ARM Cortex-A7 Exception Vector Table Entries and Execution Flow

ARM Cortex-A7 Exception Vector Table: Entries as Instructions or Addresses?

The ARM Cortex-A7 processor, like other ARMv7-A architecture processors, utilizes an exception vector table to handle exceptions and interrupts. The vector table contains entries that define the starting points for exception handlers. Each entry in the vector table corresponds to a specific exception type, such as reset, undefined instruction, software interrupt (SWI), prefetch abort, data abort, IRQ, and FIQ. The primary question revolves around whether these entries are direct addresses for the Program Counter (PC) or executable instructions.

In the case of the Cortex-A7, the exception vector table entries are instructions, not direct addresses. For example, the reset vector entry at address 0x00000000 typically contains an instruction like LDR PC, [PC, #0x18]. This instruction loads the PC with the address stored at an offset from the current PC value. The confusion arises when the processor reads addresses that seem inconsistent with the expected flow, such as reading address 0xE59F1010 instead of address 0x20 after reset.

The ARMv7-A architecture reference manual clarifies that the processor loads the appropriate exception vector into the PC, which means the vector table entries are instructions that the processor executes to jump to the actual exception handler. The instruction LDR PC, [PC, #0x18] at the reset vector (address 0x00000000) causes the processor to load the PC with the value stored at address 0x20. This value is the address of the reset handler. The processor reads address 0xE59F1010 because it is executing the instruction at address 0x00000000, which involves calculating the address for the load operation.

Misalignment Between Expected and Actual Address Reads During Exception Handling

The discrepancy between the expected and actual address reads during exception handling can be attributed to the instruction execution flow and the way the ARM Cortex-A7 processor handles the Program Counter (PC). The ARM architecture uses a pipeline, which means the PC value used in instructions is often ahead of the currently executing instruction. This pipeline effect can lead to confusion when interpreting address reads.

For instance, when the processor executes the instruction LDR PC, [PC, #0x18] at address 0x00000000, the effective address for the load operation is calculated as PC + 0x18. Due to the pipeline, the PC value used in this calculation is the address of the current instruction plus 8 (in ARM state) or plus 4 (in Thumb state). Therefore, the effective address becomes 0x00000000 + 8 + 0x18 = 0x00000020. However, the processor reads address 0xE59F1010 because it is executing the instruction at address 0x00000000, and the instruction itself is part of the address calculation process.

This behavior is consistent with the ARM architecture’s pipeline design, where the PC value used in instructions is typically ahead of the currently executing instruction. The ARMv7-A architecture reference manual provides detailed information on how the PC behaves in different states and how it affects address calculations. Understanding this pipeline effect is crucial for correctly interpreting the address reads during exception handling.

Correcting Exception Vector Table Configuration and Debugging Execution Flow

To ensure proper exception handling and correct execution flow, it is essential to configure the exception vector table correctly and understand the execution flow during exception handling. The following steps outline the process for configuring the vector table and debugging the execution flow:

1. Vector Table Configuration: Ensure that the exception vector table is correctly populated with the appropriate instructions. For example, the reset vector at address 0x00000000 should contain an instruction like LDR PC, [PC, #0x18], which loads the PC with the address of the reset handler stored at address 0x00000020. Similarly, other exception vectors should contain instructions that load the PC with the addresses of their respective handlers.

2. Handler Address Placement: Place the addresses of the exception handlers at the correct offsets in the vector table. For example, the reset handler address should be placed at address 0x00000020, the undefined instruction handler address at address 0x00000024, and so on. This ensures that the instructions in the vector table correctly load the PC with the addresses of the handlers.

3. Pipeline Effect Consideration: When debugging the execution flow, consider the pipeline effect on the PC value. Remember that the PC value used in instructions is typically ahead of the currently executing instruction. This affects address calculations and can lead to seemingly unexpected address reads. Use a debugger to step through the instructions and observe the PC values and address reads to understand the execution flow.

4. Debugging Tools: Utilize debugging tools such as JTAG debuggers and software debuggers to step through the exception handling process. Set breakpoints at the exception vector table entries and the exception handlers to observe the execution flow and verify that the correct addresses are being loaded into the PC.

5. Memory Map Verification: Verify the memory map to ensure that the exception vector table and the exception handlers are placed at the correct addresses. Any misalignment in the memory map can lead to incorrect address reads and execution flow issues.

6. Exception Handler Implementation: Implement the exception handlers correctly, ensuring that they handle the exceptions as expected. For example, the reset handler should initialize the system, the undefined instruction handler should handle undefined instructions, and so on. Proper implementation of the handlers ensures that the system behaves correctly during exceptions.

By following these steps, you can ensure that the exception vector table is correctly configured and that the execution flow during exception handling is as expected. Understanding the pipeline effect on the PC value and using debugging tools to observe the execution flow are crucial for identifying and resolving issues related to exception handling in the ARM Cortex-A7 processor.

In conclusion, the ARM Cortex-A7 exception vector table entries are instructions that load the PC with the addresses of the exception handlers. The pipeline effect on the PC value can lead to seemingly unexpected address reads, but understanding this behavior and correctly configuring the vector table and handlers ensures proper exception handling. Utilizing debugging tools and verifying the memory map are essential steps in debugging and resolving execution flow issues.