Hierarchical Reservation Station for Processor Instruction Scheduling
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Solution Overview
Problem
Modern processors face challenges in efficiently scheduling instructions for execution due to data dependencies, instruction latencies, and the limitations of existing microarchitecture techniques, which can lead to increased execution time and reduced operating clock frequency.
Innovation Solution
A processor design that incorporates a hierarchical reservation station with a primary and secondary buffer, where instructions in the primary buffer have higher priority, and control logic determines which instructions to issue based on ready source operands and available execution units, allowing for efficient scheduling and issue width management across multiple clock cycles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the size of queues in the reservation station is increased to consider more candidate instructions for selection, then the number of instructions that can be processed per clock cycle is improved, but the number of levels of logic used to schedule instructions increases and the execution time of the circuit increases
Solution Approach 1:
The reservation station queues are divided into multiple segments or levels (e.g., first queue, second queue, third queue). Each queue manages a subset of candidate instructions, allowing the selection logic to operate on smaller, divided sets rather than a single large queue. This segmentation reduces the complexity of the scheduling logic while maintaining the ability to process multiple instructions per clock cycle.
2Productivity
If the number of levels of logic used to schedule instructions for issue is increased, then more instructions can be selected for execution, but the execution time of the circuit increases and the operating clock frequency is reduced
Solution Approach 1:
The instruction selection process is segmented into multiple independent queue structures. Each queue can be evaluated in parallel or in a structured sequence that limits the critical path delay. This allows the system to consider more candidate instructions without proportionally increasing the logic depth, thereby maintaining higher clock frequencies.
Solution Approach 2:
Instead of increasing logic levels in a single dimension, the patent introduces a multi-dimensional queue structure with multiple queues at different levels. This transforms the problem from a single deep logic path to a broader, shallower structure with multiple parallel paths, reducing the critical path delay while increasing instruction selection capacity.
3Productivity
If more candidate instructions are considered for selection, then instruction throughput is improved, but the queue size and logic complexity increase leading to longer execution time
Solution Approach 1:
The candidate instruction pool is segmented across multiple queues rather than stored in a single large queue. This allows the selection logic to efficiently scan and select instructions from distributed queues, reducing the time required to evaluate all candidates while maintaining high instruction throughput.
Data Source
AI summary
Systems, apparatuses, and methods for efficiently scheduling processor instructions for execution. The reservation station in a processor stores instructions in each of a primary buffer and a secondary buffer. Control logic selects a first number of instructions with ready source operands in the primary buffer and a second number of instructions with ready source operands in the secondary buffer. If a third number of instructions to issue from the reservation station is greater than the first number of instructions, then the reservation station issues one or more instructions of the second number of instructions from the secondary buffer to the one or more execution units. Control logic selects a fourth number of instructions in the secondary buffer to transfer to the primary buffer, and cancels the transfer of a given instruction in response to determining the given instruction has issued to the one or more execution units.