CAP6 - Computer Architecture

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Questions and Answers

What is the primary issue with the lower bits address in Listing 6.2?

It always results in the same index.

It is only applicable to large memory addresses.

It may result in the same index for distinct branch instructions. (correct)

It is only applicable to small memory addresses.

Why does the 1-bit prediction scheme mispredict twice in a loop?

Because it always predicts the branch will be taken.

Because it always predicts the branch will not be taken.

Because the prediction bit is flipped after a misprediction. (correct)

Because the prediction bit is not flipped after a misprediction.

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

It never results in mispredictions.

It always results in one misprediction.

It always results in two mispredictions.

It may result in one or two mispredictions. (correct)

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

The branch instruction address determines the small memory address. Signup and view all the answers

What is the primary concern regarding the use of lower bits of the branch instruction address?

It may result in distinct branch instructions having the same index. Signup and view all the answers

What is the significance of the lower bits in the branch instruction addresses, and how do they affect the indexing of small memory?

The lower bits in the branch instruction addresses are used to index small memory, which can lead to problems if different branch instructions have the same lower bits, resulting in the same index. Signup and view all the answers

How does the 1-bit prediction scheme mispredict a branch in a loop, and what is the consequence of this misprediction?

The 1-bit prediction scheme mispredicts a branch in a loop because the prediction bit is flipped after a misprediction, causing the scheme to mispredict twice rather than once. This leads to a significant performance degradation. Signup and view all the answers

What is the relationship between the branch instruction address and the small memory address in Listing 6.2, and how does this affect the indexing of small memory?

The lower bits of the branch instruction address are used to index small memory, which can lead to problems if different branch instructions have the same lower bits, resulting in the same index. Signup and view all the answers

What is the purpose of the inner loop in Listing 6.4, and how does it relate to the 1-bit prediction scheme?

The inner loop in Listing 6.4 illustrates a scenario where the 1-bit prediction scheme is likely to mispredict twice, rather than once, due to the prediction bit being flipped after a misprediction. Signup and view all the answers

What are the implications of using the lower bits of the branch instruction address to index small memory, and how can this be mitigated?

Using the lower bits of the branch instruction address to index small memory can lead to problems if different branch instructions have the same lower bits, resulting in the same index. This can be mitigated by using more sophisticated prediction schemes or alternative indexing methods. Signup and view all the answers

What is the function of the branch-target buffer in the IF phase?

To use the branch address instruction as an index Signup and view all the answers

What is the purpose of correlating predictors?

To determine the behavior of the next branch based on a previous branch Signup and view all the answers

In Listing 6.5, what is the purpose of the code in lines 1 and 3?

To create two predictions for branch 2 based on branch 1 Signup and view all the answers

What is the relationship between the branch address instruction and the branch-target buffer?

The branch-target buffer uses the branch address instruction as an index Signup and view all the answers

What is the advantage of using correlating predictors?

It can create multiple predictions for a single branch Signup and view all the answers

What is the significance of determining the next predicted PC at the end of the IF phase in the context of branch-target buffer?

It allows for faster prediction of the next instruction, as it can determine the next predicted PC without waiting for the ID phase. Signup and view all the answers

How does the correlating predictors technique improve branch prediction accuracy?

It improves branch prediction accuracy by creating separate predictions for a branch based on the outcome of a previous branch, allowing for more informed predictions. Signup and view all the answers

What is the benefit of using the branch-target buffer in terms of pipeline performance?

It allows for faster prediction of the next instruction, reducing the delay between the IF and ID phases and improving pipeline performance. Signup and view all the answers

How does the correlating predictors technique address the limitation of single branch prediction?

It addresses the limitation by creating multiple predictions for a branch based on the outcome of previous branches, allowing for more accurate predictions in complex branch patterns. Signup and view all the answers

What is the advantage of using the branch-target buffer over waiting for the ID phase to determine the next predicted PC?

It allows for faster prediction of the next instruction, reducing the delay between the IF and ID phases and improving pipeline performance. Signup and view all the answers

What is the purpose of the `bnez x3, L1` instruction in the generated RISC-V code?

To branch to label L1 if the value in x3 is not equal to zero, indicating that the value of aa is not equal to 2. Signup and view all the answers

What is the effect of the `addi x3, x1, -2` instruction on the value of x3?

The value of x3 is set to x1 minus 2, which will be used to determine the branch condition. Signup and view all the answers

What is the purpose of the `add x1, x0, x0` instruction?

To set the value of x1 to zero, effectively terminating the execution of the program. Signup and view all the answers

What is the relationship between the `bnez x3, L1` instruction and the value of aa?

The <code>bnez x3, L1</code> instruction branches to L1 if the value of aa is not equal to 2. Signup and view all the answers

What is the condition that determines the branch taken by the `bnez x3, L1` instruction?

The branch is taken if x3 is not equal to zero, which means aa is not equal to 2. Signup and view all the answers

What is the value of x3 after executing the `addi x3, x1, -2` instruction?

aa - 2 Signup and view all the answers

What is the purpose of the `bnez x3, L1` instruction in the generated RISC-V code?

To branch to label L1 if the value of x3 is not equal to 2 Signup and view all the answers

What is the effect of the `add x1, x0, x0` instruction on the value of aa?

The value of aa is set to 0 Signup and view all the answers

What is the relationship between the `bnez x3, L1` instruction and the value of aa?

The branch is taken if the value of aa is not equal to 2 Signup and view all the answers

What is the purpose of assigning the variable aa to the RISC-V register x1?

To store the value of aa in a register Signup and view all the answers

What is the primary reason for issuing multiple instructions simultaneously in a superscalar processor?

To increase the CPI below 1 Signup and view all the answers

What is a key challenge in implementing a multiple issue superscalar processor?

Identifying dependencies among instructions in the sequence Signup and view all the answers

What is the purpose of duplicating or multiplying the CDB bandwidth in a multiple issue superscalar processor?

To support parallel instruction issue Signup and view all the answers

What is a key characteristic of a conditional branch instruction in a multiple issue superscalar processor?

It is never executed in parallel with another instruction Signup and view all the answers

What is the purpose of checking dependencies among instructions in a multiple issue superscalar processor?

To ensure correctness of instruction execution Signup and view all the answers

What is a key benefit of using out-of-order and speculative executions in a superscalar processor?

It allows the CPI to approach 1 but not below Signup and view all the answers

What is the purpose of the decode cycle in a multiple issue superscalar processor?

To identify dependencies among instructions Signup and view all the answers

What is a key challenge in achieving a CPI less than 1 in a superscalar processor?

Issuing multiple instructions simultaneously Signup and view all the answers

What is the relationship between the CDB bandwidth and the instruction fetch and decode hardware units?

The CDB bandwidth is dependent on the instruction fetch and decode hardware units Signup and view all the answers

What is the primary benefit of using speculative execution in a superscalar processor?

It improves instruction-level parallelism Signup and view all the answers

What technique aids in getting the CPI closer to 1, but not below it?

Out-of-order and speculative executions Signup and view all the answers

What is the primary requirement to achieve a CPI less than 1?

Issuing multiple instructions simultaneously (multiple issue) Signup and view all the answers

What problem must be fixed to issue multiple instructions in the same clock cycle?

Identifying dependencies among instructions in the sequence (decode cycle) Signup and view all the answers

Why is it not possible to issue a conditional branch with another instruction in the same clock cycle?

Because a conditional branch cannot be issued with another instruction in the same clock cycle Signup and view all the answers

What is the purpose of duplicating or multiplying the CDB bandwidth in a multiple issue superscalar processor?

To accommodate parallel instructions issue Signup and view all the answers

What is the benefit of multiple issue superscalar processors?

Achieving a CPI less than 1 Signup and view all the answers

What is the purpose of checking dependencies among instructions in a multiple issue superscalar processor?

To ensure correct execution of instructions Signup and view all the answers

What is the key challenge in implementing a multiple issue superscalar processor?

Identifying dependencies among instructions in the sequence (decode cycle) Signup and view all the answers

What is the purpose of the decode cycle in a multiple issue superscalar processor?

To identify dependencies among instructions Signup and view all the answers

What is a key benefit of using speculative execution in a superscalar processor?

Improved performance by reducing CPI Signup and view all the answers

Study Notes

Speculative Execution

Speculative execution is required when a conditional branch instruction has a RAW dependence on the loop iteration.
It needs more hardware, such as reservation stations and functional units, more complex control and dependencies detection, and additional memory to correct prediction errors.
The architecture requires 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 instruction flow also increases, leading to decreased CPI.
Delays caused by branches can seriously impact performance, making it essential to minimize them.

Dynamic Branch Prediction

Dynamic branch prediction considers the fact that branch instructions can be executed many times during program execution, such as inside loops.
The predicted-not-taken approach may be inefficient in loops, leading to many branch prediction misses.

Branch-Prediction Buffers

1-bit prediction is a simple dynamic branch-prediction scheme that uses a bit to relate to each branch decision.
The prediction is based on the last loop iteration, and the target branch address needs to be computed.
Branch-prediction buffers are a type of branch history table that uses 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.
The prediction is a hint that is 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 Branch-Prediction Buffer

The 1-bit branch-prediction buffer is like a 2-state finite-state machine (FSM) with states: TAKEN and UNTAKEN.
Using only the low-order address bits for indexing can lead to variations and different addresses.
However, using low-order address bits can result in problematic cases, such as two distinct branch instructions with the same index.
The 1-bit prediction scheme has the shortcoming of mispredicting twice, rather than once, when a branch is almost always taken in a loop.

Speculative Execution

Speculative execution is required when a conditional branch instruction has a RAW dependence on the loop iteration.
It needs more hardware, such as reservation stations and functional units, more complex control and dependencies detection, and additional memory to correct prediction errors.
The architecture requires 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 instruction flow also increases, leading to decreased CPI.
Delays caused by branches can seriously impact performance, making it essential to minimize them.

Dynamic Branch Prediction

Dynamic branch prediction considers the fact that branch instructions can be executed many times during program execution, such as inside loops.
The predicted-not-taken approach may be inefficient in loops, leading to many branch prediction misses.

Branch-Prediction Buffers

1-bit prediction is a simple dynamic branch-prediction scheme that uses a bit to relate to each branch decision.
The prediction is based on the last loop iteration, and the target branch address needs to be computed.
Branch-prediction buffers are a type of branch history table that uses 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.
The prediction is a hint that is 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 Branch-Prediction Buffer

The 1-bit branch-prediction buffer is like a 2-state finite-state machine (FSM) with states: TAKEN and UNTAKEN.
Using only the low-order address bits for indexing can lead to variations and different addresses.
However, using low-order address bits can result in problematic cases, such as two distinct branch instructions with the same index.
The 1-bit prediction scheme has the shortcoming of mispredicting twice, rather than once, when a branch is almost always taken in a loop.

Branch Prediction & Speculative Superscalar Processors

Using a 2-bit prediction buffer can reduce misprediction to one case.

Branch-Target Buffers

The branch-target buffer technique stores the predicted address for the next instruction following a branch.
The buffer has two columns: the first contains addresses of predicted-taken branches, and the second contains the predicted PC (next PC after the branch).
A "new" fetching begins immediately at the predicted PC when a match is found in the buffer.
The branch-target buffer is similar to a cache memory.

Branch-Target Buffer Operation

The PC of the instruction to fetch is searched for a match in the buffer.
A match indicates the branch is taken with the predicted PC as the next PC.
The buffer determines the next predicted PC at the end of the IF phase.

Correlating Predictors

Correlating predictors use a previous branch to determine the behavior of the next branch.
Two predictions can be made for the next branch based on whether the previous branch was taken or not taken.
Examples of correlating predictors are shown in Listings 6.5, 6.6, and 6.7.

Branch Prediction & Speculative Superscalar Processors

Using a 2-bit prediction buffer can reduce misprediction to one case.

Branch-Target Buffers

The branch-target buffer technique stores the predicted address for the next instruction following a branch.
The buffer has two columns: the first contains addresses of predicted-taken branches, and the second contains the predicted PC (next PC after the branch).
A "new" fetching begins immediately at the predicted PC when a match is found in the buffer.
The branch-target buffer is similar to a cache memory.

Branch-Target Buffer Operation

The PC of the instruction to fetch is searched for a match in the buffer.
A match indicates the branch is taken with the predicted PC as the next PC.
The buffer determines the next predicted PC at the end of the IF phase.

Correlating Predictors

Correlating predictors use a previous branch to determine the behavior of the next branch.
Two predictions can be made for the next branch based on whether the previous branch was taken or not taken.
Examples of correlating predictors are shown in Listings 6.5, 6.6, and 6.7.

Branch Prediction & Speculative Superscalar Processors

RISC-V code is generated from C language code.
In the generated code, C language variables are assigned to RISC-V registers.
Specifically, variable aa is assigned to register x1, and variable bb is assigned to register x2.
The instruction addi x3, x1, -2 performs an operation on the value of aa and stores the result in register x3.
The instruction bnez x3, L1 is a branch instruction that checks if the value of aa is not equal to 2, and jumps to label L1 if true.
The instruction add x1, x0, x0 resets the value of aa to 0.

Branch Prediction & Speculative Superscalar Processors

RISC-V code is generated from C language code.
In the generated code, C language variables are assigned to RISC-V registers.
Specifically, variable aa is assigned to register x1, and variable bb is assigned to register x2.
The instruction addi x3, x1, -2 performs an operation on the value of aa and stores the result in register x3.
The instruction bnez x3, L1 is a branch instruction that checks if the value of aa is not equal to 2, and jumps to label L1 if true.
The instruction add x1, x0, x0 resets the value of aa to 0.

Multiple Issue Superscalar Processors

Techniques like out-of-order and speculative executions help to reduce CPI, but it cannot be less than 1.
To achieve CPI less than 1, multiple instructions must be issued simultaneously, which is known as multiple issue.
Multiple issue allows issuing various instructions in the same clock cycle, but dependencies among instructions must be identified.
Duplicating or multiplying instruction fetch and decode hardware units is necessary to issue multiple instructions in parallel.
The common data bus (CDB) bandwidth must also be duplicated or multiplied to accommodate multiple instructions.

Branch Prediction

A conditional branch is not issued with another instruction in the same clock cycle.
Dependency checks are performed to ensure that the register written by instruction i+1 is not read by instruction i+2.
The reservation station or ROB is aware of dependencies on the first instruction.

Code Example

The code example (Listing 6.11) increments each element of an integer array.
The code has a loop that loads data from an array, performs an operation, and stores the result back into the array.
The RISC-V assembly code (Listing 6.13) is the translated version of the C code.
The assembly code includes a loop that loads data from an array using the fld instruction.

Multiple Issue Superscalar Processors

Techniques like out-of-order and speculative executions help to reduce CPI, but it cannot be less than 1.
To achieve CPI less than 1, multiple instructions must be issued simultaneously, which is known as multiple issue.
Multiple issue allows issuing various instructions in the same clock cycle, but dependencies among instructions must be identified.
Duplicating or multiplying instruction fetch and decode hardware units is necessary to issue multiple instructions in parallel.
The common data bus (CDB) bandwidth must also be duplicated or multiplied to accommodate multiple instructions.

Branch Prediction

A conditional branch is not issued with another instruction in the same clock cycle.
Dependency checks are performed to ensure that the register written by instruction i+1 is not read by instruction i+2.
The reservation station or ROB is aware of dependencies on the first instruction.

Code Example

The code example (Listing 6.11) increments each element of an integer array.
The code has a loop that loads data from an array, performs an operation, and stores the result back into the array.
The RISC-V assembly code (Listing 6.13) is the translated version of the C code.
The assembly code includes a loop that loads data from an array using the fld instruction.

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Quiz on speculative execution, branch prediction, and superscalar processors in computer architecture. Covers loop unrolling and RAW dependencies.

CAP6 - Computer Architecture

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Questions and Answers

What is the primary issue with the lower bits address in Listing 6.2?

Why does the 1-bit prediction scheme mispredict twice in a loop?

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

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

What is the primary concern regarding the use of lower bits of the branch instruction address?

What is the significance of the lower bits in the branch instruction addresses, and how do they affect the indexing of small memory?

How does the 1-bit prediction scheme mispredict a branch in a loop, and what is the consequence of this misprediction?

What is the relationship between the branch instruction address and the small memory address in Listing 6.2, and how does this affect the indexing of small memory?

What is the purpose of the inner loop in Listing 6.4, and how does it relate to the 1-bit prediction scheme?

What are the implications of using the lower bits of the branch instruction address to index small memory, and how can this be mitigated?

What is the function of the branch-target buffer in the IF phase?

What is the purpose of correlating predictors?

In Listing 6.5, what is the purpose of the code in lines 1 and 3?

What is the relationship between the branch address instruction and the branch-target buffer?

What is the advantage of using correlating predictors?

What is the significance of determining the next predicted PC at the end of the IF phase in the context of branch-target buffer?

How does the correlating predictors technique improve branch prediction accuracy?

What is the benefit of using the branch-target buffer in terms of pipeline performance?

How does the correlating predictors technique address the limitation of single branch prediction?

What is the advantage of using the branch-target buffer over waiting for the ID phase to determine the next predicted PC?

What is the purpose of the bnez x3, L1 instruction in the generated RISC-V code?

What is the effect of the addi x3, x1, -2 instruction on the value of x3?

What is the purpose of the add x1, x0, x0 instruction?

What is the relationship between the bnez x3, L1 instruction and the value of aa?

What is the condition that determines the branch taken by the bnez x3, L1 instruction?

What is the value of x3 after executing the addi x3, x1, -2 instruction?

What is the purpose of the bnez x3, L1 instruction in the generated RISC-V code?

What is the effect of the add x1, x0, x0 instruction on the value of aa?

What is the relationship between the bnez x3, L1 instruction and the value of aa?

What is the purpose of assigning the variable aa to the RISC-V register x1?

What is the primary reason for issuing multiple instructions simultaneously in a superscalar processor?

What is a key challenge in implementing a multiple issue superscalar processor?

What is the purpose of duplicating or multiplying the CDB bandwidth in a multiple issue superscalar processor?

What is a key characteristic of a conditional branch instruction in a multiple issue superscalar processor?

What is the purpose of checking dependencies among instructions in a multiple issue superscalar processor?

What is a key benefit of using out-of-order and speculative executions in a superscalar processor?

What is the purpose of the decode cycle in a multiple issue superscalar processor?

What is a key challenge in achieving a CPI less than 1 in a superscalar processor?

What is the relationship between the CDB bandwidth and the instruction fetch and decode hardware units?

What is the primary benefit of using speculative execution in a superscalar processor?

What technique aids in getting the CPI closer to 1, but not below it?

What is the primary requirement to achieve a CPI less than 1?

What problem must be fixed to issue multiple instructions in the same clock cycle?

Why is it not possible to issue a conditional branch with another instruction in the same clock cycle?

What is the purpose of duplicating or multiplying the CDB bandwidth in a multiple issue superscalar processor?

What is the benefit of multiple issue superscalar processors?

What is the purpose of checking dependencies among instructions in a multiple issue superscalar processor?

What is the key challenge in implementing a multiple issue superscalar processor?

What is the purpose of the decode cycle in a multiple issue superscalar processor?

What is a key benefit of using speculative execution in a superscalar processor?

Study Notes

Speculative Execution

Control Dependencies

Dynamic Branch Prediction

Branch-Prediction Buffers

1-bit Branch-Prediction Buffer

Speculative Execution

Control Dependencies

Dynamic Branch Prediction

Branch-Prediction Buffers

1-bit Branch-Prediction Buffer

Branch Prediction & Speculative Superscalar Processors

Branch-Target Buffers

Branch-Target Buffer Operation

Correlating Predictors

Branch Prediction & Speculative Superscalar Processors

Branch-Target Buffers

Branch-Target Buffer Operation

Correlating Predictors

Branch Prediction & Speculative Superscalar Processors

Branch Prediction & Speculative Superscalar Processors

Multiple Issue Superscalar Processors

Branch Prediction

Code Example

Multiple Issue Superscalar Processors

Branch Prediction

What is the purpose of the `bnez x3, L1` instruction in the generated RISC-V code?

What is the effect of the `addi x3, x1, -2` instruction on the value of x3?

What is the purpose of the `add x1, x0, x0` instruction?

What is the relationship between the `bnez x3, L1` instruction and the value of aa?

What is the condition that determines the branch taken by the `bnez x3, L1` instruction?

What is the value of x3 after executing the `addi x3, x1, -2` instruction?

What is the purpose of the `bnez x3, L1` instruction in the generated RISC-V code?

What is the effect of the `add x1, x0, x0` instruction on the value of aa?

What is the relationship between the `bnez x3, L1` instruction and the value of aa?