Snoop Bus Commands for Cache Debug Visibility
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Solution Overview
Problem
Current data processing systems face challenges in allowing real-time debuggers to access the latest values of variables hidden in caches or cache hierarchies without intrusive operations, which can alter the cache state, limiting debugging capabilities.
Innovation Solution
The introduction of new snoop bus commands, such as 'show line' and 'show doubleword', enables debug circuitry to access cached values without modifying the cache state, allowing visibility into cache contents by placing cached data on the system bus for capture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of information
If traditional debugger access methods are used to retrieve variable values from cache, then the debugger can obtain cache contents, but the cache state is modified which affects debugging accuracy
Solution Approach 1:
A debug interface is introduced as an intermediary between the debugger and the cache system. This interface provides a specialized read allocation path that allows the debugger to access cache contents without triggering traditional read operations that would modify cache state. The debug interface acts as a mediator that enables observation without interference.
Solution Approach 2:
The cache read access path is segmented into two distinct paths: a normal read path for processor operations and a debug read path for debugger access. The debug read path is specifically designed to provide visibility into cache contents without affecting cache state, while the normal path maintains existing cache behavior. This segmentation allows simultaneous debugging and normal operation without interference.
2Ease of operation
If intrusive operations are performed to extract variable values from cache, then the debugger can access cached data, but the operation halts the processor and alters cache state
Solution Approach 1:
The debug interface is designed to work seamlessly with the existing cache hierarchy without requiring external control logic or complex coordination. The cache system itself provides the debugging capability through its own debug read allocation mechanism, eliminating the need for external intervention or complex debugging infrastructure.
3Speed
If a cache hierarchy exists between the processor and debugger, then data access speed is improved, but the debugger cannot access the latest variable values
Solution Approach 1:
The debug interface is designed to work with any cache hierarchy configuration, providing universal debugging capability across different cache levels and architectures. The same debug read allocation mechanism can access contents at any level of the cache hierarchy, making the debugging system adaptable to various cache configurations without requiring modification to the fundamental approach.
Data Source
AI summary
A plurality of new snoop transaction types are described. Some include address information in the requests, and others include cache entry information in the requests. Some responses include tag address information, and some do not. Some provide tag address content on the data bus lines during the data portion of the transaction. These new snoop transaction types are very helpful during debug of a data processing system.