Stack Pointer Alignment Checking Circuitry for Memory Access Control
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Solution Overview
Problem
Existing data processing systems face challenges in enforcing memory access alignment, particularly when dealing with packed data structures and stack memory accesses, as alignment constraints may be inappropriate for non-stack memory accesses, leading to inefficiencies and potential corruption of stack data.
Innovation Solution
The implementation of stack alignment checking circuitry that can be selectively disabled independently of general alignment checking, allowing for alignment enforcement only for stack data stores while allowing unaligned accesses for other memory operations, combined with general purpose alignment checking circuitry for comprehensive alignment management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If alignment constraints are enforced for all memory accesses, then high speed memory accesses are achieved through wide data buses, but packed data structures cannot be manipulated and stack pointer corruption cannot be detected
Solution Approach 1:
The alignment checking mechanism is segmented into two independent parts: stack alignment checking circuitry and general purpose alignment checking circuitry. This segmentation allows different alignment policies to be applied to stack memory accesses versus other memory accesses, resolving the contradiction between enforcing alignment for speed and allowing unaligned access for packed data structures.
Solution Approach 2:
Different alignment checking behaviors are applied to different memory regions: strict alignment checking is applied locally to stack memory accesses to detect pointer corruption, while no alignment checking is applied to general memory accesses to allow packed data structures. This local differentiation resolves the contradiction by making the alignment policy adaptive to the specific memory region being accessed.
2Reliability
If alignment constraints are enforced for stack memory accesses, then stack pointer corruption is detected, but inefficiencies occur for non-stack memory accesses
Solution Approach 1:
The alignment checking system is divided into separate stack alignment checking circuitry and general purpose alignment checking circuitry, each independently controllable. This allows the reliable detection of stack pointer corruption to be maintained while avoiding the performance penalty of applying the same checking to all memory accesses.
Solution Approach 2:
Alignment checking is applied partially - only to stack memory accesses where it provides reliability benefits for detecting pointer corruption, and not applied to general memory accesses where it would cause performance degradation without providing corresponding benefits.
3Reliability
If alignment checking is performed on all memory accesses, then comprehensive alignment enforcement is achieved, but unnecessary constraints are imposed on non-stack memory operations
Solution Approach 1:
The alignment checking functionality is segmented into stack-specific and general-purpose components, allowing independent control. This enables comprehensive alignment enforcement for stack operations while maintaining flexibility for non-stack memory operations, resolving the contradiction between reliability and ease of operation.
Solution Approach 2:
Different alignment checking policies are applied locally to different types of memory accesses: strict checking for stack accesses to ensure reliability, and no checking for general memory accesses to maintain operational flexibility and ease of use.
Data Source
AI summary
A data processing system 2 includes a stack pointer register 26, 28, 30, 32 storing a stack pointer value for use in stack access operations to a stack data store 44, 46, 48, 50. Stack alignment checking circuitry 36 which is selectively disabled may be provided to check memory address alignment of the stack pointer value associated with a stack memory access. The action of the stack alignment checking circuitry 36 is independent of any further other alignment checking performed in respect of all memory accesses. Thus, general alignment checking circuitry 38 may be provided and independently selectively disabled in respect of any memory access.