| 摘要 |
In a first aspect of the invention, branch prediction hardware, comprising logic and interconnect, is configurable via a control line to alter the manner in which the branch prediction is generated. The configuration can be done programmatically in software. Or, the configuration can be done by hardware in response to processor events. Such processor events include the loading of the CS register and changes in the instruction workload. In a second aspect of the invention, related to speculative execution, the directions of a plurality of branches are predicted based partly on resolved branch history information. Tentative branch history information is then stored for each of the predicted branches. When a predicted branch is resolved, the resolved branch history information is updated based on the stored tentative branch history information for the branch most recently resolved. Additionally, the predictions may be partly based on preceding unresolved branch predictions if any are outstanding. In a third aspect of the invention, Hit/Miss Information from a Branch Prediction Cache (BPC) can optionally be used in formulating the next state value of an addressed two-bit counter stored in a correlation-based branch history table. Since a Miss in the BPC may indicate that this branch has not been encountered recently, whatever state currently exists can be optionally forced to a state that is based solely on whether the branch is resolved taken or not. This feature may be enabled and disabled under software control. In a fourth aspect of the invention, information from the instruction decoder is optionally used to override the correlation-based branch history table based prediction for select branch instructions. This feature may be enabled and disabled under software or hardware control.
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