Microprocessor Segment Register Speculative Execution
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Solution Overview
Problem
Out-of-order execution superscalar pipelined microprocessors face performance limitations due to the need for register renaming, which increases hardware complexity and power consumption.
Innovation Solution
The microprocessor employs a comparator and control unit to re-execute instructions using new values in architectural segment registers, predicting that new values are the same as old values for DS/ES segment registers, allowing speculative execution of dependent instructions without serializing them, and flushing the pipeline if the prediction is incorrect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If register renaming is used to enable out-of-order execution, then instruction independence and execution performance are improved, but hardware complexity and power consumption increase
Solution Approach 1:
The patent extracts the register renaming mechanism specifically for segment registers (DS, ES, FS, GS) while maintaining traditional renaming for other registers. By selectively applying renaming only where needed for segment registers, the system achieves the performance benefits of out-of-order execution while reducing the overall hardware complexity and power consumption associated with full register renaming.
Solution Approach 2:
The patent segments the register renaming approach by treating segment registers differently from general-purpose registers. Segment registers are renamed and tracked separately, allowing independent management of their values. This segmentation enables the system to optimize segment register access patterns without requiring comprehensive renaming of all registers, thereby reducing hardware complexity.
2Productivity
If segment register values are updated immediately in architectural state, then program correctness is maintained, but speculative execution of dependent instructions is prevented
Solution Approach 1:
The patent applies preliminary action by allowing dependent instructions to be executed speculatively before the segment register value is officially updated in the architectural state. The system predicts that the new segment register value will be the same as the old value, enabling dependent instructions to proceed in advance. This speculative execution is later verified when the actual value update occurs, ensuring program correctness.
Solution Approach 2:
The patent implements feedback by comparing the new segment register value with the old value after the speculative execution. If the values differ, the system uses this feedback to correct any instructions that were executed with the incorrect value. This feedback mechanism ensures that program correctness is maintained while still allowing speculative execution to improve performance.
Data Source
AI summary
An out-of-order execution microprocessor executes an architectural segment register-loading instruction that instructs the microprocessor to load a new value into an architectural segment register of the microprocessor. A comparator compares the new value specified by the architectural segment register-loading instruction with a current contents of the architectural segment register. A control unit causes to be re-executed using the new value all instructions in the microprocessor that used the current architectural segment register contents as a source operand and that are newer in program order than the architectural segment register-loading instruction whenever the comparator indicates the new value does not equal the current contents. An instruction scheduler retrieves the current contents and issues for execution instructions that use the retrieved current contents, even though the instructions are newer in program order than the register-loading instruction and the register-loading instruction has not yet written the new value to the architectural segment register.