Debug Circuitry Transparent Halted Mode Exception Handling
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Solution Overview
Problem
Processing circuitry hardware often cannot natively respond to certain debug instructions, especially in complex environments, requiring additional operations that cannot be performed while in a halted mode.
Innovation Solution
The processing circuitry is designed to temporarily exit the halted mode transparently to the debugger, allowing exception handler code to be executed from memory to perform necessary operations, and then re-enter the halted mode to complete the debug instruction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If processing circuitry enters halted mode to execute debug instructions, then debug capability is enabled, but the circuitry cannot perform additional operations required by complex debug instructions
Solution Approach 1:
The processing circuitry dynamically transitions between halted mode and normal execution mode based on the specific debug instruction being executed. When a complex debug instruction requiring additional operations is detected, the system temporarily exits halted mode to perform these operations, then returns to halted mode. This dynamic state adjustment enables the system to handle versatile debug instructions while maintaining the simplicity of halted mode operation when possible.
Solution Approach 2:
Exception handler code acts as an intermediary between the debug instruction and the processing circuitry. When a complex debug instruction is encountered, the system triggers an exception that transfers control to pre-prepared exception handler code stored in memory. This intermediary code performs the necessary additional operations (such as realm transitions) that cannot be executed directly in halted mode, thereby enabling complex debug functionality without modifying the halted mode itself.
2Adaptability or versatility
If processing circuitry executes additional operations for complex debug instructions, then functionality is improved, but debugger transparency is compromised
Solution Approach 1:
Exception handler code is prepared and stored in memory in advance, before any debug instructions are executed. This pre-prepared code contains all necessary logic for handling complex debug instructions and performing additional operations. By having the solution prepared beforehand, the system can execute additional operations without requiring the debugger to be aware of or modified for each specific case, thereby maintaining debugger transparency while supporting enhanced functionality.
Data Source
AI summary
An apparatus and method are provided for executing debug instructions. The apparatus has processing circuitry for executing instructions fetched from memory, and a debug interface. The processing circuitry is responsive to a halt event to enter a halted mode where the processing circuitry stops executing the instructions fetched from memory, and instead is arranged to execute debug instructions received from a debugger via the debug interface. The processing circuitry is responsive to detection of a trigger condition when executing a given debug instruction to exit the halted mode transparently to the debugger, and to take an exception in order to execute exception handler code comprising a sequence of instructions fetched from memory. On return from the exception, the processing circuitry then re-enters the halted mode and performs any additional processing required to complete execution of the given debug instruction. This provides a mechanism for allowing an apparatus to perform operations required by debug instructions in situations where the processing circuitry hardware is not able to natively perform those operations in response to the specified debug instruction.