Two-Cycle Micro-Operation Scheduler with Speculative Picking
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Solution Overview
Problem
Schedulers in processors are difficult to design for higher-throughput operations due to complex and time-consuming processes for identifying and picking ready micro-operations for execution, especially with a limited number of execution units.
Innovation Solution
A two-cycle pipelined scheduler that speculatively identifies micro-operations as ready for issue to execution units, using early and late wake circuitry to anticipate dependencies and correct mis-speculations, allowing for efficient micro-operation scheduling across two clock cycles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the scheduler uses complex dependency checking to ensure correctness, then micro-operation execution accuracy is improved, but scheduler complexity and time consumption increase
Solution Approach 1:
The patent applies preliminary action by speculatively identifying micro-operations as ready for issue before all dependencies are fully resolved. The scheduler anticipates future dependency states and marks micro-operations as ready in advance, then corrects any mis-speculations later. This allows the scheduler to avoid complex real-time dependency checking while maintaining execution accuracy through subsequent correction mechanisms.
Solution Approach 2:
The patent implements feedback by using late wake circuitry to detect and correct mis-speculated micro-operations after they have been initially identified as ready. The system continuously monitors execution status and provides feedback to adjust the ready status of micro-operations, ensuring accuracy while keeping the initial scheduling process simple and fast.
2Reliability
If the scheduler checks all dependencies before picking micro-operations, then execution correctness is improved, but throughput decreases due to time-consuming operations
Solution Approach 1:
The scheduler performs preliminary identification of ready micro-operations based on current dependency status without waiting for all dependencies to be fully resolved. By marking micro-operations as ready speculatively and correcting later, the system achieves high throughput while maintaining correctness through the feedback mechanism.
Solution Approach 2:
The late wake circuitry provides continuous feedback to correct any mis-speculated micro-operations, ensuring execution correctness is maintained even though the initial ready identification is done quickly without exhaustive dependency checking.
3Reliability
If the scheduler waits for all dependencies to resolve before issuing micro-operations, then execution safety is improved, but processor throughput decreases
Solution Approach 1:
The scheduler issues micro-operations preliminarily before all dependencies are confirmed resolved, based on speculative readiness assessment. This enables continuous throughput by not waiting for complete dependency resolution, while execution safety is maintained through subsequent correction of any premature issuances.
Solution Approach 2:
The feedback mechanism in late wake circuitry monitors issued micro-operations and corrects any that were issued prematurely before their dependencies were actually resolved, thereby maintaining execution safety without sacrificing throughput.
4Productivity
If the scheduler uses speculative picking, then processor throughput is improved, but risk of mis-speculation increases
Solution Approach 1:
The scheduler speculatively marks micro-operations as ready for issue before all dependencies are confirmed, enabling high throughput operation. The preliminary action of speculative marking is followed by correction mechanisms that maintain reliability despite the initial speculation.
Solution Approach 2:
The late wake circuitry provides feedback to detect and correct mis-speculated micro-operations, reducing the risk of errors introduced by speculative picking while maintaining the throughput benefits of early identification of ready operations.
Data Source
AI summary
A multi-cycle scheduler for a processor includes early wake circuitry, late wake circuitry, and picker circuitry. In a first cycle of a clock, the early wake circuitry speculatively identifies child micro-operations as ready whose dependencies are satisfied by a set of ready parent micro-operations. In a second cycle of the clock, the picker circuitry picks at least one of the child micro-operations identified as ready for issue to execution circuitry. In addition, the late wake circuitry blocks from issue at least one picked child micro-operation speculatively identified as ready upon determining that a respective parent micro-operation did not issue to execution circuitry.