1 bit branch prediction


1-bit Branch Prediction Technique – In this technique hardware changes its assumption just after one false assumption.For example if hardware assumes branch to be taken but actually branch is not taken, then in next step hardware assumes branch to be not taken and vice-versa. 1-bit branch prediction machine is shown in the fig below: Explanation –

1-bit Branch Prediction Technique –

In this technique hardware changes its assumption just after one false assumption. For example if hardware assumes branch to be taken but actually branch is not taken, then in next step hardware assumes branch to be not taken and vice-versa.Jun 13, 2022


What is a 1-bit branch-prediction buffer?

1-bit Branch-Prediction Buffer: In this case, the Branch History Table (BHT) or Branch Prediction Buffer stores 1-bit values to indicate whether the branch is predicted to be taken / not taken. The lower bits of the PC address index this table of 1-bit values and get the prediction. This says whether the branch was recently taken or not.

What is the difference between a 1 bit and 2 bit predictor?

A 1-bit predictor uses one bit prediction, but is not very accurate. A 2-bit predictor has improved prediction accuracies and changes prediction only when there are two successful mispredictions. Correlating branch predictors correlate recently executed branches with the next branch.

What is the importance of bit branch prediction?

Branch prediction became more important with the introduction of pipelined superscalar processors like the Intel Pentium, DEC Alpha 21064, the MIPS R8000, and the IBM POWER series. These processors all rely on one-bit or simple bimodal predictors.

What is the difference between 2-bit branch predictor and correlating branch predictor?

We cannot get significant accuracy from 2-bit branch predictor also due to interference with other branches.So correlating branch prediction comes into picture which is also known as two-level branch predictor in which prediction accuracy is improved as it takes into consideration the recent behavior of other branches also.


What is a 1-bit predictor?

1-bit predictor: when we execute a branch, first check if it was taken when it was last executed: if yes, predict it will be taken this time; if no, predict it will not be taken this time.

What is an example of branch prediction?

The branch predictor may, for example, recognize that the conditional jump is taken more often than not, or that it is taken every second time. Branch prediction is not the same as branch target prediction. Branch prediction attempts to guess whether a conditional jump will be taken or not.

How does a 2 bit branch predictor work?

2-bit predictor: This predictor changes prediction only on two successive mispredictions. Two bits are maintained in the prediction buffer and there are four different states. Two states corresponding to a taken state and two corresponding to not taken state.

How accurate is branch prediction?

This scheme was implemented in the MIPS R10000 processor and the results showed prediction accuracy of ~90%.

What is branch prediction method?

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.

How fast is branch prediction?

on M1 the predicted-taken branch generally takes 3 cycles and unpredicted but taken has varying cost, depending on jmp length.

What are the two types of branch prediction techniques available?

Branch prediction schemes are of two types: static branch schemes and dynamic branch schemes. branch scheme (hardware techniques) is based on the hardware and it assembles the information during the run-time of the program.

Will branch prediction always take only 1 cycle?

If you don’t already use tricks like MIPS I early eval of branch conditions, your branch latency would be 2 cycles (IF to EX) for conditional branches. Static always-taken prediction would shorten that to 1 cycle (IF to ID).

What is PHT in computer architecture?

Taken. Not taken. Not taken. Page 8. Inf3 Computer Architecture – 2017-2018.

How can branch prediction be improved?

One thing you can do in a high-level language is to eliminate branches by expressing the problem in terms of lookups or arithmetic. This helps branch prediction work better on the remaining branches, because there’s more “history” available. I’ve made huge performance improvements to bottleneck code with this approach.

What is 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. Instr address Predicted PC. BTB is a cache that holds. (instr addr, predicted PC)

What is branch prediction?

1. Integrated branch prediction —The branch predictor becomes part of the instruction fetch unit and is constantly predicting branches, so as to drive the fetch pipeline. 2. Instruction prefetch —To deliver multiple instructions per clock, the instruction fetch unit will likely need to fetch ahead.

What happens when a prediction is wrong?

If the prediction is wrong, flush the pipeline and also flip prediction. So, every time a wrong prediction is made, the prediction bit is flipped. Usage of only some of the address bits may give us prediction about a wrong branch. But, the best option is to use only some of the least significant bits of the PC address.

Is the behavior of one branch dependent on the behavior of other branches?

But, many a times, we find that the behavior of one branch is dependent on the behavior of other branches. There is a correlation between different branches . Branch Predictors that use the behavior of other branches to make a prediction are called Correlating or two-level predictors.

When can a branch predictor base a prediction on a history?

When it encounters a conditional jump that has been seen several times before, then it can base the prediction on the history. The branch predictor may, for example, recognize that the conditional jump is taken more often than not, or that it is taken every second time.

Why do we use branch prediction hints?

Some processors allow branch prediction hints to be inserted into the code to tell whether the static prediction should be taken or not taken.

What is an agree predictor?

An agree predictor is a two-level adaptive predictor with globally shared history buffer and pattern history table, and an additional local saturating counter. The outputs of the local and the global predictors are XORed with each other to give the final prediction. The purpose is to reduce contentions in the pattern history table where two branches with opposite prediction happen to share the same entry in the pattern history table .

How many clock cycles does a branch misprediction take?

Modern microprocessors tend to have quite long pipelines so that the misprediction delay is between 10 and 20 clock cycles.

What is static branch prediction?

Static prediction is the simplest branch prediction technique because it does not rely on information about the dynamic history of code executing. Instead, it predicts the outcome of a branch based solely on the branch instruction.

What is the advantage of global branch predictor?

A global branch predictor does not keep a separate history record for each conditional jump. Instead it keeps a shared history of all conditional jumps . The advantage of a shared history is that any correlation between different conditional jumps is part of making the predictions. The disadvantage is that the history is diluted by irrelevant information if the different conditional jumps are uncorrelated, and that the history buffer may not include any bits from the same branch if there are many other branches in between. It may use a two-level adaptive predictor.

What is two way branching?

Two-way branching is usually implemented with a conditional jump instruction. A conditional jump can either be “not taken” and continue execution with the first branch of code which follows immediately after the conditional jump, or it can be “taken” and jump to a different place in program memory where the second branch of code is stored.


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