Cortex-R52+ Floating-Point Register Corruption in ISRs

The Cortex-R52+ processor, based on the Armv8-R AArch32 architecture, exhibits unexpected behavior when floating-point calculations are performed within interrupt service routines (ISRs). Specifically, floating-point register values may become corrupted or inconsistent across multiple ISR invocations. This issue arises due to the architecture’s design, which does not automatically save and restore floating-point registers during exception handling. The problem is particularly pronounced in systems where multiple ISRs perform floating-point operations, leading to intermittent calculation errors and unpredictable behavior.

The root cause of this issue lies in the architecture’s exception handling mechanism. Unlike some other ARM cores, the Cortex-R52+ does not automatically preserve the state of floating-point registers (D0-D15) or the Floating-Point Status and Control Register (FPSCR) when an interrupt occurs. This omission is intentional, as it reduces the overhead of context switching in ISRs, which are often designed to avoid floating-point operations for performance reasons. However, when floating-point operations are necessary within ISRs, the lack of automatic register preservation can lead to data corruption.

To address this issue, developers must manually save and restore the floating-point registers and FPSCR at the beginning and end of each ISR. This ensures that the floating-point state is consistent across ISR invocations and prevents corruption of floating-point data. The use of VPUSH and VPOP instructions to manage the floating-point register stack is a common solution, but it requires careful implementation to avoid introducing performance bottlenecks or stack overflow issues.

Manual Floating-Point Register Preservation in Cortex-R52+ ISRs

The Cortex-R52+ processor’s exception handling mechanism does not include automatic preservation of floating-point registers. This design choice is rooted in the Armv8-R AArch32 architecture, which prioritizes performance and simplicity in exception handling by minimizing the overhead associated with context switching. As a result, the processor only automatically saves and restores a subset of general-purpose registers (e.g., R0-R3, R12, LR, and PC) during exception entry and exit. Floating-point registers, including D0-D15 and FPSCR, are excluded from this automatic preservation process.

This architectural decision has significant implications for software design, particularly in systems where ISRs perform floating-point calculations. When an interrupt occurs, the processor switches to the exception handler without saving the current state of the floating-point registers. If the ISR modifies these registers, the changes will persist after the ISR completes, potentially corrupting the floating-point state of the interrupted code. This can lead to incorrect calculations, data corruption, and other unpredictable behavior.

The absence of automatic floating-point register preservation is documented in the Arm Architecture Reference Manual for Armv8-R, but it is not explicitly highlighted in the Cortex-R52+ programming guide. This omission can lead to confusion among developers, particularly those transitioning from other ARM cores where floating-point register preservation is handled automatically. As a result, developers must be aware of this architectural detail and take appropriate measures to ensure the integrity of floating-point operations in ISRs.

Implementing VPUSH and VPOP for Floating-Point Register Management

To address the issue of floating-point register corruption in Cortex-R52+ ISRs, developers must manually save and restore the floating-point registers and FPSCR at the beginning and end of each ISR. This can be achieved using the VPUSH and VPOP instructions, which allow for efficient stacking and unstacking of floating-point registers. The following steps outline the recommended approach for implementing this solution:

Save Floating-Point Registers at ISR Entry: At the beginning of the ISR, use the VPUSH instruction to save the contents of the floating-point registers (D0-D15) to the stack. This ensures that the current state of the floating-point unit is preserved before any floating-point operations are performed within the ISR. For example:
```
VPUSH {D0-D15}
```
Save FPSCR at ISR Entry: In addition to the floating-point registers, the FPSCR must also be saved to ensure that the floating-point control and status state is preserved. This can be done using the VMRS instruction to transfer the contents of FPSCR to a general-purpose register, followed by a PUSH instruction to save the value to the stack. For example:
```
VMRS R0, FPSCR
PUSH {R0}
```
Perform Floating-Point Operations: Once the floating-point registers and FPSCR have been saved, the ISR can safely perform any necessary floating-point calculations. The saved state ensures that the interrupted code’s floating-point context is preserved and will be restored after the ISR completes.
Restore FPSCR at ISR Exit: Before exiting the ISR, the FPSCR must be restored to its original state. This can be done using the POP instruction to retrieve the saved FPSCR value from the stack, followed by the VMSR instruction to transfer the value back to FPSCR. For example:
```
POP {R0}
VMSR FPSCR, R0
```
Restore Floating-Point Registers at ISR Exit: Finally, use the VPOP instruction to restore the contents of the floating-point registers (D0-D15) from the stack. This ensures that the floating-point state is consistent with the state at the time the ISR was entered. For example:
```
VPOP {D0-D15}
```

By following these steps, developers can ensure that the floating-point state is preserved across ISR invocations, preventing data corruption and ensuring the correctness of floating-point calculations. However, it is important to note that this approach introduces additional overhead in the form of stack usage and execution time. Developers should carefully consider the trade-offs between performance and reliability when implementing this solution, particularly in systems with tight timing constraints or limited stack space.

Performance Considerations and Optimizations

While the use of VPUSH and VPOP instructions provides a reliable solution for preserving floating-point registers in Cortex-R52+ ISRs, it is important to consider the performance implications of this approach. Saving and restoring 16 double-precision floating-point registers (D0-D15) and the FPSCR can significantly increase the execution time of the ISR, particularly in systems with frequent interrupts. Additionally, the increased stack usage may lead to stack overflow issues in systems with limited memory resources.

To mitigate these performance concerns, developers can consider the following optimizations:

Selective Register Preservation: In some cases, it may be possible to reduce the number of floating-point registers that need to be saved and restored. For example, if an ISR only uses a subset of the floating-point registers (e.g., D0-D7), the VPUSH and VPOP instructions can be modified to only preserve the necessary registers. This reduces both execution time and stack usage. For example:
```
VPUSH {D0-D7}
VPOP {D0-D7}
```
Lazy Context Switching: In systems where floating-point operations are rarely performed within ISRs, developers can implement a lazy context switching mechanism. This approach involves checking whether floating-point operations are required within the ISR and only saving/restoring the floating-point registers if necessary. This can significantly reduce the overhead of context switching in ISRs that do not perform floating-point calculations.
Stack Usage Monitoring: To avoid stack overflow issues, developers should carefully monitor stack usage and ensure that sufficient stack space is allocated for the worst-case scenario. This includes accounting for the additional stack space required for floating-point register preservation, as well as any other context switching overhead.
Compiler Optimizations: Modern compilers often provide optimizations for floating-point operations and context switching. Developers should ensure that these optimizations are enabled and configured appropriately to minimize the performance impact of floating-point register preservation.

By carefully considering these optimizations, developers can strike a balance between performance and reliability, ensuring that the Cortex-R52+ processor operates correctly and efficiently in systems with floating-point ISRs.

Conclusion

The Cortex-R52+ processor’s lack of automatic floating-point register preservation in ISRs can lead to data corruption and unpredictable behavior in systems that perform floating-point calculations within interrupt handlers. To address this issue, developers must manually save and restore the floating-point registers and FPSCR using VPUSH and VPOP instructions. While this approach introduces additional overhead, it ensures the correctness of floating-point operations and prevents data corruption. By carefully considering performance optimizations and stack usage, developers can implement a robust solution that meets the requirements of their specific application.

Cortex-R52+ Floating-Point Register Corruption in ISRs