Multistep-Ahead Branch Predictor for Microprocessor Instruction Fetching
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Solution Overview
Problem
In pipelined microprocessor architectures, branch predictors face delays due to the need for multiple timing cycles to generate branch prediction results, leading to meaningless predictions for subsequent instruction addresses that should be skipped, causing inefficiencies in instruction fetching.
Innovation Solution
A multistep-ahead branch prediction technology that performs parallel branch prediction on consecutive instruction-address blocks, using a branch predictor with a pipelined architecture and memory structures like branch target address cache and history table to skip meaningless predictions and accelerate instruction fetching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a traditional single-step branch predictor is used, then the device complexity is low, but the instruction fetching speed is slow due to sequential processing delays
Solution Approach 1:
The branch predictor is divided into multiple independent prediction units, each handling a specific instruction-address block. These units operate in parallel to generate predictions for multiple future blocks simultaneously, transforming a sequential single-step predictor into a parallel multistep-ahead system that accelerates instruction fetching without proportionally increasing complexity
Solution Approach 2:
The branch predictor performs predictions for N-1 instruction-address blocks in advance before they are actually needed for execution. By generating predictions for future blocks ahead of time and storing them in buffers, the system eliminates waiting delays during instruction fetching, allowing the pipeline to continue operating at full speed
2Loss of time
If branch prediction is performed for multiple instruction-address blocks in parallel, then the prediction accuracy improves by skipping meaningless predictions, but the memory access complexity increases
Solution Approach 1:
The prediction memory is segmented into multiple banks, with each bank storing predictions for specific instruction-address blocks. This segmentation allows parallel access to multiple prediction entries simultaneously without creating memory conflicts, as each bank can be accessed independently by different prediction units at the same time
Solution Approach 2:
The memory system transitions from a single-dimensional sequential access structure to a multi-dimensional parallel access structure. By organizing memory banks and prediction entries in a way that enables simultaneous access from multiple dimensions (different banks, different entry points), the system achieves O(1) access time for multiple predictions without proportionally increasing access complexity
3Productivity
If the branch predictor waits for complete pipeline execution, then prediction accuracy is maintained, but instruction fetching efficiency decreases due to delays
Solution Approach 1:
The system performs branch predictions in advance for instruction-address blocks that are ahead of the current pipeline position. These preliminary predictions are stored and held until needed, allowing the instruction fetching to proceed without waiting for pipeline completion while maintaining prediction accuracy through the use of valid prediction data
Solution Approach 2:
The branch predictor implements a feedback mechanism where prediction results are validated and updated based on actual pipeline execution outcomes. When predictions are made in advance, the system receives feedback from the pipeline execution and uses this information to confirm or correct predictions, ensuring reliability while maintaining high fetching efficiency
Data Source
AI summary
A microprocessor with a multistep-ahead branch predictor is shown. The branch predictor is coupled to an instruction cache and has an N-stage pipelined architecture, which is configured to perform branch prediction to control the instruction fetching of the instruction cache. The branch predictor performs branch prediction for (N−1) instruction-address blocks in parallel, wherein the (N−1) instruction-address blocks include a starting instruction-address block and (N−2) subsequent instruction-address blocks. The branch predictor is thereby ahead of branch prediction of the starting instruction-address block. The branch predictor stores reference information about branch prediction in at least one memory and performs a parallel search of the memory for the branch prediction of the (N-1) instruction-address blocks.