Grabline Instruction for Zero-Beat Cache Line Ownership
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Solution Overview
Problem
The REP STOS instruction in microprocessors faces inefficiencies due to cache line ownership issues and potential data corruption when performing large string stores, especially when cache lines are not already in the cache, leading to slower performance and potential processor hangs.
Innovation Solution
Implementing a grabline instruction that initiates a zero-beat read-invalidate transaction on the bus to obtain cache line ownership without actual data access, ensuring ownership before executing store operations, while avoiding exceptions and ensuring architectural state integrity through microcode routines and exception handling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the processor performs a normal bus cycle to obtain cache line ownership, then data integrity is ensured, but the operation speed decreases due to additional data cycles
Solution Approach 1:
The patent extracts the data transfer phase from the cache line allocation process. Instead of performing a complete read cycle that includes data transfer, the processor performs only the ownership acquisition phase (read-invalidate transaction) without actually reading the data. This separates the ownership acquisition function from the data transfer function, allowing faster cache line allocation for store operations.
Solution Approach 2:
The patent performs preliminary cache line ownership acquisition before the actual store operation begins. By using a read-invalidate transaction to obtain ownership in advance, the cache lines are prepared and ready before the REP STOS instruction starts writing data, eliminating the need for slow cache line allocation during the critical store path.
2Productivity
If the processor performs zero-beat read-invalidate transactions to pre-allocate cache lines, then store operation speed increases, but the risk of data corruption and processor hangs increases
Solution Approach 1:
The processor performs preliminary validation checks before executing zero-beat read-invalidate transactions. The microcode routine verifies that the memory region is suitable for this optimization (e.g., checking memory type, cacheability attributes) and that no pending exceptions or interrupts might cause problems. This preliminary validation prevents unreliable operations from being performed.
Solution Approach 2:
The system implements feedback mechanisms to monitor the execution of grabline operations. If an exception or error condition is detected during or after a zero-beat read-invalidate transaction, the system can detect and handle the error, preventing processor hangs. The microcode routine includes exception handling that responds to feedback from the memory subsystem about the success or failure of grabline operations.
3Reliability
If the processor checks for exceptions before each grabline operation, then reliability is maintained, but the complexity of the microcode routine increases
Solution Approach 1:
The patent segments the exception checking and handling into distinct phases and locations. Rather than checking for every possible exception at every step, the microcode routine is divided into segments: initial validation segment, grabline execution segment, and exception handling segment. Each segment has a specific responsibility, reducing the complexity within each segment while maintaining overall reliability through the segmented structure.
Solution Approach 2:
The patent introduces an intermediary validation layer between the grabline operation and the actual store operation. The microcode routine acts as an intermediary that performs necessary checks and prepares the system for grabline execution. This intermediary layer handles the complexity of exception checking in a centralized manner, shielding the main grabline execution logic from complex exception handling details.
Data Source
AI summary
A microprocessor includes a cache memory and a grabline instruction. The grabline instruction specifies a memory address that implicates a cache line of the memory. The grabline instruction instructs the microprocessor to initiate a zero-beat read-invalidate transaction on the bus to obtain ownership of the cache line. The microprocessor foregoes initiating the transaction on the bus when executing the grabline instruction if the microprocessor determines that a store to the cache line would cause an exception.