What is the difference between branch target buffer and branch prediction buffer?

What is the difference between branch target buffer and branch prediction buffer?

Branch prediction buffers contain prediction about whether the next branch will be taken (T) or not (NT), but it does not supply the target PC value. A Branch Target Buffer (BTB) does this. The control unit looks up the branch target buffer during the “F” phase.

What is branch target prediction buffer?

A branch target buffer (BTB) can reduce the performance penalty of branches in pipelined processors by predicting the path of the branch and caching information used by the branch.

What is meant by branch prediction?

Branch prediction is an approach to computer architecture that attempts to mitigate the costs of branching. Branch predication speeds up the processing of branch instructions with CPUs using pipelining. The technique involves only executing certain instructions if certain predicates are true.

What is branch target in computer architecture?

In computer architecture, a branch target predictor is the part of a processor that predicts the target of a taken conditional branch or an unconditional branch instruction before the target of the branch instruction is computed by the execution unit of the processor.

Why is branch prediction so important?

The purpose of the branch predictor is to improve the flow in the instruction pipeline. Branch predictors play a critical role in achieving high effective performance in many modern pipelined microprocessor architectures such as x86.

Do unconditional jumps need branch prediction?

Unconditional branches are not a problem for the branch predictor, because the branch predictor doesn’t have to predict them.

Why do we need branch prediction?

What are the different ways for branch prediction?

Implementation

• Static branch prediction.
• Dynamic branch prediction.
• Random branch prediction.
• Next line prediction.
• One-level branch prediction.
• Two-level predictor.
• Local branch prediction.
• Global branch prediction.

How accurate is branch prediction?

On the SPEC’89 benchmarks, very large bimodal predictors saturate at 93.5% correct, once every branch maps to a unique counter. The predictor table is indexed with the instruction address bits, so that the processor can fetch a prediction for every instruction before the instruction is decoded.

When to use branch predictor without branch prediction?

Without branch prediction, the processor would have to wait until the conditional jump instruction has passed the execute stage before the next instruction can enter the fetch stage in the pipeline. The branch predictor attempts to avoid this waste of time by trying to guess whether the conditional jump is most likely to be taken or not taken.

How does branch target prediction help the CPU?

Branch Target Prediction predicts the answer to “Where will I branch to?” Both of these are considered at the same time. An optimal CPU will be able to not only predict wether or not a branch will happen, but also where it will branch to. This would allow it to start pushing instructions through the pipeline that it predicts will be coming up.

How are branch delay slots used in Static prediction?

Both architectures define branch delay slots in order to utilize these fetched instructions. A more advanced form of static prediction presumes that backward branches will be taken and that forward branches will not. A backward branch is one that has a target address that is lower than its own address.

How does branch target buffer predict target address?

The Branch Target Buffer predicts the target address way ahead of this, so code fetch can start ASAP. A sophisticated BTB can recognize patterns, like an indirect jump that alternates between two targets. Good BTB performance is critical for indirect jumps.