Unaligned Memory Access Fault with STRH on Cortex-M7: Causes and Solutions

ARM Cortex-M7 Unaligned Memory Access Fault with STRH Instruction

The ARM Cortex-M7 processor is a high-performance microcontroller core designed for real-time applications. One of its key features is its ability to handle unaligned memory accesses efficiently, which can improve performance in certain scenarios. However, unaligned memory accesses can also lead to unexpected faults if not properly managed. In this post, we will delve into the issue of unaligned memory access faults specifically related to the STRH (Store Halfword) instruction on the Cortex-M7, explore the possible causes, and provide detailed troubleshooting steps and solutions.

Cortex-M7 Memory Access Alignment Requirements and STRH Behavior

The Cortex-M7 processor, like other ARM Cortex-M series processors, has specific requirements for memory access alignment. For halfword (16-bit) accesses, the address should be aligned to a 2-byte boundary. When an unaligned access occurs, the behavior can vary depending on the configuration of the processor and the specific instruction being executed.

The STRH instruction is used to store a halfword (16-bit) value from a register to memory. According to the ARM Architecture Reference Manual, the STRH instruction should support unaligned memory accesses, but this support can be influenced by the configuration of the Cortex-M7’s Configuration and Control Register (CCR). Specifically, the UNALIGN_TRP bit in the CCR determines whether unaligned accesses should trap (cause a fault) or not.

In the case described, the STRH instruction is attempting to store a halfword to an unaligned memory address, which results in a hard fault. This is unexpected because the UNALIGN_TRP bit in the CCR is set to 0, indicating that unaligned accesses should not trap. This discrepancy suggests that there may be other factors at play, such as a potential implementation issue with the NXP microcontroller or a misconfiguration in the system.

Configuration and Control Register (CCR) Settings and NXP Implementation Considerations

The Configuration and Control Register (CCR) in the Cortex-M7 plays a crucial role in determining how the processor handles unaligned memory accesses. The UNALIGN_TRP bit (bit 3) in the CCR controls whether unaligned word or halfword accesses should trap. When this bit is set to 0, unaligned accesses should not cause a fault, and the processor should handle them transparently, albeit with a potential performance penalty.

In the scenario described, the UNALIGN_TRP bit is correctly set to 0, which should allow the STRH instruction to proceed without faulting. However, the fault still occurs, which raises questions about the implementation of the Cortex-M7 by NXP. It is possible that NXP’s implementation of the Cortex-M7 does not fully support unaligned memory accesses as described in the ARM reference manual. This could be due to a variety of reasons, such as silicon errata, design choices, or limitations in the memory controller.

To further investigate this issue, it is important to review the NXP documentation for the specific microcontroller being used. NXP may provide additional information or errata related to unaligned memory accesses on their Cortex-M7-based devices. Additionally, it may be necessary to consult with NXP support to determine if there are any known issues or workarounds related to this behavior.

Implementing Data Alignment and Memory Access Best Practices

Given the potential issues with unaligned memory accesses on the Cortex-M7, it is important to implement best practices for data alignment and memory access in your firmware. This will help avoid unexpected faults and ensure optimal performance. Below are some detailed steps and solutions to address the unaligned memory access fault with the STRH instruction:

1. Ensure Proper Data Alignment: The simplest way to avoid unaligned memory access faults is to ensure that all data is properly aligned in memory. For halfword (16-bit) accesses, this means ensuring that the address is aligned to a 2-byte boundary. This can be achieved by carefully designing your data structures and using alignment specifiers in your code. For example, in C/C++, you can use the __align keyword or the alignas specifier to ensure that variables are properly aligned.

2. Review and Modify the CCR Settings: Although the UNALIGN_TRP bit is already set to 0 in the described scenario, it is important to double-check the CCR settings to ensure that no other bits are inadvertently causing the fault. Additionally, you can experiment with setting the UNALIGN_TRP bit to 1 to see if the fault behavior changes. This can provide additional insight into whether the issue is related to the CCR configuration or the NXP implementation.

3. Use Alternative Instructions: If unaligned memory accesses are unavoidable in your application, consider using alternative instructions that are guaranteed to support unaligned accesses. For example, the LDRD and STRD instructions (Load and Store Doubleword) can be used to perform 64-bit accesses, which are always aligned to an 8-byte boundary. While this may not directly solve the issue with the STRH instruction, it can help mitigate the risk of unaligned access faults in other parts of your code.

4. Implement Software Workarounds: If the NXP implementation does not fully support unaligned memory accesses, you may need to implement software workarounds to handle unaligned data. This can involve manually aligning data in memory or using a series of byte-level accesses to perform the desired operation. For example, instead of using a single STRH instruction to store a halfword to an unaligned address, you could use two STRB (Store Byte) instructions to store the individual bytes.

5. Consult NXP Documentation and Support: As mentioned earlier, it is crucial to review the NXP documentation for your specific microcontroller to determine if there are any known issues or limitations related to unaligned memory accesses. Additionally, reaching out to NXP support can provide valuable insights and potential solutions. NXP may have specific recommendations or patches to address this behavior.

6. Enable and Analyze Fault Handlers: To gain more insight into the fault, you can enable and analyze the fault handlers in your firmware. The Cortex-M7 provides detailed fault status registers (e.g., CFSR, HFSR, DFSR) that can help identify the root cause of the fault. By examining these registers, you can determine if the fault is indeed caused by an unaligned memory access or if there are other contributing factors. This information can guide your troubleshooting efforts and help you implement the appropriate fixes.

7. Consider Hardware Solutions: In some cases, the issue may be related to the memory controller or other hardware components in the system. If software workarounds are not sufficient, you may need to consider hardware solutions, such as using a different memory controller or modifying the hardware design to ensure proper alignment of memory accesses. This is typically a last resort, but it may be necessary in certain high-performance or safety-critical applications.

Conclusion

Unaligned memory access faults with the STRH instruction on the Cortex-M7 can be a challenging issue to diagnose and resolve, especially when the expected behavior does not align with the actual behavior of the processor. By understanding the alignment requirements of the Cortex-M7, reviewing the CCR settings, and implementing best practices for data alignment and memory access, you can mitigate the risk of these faults and ensure reliable operation of your firmware.

If the issue persists, it is important to consult the NXP documentation and support for additional guidance. In some cases, software workarounds or hardware modifications may be necessary to fully address the problem. By following the detailed troubleshooting steps and solutions outlined in this post, you can effectively resolve unaligned memory access faults and optimize the performance of your Cortex-M7-based system.

This post provides a comprehensive analysis of the unaligned memory access fault with the STRH instruction on the Cortex-M7, covering the underlying causes, potential implementation issues, and detailed troubleshooting steps. By following these guidelines, you can effectively diagnose and resolve this issue, ensuring the reliable operation of your embedded system.