Illegal Mode Change Detection Circuitry for Secure Data Processing
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Solution Overview
Problem
Data processing systems with multiple modes of operation face challenges in consistently and securely preventing illegal mode changes, which can compromise system security and introduce implementation complexities due to varying behaviors across different implementations.
Innovation Solution
The implementation of illegal change detecting circuitry that suppresses unauthorized mode changes and sets an illegal change bit, treating subsequent instructions as undefined, allowing for a uniform and consistent response without introducing significant data dependency issues, and utilizing a syndrome register to tailor the response to the specific type of illegal instruction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If hardware mechanisms are provided to prevent illegal mode changes, then system security is improved, but system behavior becomes inconsistent across different implementations and illegal change types
Solution Approach 1:
The patent introduces an illegal change bit as a state parameter that changes the system's response mode. When set, this bit transforms subsequent instructions into undefined instructions, creating a consistent parameter-based response across all implementations and illegal change types, thereby resolving the behavior inconsistency problem while maintaining security.
2Adaptability or versatility
If a data dependency is introduced to trigger the illegal change response, then the response can be uniformly applied, but implementation difficulty increases significantly
Solution Approach 1:
The patent extracts the illegal change detection and response mechanism from the main instruction execution flow. By using a separate illegal change bit that operates independently of data dependencies, the system achieves uniform response behavior without introducing complex data dependency chains, thus reducing implementation difficulty while maintaining consistency.
3Stability of the object's composition
If the illegal change bit is stored in the current program status register, then the bit is properly treated as part of the current system state, but the register capacity is reduced
Solution Approach 1:
The patent utilizes an existing but currently unused bit position in the program status register to store the illegal change bit. This parameter-based approach allows the illegal change state to be properly integrated into the system state without requiring additional register capacity, as it repurposes an existing parameter space within the register structure.
Data Source
AI summary
A data processing system 2 supporting multiple modes of operation is provided with illegal change detecting circuitry 22 which detects attempts by program instructions to perform an illegal change of mode, such as a change to a higher level of privilege in response to execution of a mode changing program instruction or an exception return instruction. If such a change is detected, then an illegal change bit CPSR.IL is set. An instruction decoder 12 is responsive to the illegal change bit having a set value to treat subsequent program instructions as undefined instructions. These undefined instructions may then trigger an undefined instruction exception or other type of response.