Computer Architecture Questions Medium
Branch prediction is a technique used in computer architecture to improve the performance of processors by predicting the outcome of conditional branch instructions. Conditional branch instructions are instructions that determine the next instruction to be executed based on a condition, such as an if-else statement or a loop.
The concept of branch prediction is based on the observation that in many cases, the outcome of a branch instruction can be predicted with a high degree of accuracy. This prediction is made based on the history of previous branch instructions and their outcomes.
There are two main types of branch prediction techniques: static branch prediction and dynamic branch prediction.
Static branch prediction involves making predictions based on the characteristics of the branch instruction itself, without considering the history of previous branch instructions. For example, if a branch instruction always takes the same path, it can be predicted as always taken or always not taken.
Dynamic branch prediction, on the other hand, uses the history of previous branch instructions to make predictions. It maintains a record of the outcomes of previous branch instructions and uses this information to predict the outcome of future branch instructions. This prediction is typically done using a branch prediction table or a branch history table.
The branch prediction table stores the history of branch instructions and their outcomes. It is indexed by the address of the branch instruction and contains a prediction for each branch instruction. The prediction can be taken (branch is predicted to be taken) or not taken (branch is predicted to not be taken).
When a branch instruction is encountered, the branch prediction unit looks up the prediction in the branch prediction table. If the prediction matches the actual outcome of the branch instruction, the processor continues executing instructions from the predicted path, resulting in a performance improvement. However, if the prediction is incorrect, a pipeline flush occurs, and the processor has to discard the instructions that were fetched based on the incorrect prediction and start fetching instructions from the correct path.
To improve the accuracy of branch prediction, various techniques are used, such as branch history tables, branch target buffers, and pattern history tables. These techniques aim to capture more accurate patterns and improve the prediction accuracy.
Overall, branch prediction plays a crucial role in computer architecture by reducing the impact of conditional branch instructions on processor performance. By accurately predicting the outcome of branch instructions, processors can minimize pipeline stalls and improve instruction throughput, resulting in better overall performance.