Computer Organisation Chapter 6

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Questions and Answers

What is the primary issue with the lower bits address indexing in the example provided?

Two distinct branch instructions may end up with the same index (correct)

The indexing is too slow

The indexing is not compatible with all memory types

The indexing is not scalable

What is the limitation of the 1-bit prediction scheme in the context of a loop?

It is not suitable for conditional branches

It is not efficient in terms of memory usage

It is too complex to implement

It is likely to mispredict twice when a branch is not taken (correct)

What is the main purpose of the example shown in Listing 6.2?

To illustrate the 1-bit prediction scheme

To show how to optimize memory usage

To explain the concept of dynamic branch prediction

To demonstrate the limitation of the lower bits address indexing (correct)

What is the consequence of the misprediction in the 1-bit prediction scheme?

The prediction bit is flipped Signup and view all the answers

What is the primary issue with the indexing scheme shown in Listing 6.3?

Two distinct branch instructions may end up with the same index Signup and view all the answers

What is the purpose of the code shown in Listing 6.4?

To illustrate the limitation of the 1-bit prediction scheme in a loop Signup and view all the answers

What is the main reason why speculative execution is required in certain types of loops?

To handle conditional branch instructions with RAW dependence on the loop iteration Signup and view all the answers

What is the consequence of not having efficient branch prediction in speculative superscalar execution?

Decreased performance due to unrecovered hardware cost Signup and view all the answers

What is the main advantage of dynamic branch prediction over static branch prediction?

It considers the fact that branch instructions can be executed many times during program execution Signup and view all the answers

What is the result of increasing the number of executed instructions per clock cycle?

Decreased CPI and increased performance Signup and view all the answers

What is the maximum number of times the branch prediction will miss in the given code snippet?

5000 Signup and view all the answers

What is the purpose of reservation stations in speculative superscalar execution?

To hold instructions waiting for their operands to be available Signup and view all the answers

What is the primary function of branch-prediction buffers?

To relate one bit to each branch decision Signup and view all the answers

What is the main challenge in controlling dependencies in speculative superscalar execution?

Detecting control dependencies Signup and view all the answers

What is the consequence of a branch outcome misprediction in speculative superscalar execution?

The incorrect branch is executed, but the correct branch is eventually recovered Signup and view all the answers

What is the limitation of the 1-bit branch-prediction buffer?

The prediction is a hint assumed to be correct, but may not be Signup and view all the answers

What is the main reason why speculative execution is necessary in certain types of loops?

To handle conditional branch instructions with RAW dependence on the loop iteration Signup and view all the answers

What is the purpose of using the low-order address bits in the branch-prediction buffer?

To have variations, i.e., different addresses considering the use of just the low-order bits Signup and view all the answers

What is the state of the 1-bit branch-prediction buffer when the prediction is correct?

TAKEN Signup and view all the answers

What happens when the branch prediction is incorrect in the 1-bit branch-prediction buffer?

The prediction bit is inverted and stored back Signup and view all the answers

What is the name of the approach that considers the aid of a small memory indexed by the lower portion of the branch instruction address?

Branch History Table Signup and view all the answers

What is the primary goal of using branch prediction in the given code snippet?

To improve the accuracy of the branch predictions Signup and view all the answers

What is the issue with the addresses in Listing 6.3?

Two distinct branch instructions end up with the same index. Signup and view all the answers

What happens to the prediction bit in the 1-bit prediction scheme when a branch is mispredicted?

The prediction bit is flipped. Signup and view all the answers

What is the purpose of the example in Listing 6.2?

To demonstrate the use of lower bits addresses in indexing small memory. Signup and view all the answers

What is the consequence of the 1-bit prediction scheme's shortcoming in a loop?

The scheme will mispredict twice, rather than once when the branch is not taken. Signup and view all the answers

What is the relationship between the branch instruction address and the small memory address in Listing 6.2?

The small memory address is the lower bits of the branch instruction address. Signup and view all the answers

What is the effect of the misprediction on the branch prediction bit in the 1-bit prediction scheme?

The branch prediction bit is flipped. Signup and view all the answers

What is the primary reason why speculative execution is required in certain types of loops?

To handle the RAW dependence on the loop iteration Signup and view all the answers

What is the main challenge in controlling dependencies in speculative superscalar execution?

Detecting dependencies between instructions Signup and view all the answers

What is the primary benefit of dynamic branch prediction over static branch prediction?

It considers the fact that branch instructions can be executed many times Signup and view all the answers

What is the main purpose of additional memory in speculative superscalar execution?

To guarantee an eventual correction in case of a prediction error Signup and view all the answers

What is the result of increasing the number of executed instructions per clock cycle?

The potential instructions flow increases Signup and view all the answers

What is the main advantage of using speculative execution in certain types of loops?

It minimizes the delays caused by branches Signup and view all the answers

What is the primary function of reservation stations in speculative superscalar execution?

To schedule instructions in the correct order Signup and view all the answers

What is the consequence of not having efficient branch prediction in speculative superscalar execution?

The performance does not pay off the new hardware cost Signup and view all the answers

What is the primary objective of using a branch-prediction buffer?

To reduce the number of mispredictions Signup and view all the answers

What is the main characteristic of a 1-bit branch-prediction buffer?

It is a 2-state finite-state machine with states: TAKEN and UNTAKEN Signup and view all the answers

What is the role of the low-order address bits in the branch-prediction buffer?

To index the branch-prediction buffer Signup and view all the answers

What happens when the branch prediction is incorrect in the 1-bit branch-prediction buffer?

The prediction bit is inverted and stored back Signup and view all the answers

What is the primary benefit of using dynamic branch prediction over static branch prediction?

It can adapt to changing branch behavior Signup and view all the answers

What is the main limitation of the 1-bit branch-prediction buffer?

It can only store a single bit of information Signup and view all the answers

What is the purpose of the branch history table?

To provide a hint about the branch direction Signup and view all the answers

What is the consequence of a misprediction in the 1-bit branch-prediction buffer?

The instruction pipeline is flushed Signup and view all the answers

Study Notes

Speculative Execution

Speculative execution is required when unrolling loops with conditional branch instructions that have a RAW dependence on the loop iteration.
It requires more hardware, such as reservation stations and functional units, as well as more complex control and dependencies detection.
Additional memory is needed to guarantee an eventual correction in case of a prediction error.
The architecture needs more efficient branch prediction to pay off the new hardware cost with performance.

Control Dependencies

As the number of executed instructions per clock cycle increases, the potential instructions flow also increases, reducing CPI.
Delays caused by branches can seriously impact performance.
A "simple" strategy to minimize this problem is to not stop or reduce speed in branches.

Dynamic Branch Prediction

Dynamic branch prediction considers the fact that branch instructions can be executed many times during program execution, e.g., inside loops.
This approach is different from static branch prediction strategies, such as predicted-not-taken and predicted-taken.
The predicted-not-taken approach may be inefficient in loops, leading to many mispredictions.

Branch-Prediction Buffers

A possible solution to the problem is to use branch-prediction buffers, which relate one bit to each branch decision.
The buffer is a small memory indexed by the lower portion of the branch instruction address.
The memory contains a bit that indicates whether a branch was recently taken or untaken.
This approach is a hint assumed to be correct, and the next instruction fetch begins in the predicted direction.
If the hint is wrong, the prediction bit is inverted and stored back.

1-Bit Prediction

The 1-bit prediction scheme is a simple dynamic branch-prediction scheme.
It is a 2-state finite-state machine (FSM) with states: TAKEN and UNTAKEN.
The scheme uses just the low-order address bits to have variations, i.e., different addresses considering the use of just the low-order bits.
However, there may be an inconvenient or even problematic case regarding these low-order address bits, where two distinct branch instructions end up with the same index.
The 1-bit prediction scheme has the shortcoming that it may mispredict twice, rather than once, when a branch is not taken in a loop.

Speculative Execution

Speculative execution is required when unrolling loops with conditional branch instructions that have a RAW dependence on the loop iteration.
It requires more hardware, such as reservation stations and functional units, as well as more complex control and dependencies detection.
Additional memory is needed to guarantee an eventual correction in case of a prediction error.
The architecture needs more efficient branch prediction to pay off the new hardware cost with performance.

Control Dependencies

As the number of executed instructions per clock cycle increases, the potential instructions flow also increases, reducing CPI.
Delays caused by branches can seriously impact performance.
A "simple" strategy to minimize this problem is to not stop or reduce speed in branches.

Dynamic Branch Prediction

Dynamic branch prediction considers the fact that branch instructions can be executed many times during program execution, e.g., inside loops.
This approach is different from static branch prediction strategies, such as predicted-not-taken and predicted-taken.
The predicted-not-taken approach may be inefficient in loops, leading to many mispredictions.

Branch-Prediction Buffers

A possible solution to the problem is to use branch-prediction buffers, which relate one bit to each branch decision.
The buffer is a small memory indexed by the lower portion of the branch instruction address.
The memory contains a bit that indicates whether a branch was recently taken or untaken.
This approach is a hint assumed to be correct, and the next instruction fetch begins in the predicted direction.
If the hint is wrong, the prediction bit is inverted and stored back.

1-Bit Prediction

The 1-bit prediction scheme is a simple dynamic branch-prediction scheme.
It is a 2-state finite-state machine (FSM) with states: TAKEN and UNTAKEN.
The scheme uses just the low-order address bits to have variations, i.e., different addresses considering the use of just the low-order bits.
However, there may be an inconvenient or even problematic case regarding these low-order address bits, where two distinct branch instructions end up with the same index.
The 1-bit prediction scheme has the shortcoming that it may mispredict twice, rather than once, when a branch is not taken in a loop.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz covers branch prediction and speculative superscalar processors in computer organisation, focusing on loop unrolling and speculative execution.

Computer Organisation Chapter 6

Podcast Beta

Questions and Answers

What is the primary issue with the lower bits address indexing in the example provided?

What is the limitation of the 1-bit prediction scheme in the context of a loop?

What is the main purpose of the example shown in Listing 6.2?

What is the consequence of the misprediction in the 1-bit prediction scheme?

What is the primary issue with the indexing scheme shown in Listing 6.3?

What is the purpose of the code shown in Listing 6.4?

What is the main reason why speculative execution is required in certain types of loops?

What is the consequence of not having efficient branch prediction in speculative superscalar execution?

What is the main advantage of dynamic branch prediction over static branch prediction?

What is the result of increasing the number of executed instructions per clock cycle?

What is the maximum number of times the branch prediction will miss in the given code snippet?

What is the purpose of reservation stations in speculative superscalar execution?

What is the primary function of branch-prediction buffers?

What is the main challenge in controlling dependencies in speculative superscalar execution?

What is the consequence of a branch outcome misprediction in speculative superscalar execution?

What is the limitation of the 1-bit branch-prediction buffer?

What is the main reason why speculative execution is necessary in certain types of loops?

What is the purpose of using the low-order address bits in the branch-prediction buffer?

What is the state of the 1-bit branch-prediction buffer when the prediction is correct?

What happens when the branch prediction is incorrect in the 1-bit branch-prediction buffer?

What is the name of the approach that considers the aid of a small memory indexed by the lower portion of the branch instruction address?

What is the primary goal of using branch prediction in the given code snippet?

What is the issue with the addresses in Listing 6.3?

What happens to the prediction bit in the 1-bit prediction scheme when a branch is mispredicted?

What is the purpose of the example in Listing 6.2?

What is the consequence of the 1-bit prediction scheme's shortcoming in a loop?

What is the relationship between the branch instruction address and the small memory address in Listing 6.2?

What is the effect of the misprediction on the branch prediction bit in the 1-bit prediction scheme?

What is the primary reason why speculative execution is required in certain types of loops?

What is the main challenge in controlling dependencies in speculative superscalar execution?

What is the primary benefit of dynamic branch prediction over static branch prediction?

What is the main purpose of additional memory in speculative superscalar execution?

What is the result of increasing the number of executed instructions per clock cycle?

What is the main advantage of using speculative execution in certain types of loops?

What is the primary function of reservation stations in speculative superscalar execution?

What is the consequence of not having efficient branch prediction in speculative superscalar execution?

What is the primary objective of using a branch-prediction buffer?

What is the main characteristic of a 1-bit branch-prediction buffer?

What is the role of the low-order address bits in the branch-prediction buffer?

What happens when the branch prediction is incorrect in the 1-bit branch-prediction buffer?

What is the primary benefit of using dynamic branch prediction over static branch prediction?

What is the main limitation of the 1-bit branch-prediction buffer?

What is the purpose of the branch history table?

What is the consequence of a misprediction in the 1-bit branch-prediction buffer?

Study Notes

Speculative Execution

Control Dependencies

Dynamic Branch Prediction

Branch-Prediction Buffers

1-Bit Prediction

Speculative Execution

Control Dependencies

Dynamic Branch Prediction

Branch-Prediction Buffers

1-Bit Prediction

Studying That Suits You

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