This document will describe the important structures and external interfaces in the BPU, with a description granularity that delves into the code level. The structures described in the document will be consistent with the Chisel version of the Xiangshan branch prediction unit code, and the signal structures and names also come from the Chisel version code.
This document is intended for those who have already understood the basic design of the Xiangshan branch prediction unit and want to delve into the signal details. You can selectively read according to the content needed for verification or refer to it at any time.
Among them, the FTB entry and the full prediction result interface involve the way the BPU generates prediction results, which is recommended for every reader to read.
The basic prediction unit of BPU is the branch prediction block, but what is the structure of a branch prediction block? Why can a branch prediction block guide the execution of so many subsequent instructions? This section will describe the core data structure for generating branch prediction blocks—the FTB item, and the complete prediction result structure generated from the FTB item.
Redirection and update requests are the two main types of information sent from the FTQ to the BPU. This section details the specific structure of these request interfaces.
This section introduces the definition of the overall external interaction interface of the Xiangshan Branch Prediction Unit, including the presentation of global branch prediction results and single pipeline stage prediction results.
This document introduces the sub-predictor interface in the Xiangshan BPU, as well as the use of the sub-predictor base class. Reading this document can help you understand the external interactions of sub-predictors and the use of signals in the sub-predictor base class.