Rescue Buffer Circuit for Zero-Cycle Loads
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Solution Overview
Problem
Processor cores face inefficiencies in store and load operations due to the time-consuming nature of memory access, leading to stalls in program execution as store instructions queue up and load instructions wait for their corresponding data, often resulting in multiple processor cycles being wasted.
Innovation Solution
The implementation of a rescue buffer circuit that retains relevant information from store instructions until paired load instructions are ready, allowing for zero-cycle loads (ZCL) by speculatively fulfilling load instructions without waiting for the store instructions to complete, thereby reducing memory fetch time and increasing performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If store instructions are queued in a store instruction queue to allow continuous execution of other instructions, then processor throughput is improved, but load instructions must wait for store instructions to complete, causing execution stalls and increasing latency
Solution Approach 1:
The patent performs preliminary actions by detecting store instructions in advance and pre-calculating their target memory addresses and data. This information is stored in a buffer before the actual store operation completes, so when a load instruction is issued, the data is already prepared and can be returned immediately, eliminating the wait time for store completion.
Solution Approach 2:
The patent introduces an intermediary buffer structure that sits between the store instruction queue and the load execution unit. This buffer holds pre-processed store data and acts as a mediator that can directly satisfy load requests without requiring the actual store operation to complete, thus decoupling the timing between store and load operations.
2Reliability
If the processor waits for store instructions to complete before executing paired load instructions, then data correctness is ensured, but program execution stalls and performance decreases
Solution Approach 1:
The patent implements a feedback mechanism where the system detects store instructions, pre-processes them, and stores the results in a buffer. When a load instruction is encountered, the system checks the buffer for matching data and returns it if available. This feedback loop ensures data correctness by verifying the store-load pairing while enabling zero-cycle loads that dramatically improve execution speed.
Solution Approach 2:
By performing preliminary detection and preprocessing of store instructions, the system prepares data in advance so that when a load instruction is issued, the data is already validated and ready for immediate return, ensuring correctness without waiting for store completion.
3Device complexity
If traditional store and load execution is used with multiple processor cycles for memory access, then hardware complexity is kept moderate, but power consumption increases and performance is limited
Solution Approach 1:
The patent extracts the memory access operation from the critical path by detecting store instructions and pre-loading their data into a buffer. When a load instruction is issued, the data is taken directly from the buffer without accessing external memory, eliminating the power-consuming memory access cycle while keeping the overall hardware architecture relatively simple.
Solution Approach 2:
By performing preliminary detection and buffering of store data, the system avoids the need for full memory access cycles for load operations. This preliminary action reduces power consumption by eliminating redundant memory transactions while maintaining moderate hardware complexity through efficient buffer management.
Data Source
AI summary
An apparatus includes a rescue buffer circuit, a store queue circuit, and a control circuit. The rescue buffer circuit may be configured to retain address information related to store instructions. The store queue circuit may be configured to buffer dependency information related to a particular store instruction until the particular store instruction is released to be executed. The control circuit may be configured to cause a subset of the dependency information for the particular store instruction to be written to the rescue buffer circuit. The rescue buffer circuit may be configured to retain the subset after the dependency information has been released from the store queue circuit, and to perform a subsequent load instruction corresponding to a memory location associated with the particular store instruction using the subset of the dependency information from the rescue buffer circuit.