The ARM Cortex exception stack frame is the region of memory used by the processor to store context information when an exception occurs. The stack frame allows the processor to save the current state of execution, handle the exception, and then return to where it left off after the exception has been resolved.
Overview of the ARM Exception Handling
The ARM Cortex-M processors have a configurable priority-based exception model for handling interrupts and faults. When an exception event occurs, the processor automatically saves its current context to the stack, handles the exception, and then returns to the main program flow. This provides an efficient and structured way to respond to external events and error conditions.
There are several steps involved in ARM Cortex exception handling:
- The processor detects an exception condition, such as an interrupt request, memory fault, or undefined instruction.
- The processor finishes executing the current instruction.
- The processor automatically pushes registers and other context state onto the stack to create a stack frame.
- The processor loads the exception handler address into the program counter and begins executing the handler code.
- The exception handler runs and services the exception condition.
- The handler executes an exception return instruction to pop the stack frame and resume normal execution.
The stack frame created in step 3 is essential for capturing the processor state to suspend execution, run the exception handler safely, and then return to the original code as if the exception never happened. The contents of the ARM Cortex stack frame provide this context preservation.
ARM Cortex Exception Stack Frame Contents
The specific contents of the ARM Cortex exception stack frame depend on the ARM architecture version (such as Cortex-M3, Cortex-M4, etc.) and chip implementation. However, in general the stack frame consists of:
- Saved general purpose registers R0-R12
- Saved program status registers such as the program counter and program status register
- Saved stack pointer
- Saved link register (LR)
- Saved processor mode bits
- Saved exception number and type values
By pushing this context information onto the current stack, the processor preserves its entire state right before the exception occurred. The stack grows downwards to lower addresses, so the stack pointer gets decremented with each push. The exception handler then has full access to the registers and status values to service the exception.
Registers
The general purpose registers R0-R12 hold important data being used by the program at the time of the exception. Saving them allows the handler to use the registers without corrupting the main program’s data. The link register LR contains the return address back to the main program once the handler finishes. The stack pointer SP points to the top of the stack frame so the handler can access the frame.
Program Counter and Program Status Register
The program counter PC contains the address of the next instruction that would have executed after the interrupted instruction. This allows execution to resume in the main program where it left off. The program status register PSR contains important processor state like the interrupt mask, overflow flags, and processor mode bits.
Processor Mode Bits
The processor mode bits indicate the current privilege level the processor was in, such as Thread mode, Handler mode, or Fault mode. Saving this allows the exception handler to identify the mode and return to it later.
Exception Number and Type
Information about the specific exception number and type is also saved, allowing the handler to identify and service the exact exception condition. For example, the exception type could indicate a memory fault, divide by zero error, or external interrupt source.
Stack Frame Usage in Exception Handling
When an exception occurs, the processor automatically pushes the registers and status values onto the current stack to create the stack frame. This stacking occurs extremely quickly in hardware. The handler then has access to the frame to use the registers and program status values to handle the exception.
Typically, the first few instructions in the exception handler will adjust the stack pointer to allocate additional space on the stack for local variables needed by the handler. The handler will also usually save any additional registers to the stack that need to be preserved.
Once the exception processing is complete, the handler executes an exception return instruction. This instruction automatically pops the context values from the stack frame back into the original registers and program status values, restoring the state when the exception occurred. Execution then resumes in the main program right after the point of interruption.
Nested Exceptions
The Cortex-M processors allow nested exceptions, where a higher priority exception can interrupt handling of a lower priority exception. In this case, the processor pushes the current stack frame onto the stack, creating a new stack frame for the higher priority exception. This nested frame structure allows prior contexts to be restored properly.
When exception handling completes, the stack frames are unwound in reverse order. The handler executes an exception return instruction to pop its own frame, then recover the previous frame if returning to a nested handler. This continues until all nested handlers have completed and the original context frame is restored.
Allocating the Exception Stack
The exception stack space needs to be allocated properly based on the expected stack frame size and handler needs. Typically, the stack will be set up at a fixed location in memory, with the stack pointer initialized at the top of the stack. Enough stack space must be provided to handle the maximum exception nesting depth.
For Cortex-M processors, the stack is often located in dedicated SRAM memory. The size is configured based on factors such as:
- Number of registers saved in the exception context
- Size requirements for handler local variables
- Number of nested exceptions to support
- Stack alignment requirements
The ARM Cortex-M3 and M4 processors have options to either use the Main stack or create a dedicated Process stack for exception handling contexts. The Process stack helps protect and isolate the exception handling stack frames from the main program stack.
Importance of the Exception Stack Frame
The ARM Cortex exception stack frame is a key part of structured exception handling on ARM processors. It automatically captures the complete processor state without the handler needing to manually manage register saving. This simplifies exception processing and enables easy resumption of the main program.
Careful configuration of the exception stack space ensures there is enough room for multiple nested exceptions and handler local data needs. Overall, the stack frame mechanism streamlines context management during interrupts and exceptions on Cortex-M processors.
Conclusion
In summary, the key points about the ARM Cortex exception stack frame include:
- The processor automatically pushes registers and status onto the current stack when an exception occurs.
- This creates a stack frame with the context of the interrupted program.
- The frame contains registers, program counter, status registers, and exception details.
- The handler uses this information to process the exception.
- An exception return instruction pops the frame to resume execution.
- Nested exceptions are supported through stacking multiple frames.
- The exception stack needs proper configuration and memory allocation.
- The stack frame simplifies context management during exception handling.
Understanding the contents and usage of the ARM Cortex exception stack frame provides great insight into the processor’s exception handling mechanism. Proper usage of the frame is key for efficient and robust exception processing in embedded systems applications built on ARM Cortex-M processors.
