during the ID stage, as the pipeline needs to determine the correct address to fetch the next instruction in the event of a branch.}$ Signup and view all the answers

Describe the impact of a one-cycle stall on the performance of the pipeline, and explain the reason for allowing instructions issued earlier than the stalled instruction to continue executing.

$ ext{A one-cycle stall in the pipeline can have a significant impact on performance. The text states that the reason for allowing instructions issued earlier than the stalled instruction to continue executing is to avoid wasting resources and maintain efficiency in the pipeline.}$ Signup and view all the answers

Discuss the role of forwarding in the pipeline architecture described in the text, including the key benefit of this mechanism and the number of cycles earlier a result can be forwarded from.

$ ext{Forwarding is a key mechanism in the pipeline architecture to address data hazards. The text states that the key advantage of the forwarding mechanism is that it allows results to be forwarded from up to } extbf{two cycles earlier} ext{ in the pipeline, which improves performance by reducing the need for stalls.}$ Signup and view all the answers

What is the formula for the effective pipeline speedup with branch penalties, assuming an ideal CPI of 1?

Pipeline speedup = Pipeline depth / (1 + Branch frequency * Branch penalty) Signup and view all the answers

Explain why the branch penalty for conditional branches is more significant than for unconditional branches.

The text states that the branch penalty for conditional branches dominates because they are more frequent than unconditional branches. Signup and view all the answers

Calculate the effective CPI increase due to branches for the pipeline described, given the provided branch frequencies.

The effective CPI increase due to branches is 0.4 (4% unconditional branch penalty + 6% conditional untaken branch penalty + 10% conditional taken branch penalty). Signup and view all the answers

Explain why the branch-target address is not known until at least 3 pipeline stages after the branch instruction in the MIPS R4000 and later RISC processors.

The text states that for a deeper pipeline, such as in the MIPS R4000, it takes at least three pipeline stages before the branch-target address is known and an additional cycle before the branch condition is evaluated. Signup and view all the answers

How do the three simplest branch prediction schemes differ in their branch penalties according to Figure C.12?

According to Figure C.12, the branch penalties for the three simplest prediction schemes are: 1 cycle for a taken branch, 0 cycles for an untaken branch, and 2 cycles for a mispredicted branch. Signup and view all the answers

Explain why it is necessary to have an adder to compute the potential branch target address during the ID stage of the pipeline.

The text states that it is necessary to have an adder to compute the potential branch target address during the ID stage, as this information is needed before the branch condition is evaluated in the later pipeline stages. Signup and view all the answers

Explain how the effective address and execution cycles can be combined into a single clock cycle in a load-store architecture, and the rationale behind this design choice.

In a load-store architecture, the effective address and execution cycles can be combined into a single clock cycle because no instruction needs to simultaneously calculate a data address and perform an operation on the data. This design choice is made to simplify the pipeline and reduce the number of cycles required for certain instructions, improving overall performance and efficiency. Signup and view all the answers

Describe the purpose and operation of the Write-back (WB) cycle in the given pipeline implementation.

The Write-back (WB) cycle is the final stage of the pipeline. For register-register ALU instructions or load instructions, the result is written back into the register file during this cycle. For ALU instructions, the result comes from the ALU, while for load instructions, the result comes from the memory system. Signup and view all the answers

Calculate the overall CPI (Cycles Per Instruction) for the given pipeline implementation, assuming a branch frequency of 12% and a store frequency of 10%.

According to the text, in this implementation, branch instructions require three cycles, store instructions require four cycles, and all other instructions require five cycles. Given a branch frequency of 12% and a store frequency of 10%, the overall CPI can be calculated as: (0.12 × 3) + (0.10 × 4) + (0.78 × 5) = 4.66. Signup and view all the answers

Explain the purpose and operation of the forwarding mechanism described in the text, and its impact on pipeline performance.

The forwarding mechanism, also known as bypassing or data forwarding, is used to resolve data hazards in the pipeline. It allows the result of an instruction to be forwarded directly to a subsequent instruction that depends on it, rather than waiting for the result to be written back to the register file. This mechanism improves pipeline performance by reducing the number of stalls required due to data hazards. Signup and view all the answers

Describe the potential scenarios and consequences of structural hazards in modern processors, and explain why they are not considered a major performance factor according to the text.

Structural hazards occur when multiple instructions attempt to use the same hardware resource simultaneously, such as the ALU or memory unit. In modern processors, structural hazards are typically mitigated through the use of separate functional units or by stalling the pipeline. The text suggests that structural hazards are not a major performance factor because modern processors are designed with sufficient hardware resources to minimize such conflicts. Signup and view all the answers

Explain the purpose of incrementing and storing the Program Counter (PC) every clock cycle during the Instruction Fetch (IF) stage, and the potential consequences of not doing so.

Incrementing and storing the PC every clock cycle during the IF stage is crucial for maintaining the correct program flow and fetching the next instruction in the sequence. If this step is not performed correctly, the pipeline may fetch the wrong instruction, leading to incorrect program execution or even a system crash. Signup and view all the answers

Explain the key challenge in forwarding the result of a load instruction to the subsequent instructions in the pipeline, as described in the text.

The key challenge is that forwarding the result of a load instruction to the subsequent instructions would require forwarding the result in "negative time", which is not possible with current computer hardware. As the text states, "such a forwarding path would have to operate backward in timea capability not yet available to computer designers!" Signup and view all the answers

Explain why a pipeline interlock is necessary to preserve the correct execution pattern when a load instruction is followed by an instruction that depends on its result.

A pipeline interlock is necessary because the forwarding mechanism alone cannot eliminate the data hazard from using the result of a load instruction. As the text states, "the load instruction has a delay or latency that cannot be eliminated by forwarding alone. Instead, we need to add hardware, called a pipeline interlock, to preserve the correct execution pattern." Signup and view all the answers

Describe the purpose and behavior of a pipeline interlock in the context of a data hazard caused by a load instruction.

A pipeline interlock "detects a hazard and stalls the pipeline until the hazard is cleared". In the case of a data hazard from a load instruction, "the interlock stalls the pipeline, beginning with the instruction that wants to use the data until the source instruction (the load) produces it." This introduces a stall or bubble in the pipeline. Signup and view all the answers

Describe the impact of a pipeline stall caused by a data hazard on the performance of the pipeline.

The text states that a pipeline interlock "introduces a stall or bubble, just as it did for the structural hazard." This stall or bubble reduces the efficiency and throughput of the pipeline, as instructions that could otherwise be executed in parallel are held up waiting for the data hazard to be resolved. Signup and view all the answers

Explain why the forwarding mechanism described in the text is able to forward the result of a load instruction to the $and$ and $or$ instructions, but not to the $sub$ instruction.

The text states that the forwarding mechanism "can forward the result immediately to the ALU from the pipeline registers for use in the $and$ and $or$ operations", but for the $sub$ instruction, "the forwarded result arrives too lateat the end of a clock cycle, when it is needed at the beginning." This is because the load instruction has a delay or latency that cannot be eliminated by forwarding alone, and the $sub$ instruction requires the result at the beginning of the clock cycle. Signup and view all the answers

Explain the four simple compile time schemes discussed in the text for dealing with pipeline stalls caused by branch delays.

The four simple compile time schemes discussed are: 1) Freezing or flushing the pipeline, holding or deleting any instructions after the branch until the branch destination is known. 2) Treating every branch as not taken, allowing the hardware to continue as if the branch were not executed. 3) Using knowledge of the hardware scheme and branch behavior to minimize the branch penalty. 4) Using hardware-based schemes that dynamically predict branch behavior. Signup and view all the answers

Explain the key differences between the 'freeze/flush' and 'treat as not taken' approaches to handling branches in a pipeline, and the advantages and disadvantages of each.

The 'freeze/flush' approach simply holds or deletes instructions after a branch until the branch destination is known. This is simple but has a fixed branch penalty that cannot be reduced by software. The 'treat as not taken' approach allows the pipeline to continue as if the branch were not executed, but requires more complexity to 'back out' any changes if the branch is taken. The 'treat as not taken' approach allows the branch penalty to be reduced by software, but is more complex to implement. Signup and view all the answers

Describe the role of software in minimizing branch penalties according to the text, and explain how this differs from the hardware-based dynamic branch prediction schemes discussed.

The text states that in the four compile time schemes discussed, the software can try to minimize the branch penalty using knowledge of the hardware scheme and of branch behavior. This is in contrast to the hardware-based schemes that dynamically predict branch behavior mentioned later in the passage. The software-based approaches rely on static, compile-time knowledge to reduce branch penalties, while the hardware-based schemes use dynamic prediction at runtime. Signup and view all the answers

Explain the purpose and function of the forwarding paths shown in Figure C.5, and how they help address data hazards in the pipeline architecture described.

The text states that the key advantage of the forwarding mechanism described in the text is that it allows a result to be forwarded from cycles earlier in the pipeline. This forwarding of results helps address data hazards by allowing instructions to access the most up-to-date data, even if it has not yet been written back to the register file. Signup and view all the answers

Describe the role of the adder used to compute the potential branch target address during the ID stage, and explain why this is necessary even though a full ALU may not be needed for every instruction.

The text states that it is necessary to have an adder to compute the potential branch target address during ID. This is required even if a full ALU is not needed for every instruction, because the pipeline needs to be able to determine the branch target address in order to handle control hazards caused by branches. Signup and view all the answers

Explain the key purpose and benefit of pipelining the execution described in the previous section, and how this relates to the performance impact of a one-cycle stall in the pipeline.

The text states that the key advantage of pipelining the execution described in the previous section is that it improves performance. However, it also notes that a one-cycle stall in the pipeline impacts the performance. This indicates that the benefit of pipelining is to improve overall performance, but that pipeline stalls can negate some of this benefit by introducing delays. Signup and view all the answers

Study Notes

Pipelining and Hazards

Pipelining is a technique that exploits parallelism among instructions in a sequential instruction stream to increase performance.
A pipeline consists of several stages, each performing a specific function, such as instruction fetch, decode, execute, memory access, and write back.
Hazards arise when there are dependencies between instructions in the pipeline, causing stalls or incorrect results.

Types of Hazards

Structural hazards: occur when the hardware cannot support all possible combinations of instructions simultaneously in overlapped execution.
Data hazards: occur when an instruction depends on the results of a previous instruction in a way that is exposed by the overlapping of instructions in the pipeline.
Control hazards: arise from the pipelining of branches and other instructions that change the PC.

Pipeline Stages

Instruction Fetch (IF): fetches an instruction from memory and decodes it.
Instruction Decode (ID): decodes the instruction and determines the operation and operands.
Execute (EX): executes the instruction, performing the required operation.
Memory Access (MEM): accesses memory if the instruction is a load or store.
Write Back (WB): writes the result back to the register file.

Branch Prediction and Penalty

Branch prediction is crucial in pipelining, as misprediction can lead to stalls and reduced performance.
The branch penalty is the number of cycles lost due to a mispredicted branch.
Techniques for reducing branch penalties include:
- Freezing or flushing the pipeline
- Treating every branch as not taken
- Compiler-directed prediction
- Hardware-based branch prediction

Forwarding and Bypassing

Forwarding: passing the result of one instruction directly to another instruction that needs it, without storing it in a register.
Bypassing: forwarding the result of one instruction to another instruction, avoiding a stall.
Forwarding and bypassing can reduce stalls and improve pipeline performance.

Pipeline Interlocks

A pipeline interlock detects a hazard and stalls the pipeline until the hazard is cleared.
Pipeline interlocks can be used to handle data hazards, controlling the flow of instructions through the pipeline.

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Book-CAP3A - RISC Processor Pipeline Design Exercise