Interrupt Handling During Multi-Cycle Atomic Operations in ARM Cortex M3

The ARM Cortex M3 processor implements interrupt handling in a way that preserves the atomicity of multi-cycle instructions and operations. This enables robust synchronization between interrupts and program execution even for instructions that take multiple clock cycles to complete.

Multi-Cycle Instructions in Cortex M3

Certain instructions in the Cortex M3 Thumb instruction set require multiple clock cycles to execute. These include:

• 32-bit Thumb-2 instructions like conditional branching and data processing operations
• Loads and stores to memory
• Coprocessor instructions like floating point arithmetic

The Cortex M3 pipeline allows these multi-cycle instructions to be interleaved with single-cycle instructions. However, any interrupt or exception could potentially disrupt the execution flow in the middle of a multi-cycle instruction.

Interrupt Handling Challenges

If an interrupt occurs in the middle of a multi-cycle atomic instruction or operation, problems can arise:

• The instruction may only be partially executed, leaving registers or memory in an inconsistent state when the interrupt handler runs.
• Updates to memory may be only partially completed.
• After the interrupt, the instruction may restart and execute again, leading to incorrect results.

To avoid these issues, the Cortex M3 employs special handling of interrupts during multi-cycle instructions to ensure atomic execution.

Interrupt Handling Process

The Cortex M3 interrupt handling involves several steps:

1. Interrupt Detection: The processor detects an interrupt request and suspends execution of the current instruction at the end of the current clock cycle.
2. Multi-cycle Instruction Completion: If a multi-cycle instruction was interrupted, the processor first finishes executing it fully before handling the interrupt.
3. Atomic Context Saving: The processor saves a consistent register and memory context to the stack by inhibiting memory access during this step.
4. Interrupt Vector Fetch: The interrupt vector is fetched to branch to the corresponding interrupt handler.
5. Interrupt Handler Execution: The desired interrupt handler runs to completion.
6. Context Restoration: The original context is reloaded from the stack to resume interrupted instruction stream.

Multi-cycle Instruction Completion

If a multi-cycle instruction is interrupted, the Cortex M3 finishes executing it atomically before saving the context and branching to the interrupt handler. This avoids inconsistencies from partial execution.

Atomic Context Saving

All processor registers, status flags, and program counter values are saved to the stack atomically. The processor briefly stalls memory access during this step to ensure context consistency.

Interrupts Disabled During Critical Sections

The interrupted program code can also prevent interrupts using the PRIMASK register to mark critical sections. This guarantees uninterrupted execution of sensitive multi-cycle instructions.

Usage in Code

Here is example C code using intrinsic functions to implement an atomic 32-bit increment operation that will not be interrupted:

uint32_t value; 

void atomic_increment() {

  uint32_t primask;

  /* Disable interrupts */
  primask = __get_PRIMASK();
  __disable_irq();  

  /* Perform 32-bit atomic increment */
  value++;

  /* Restore interrupt mask state */
  __set_PRIMASK(primask);

}

The __get_PRIMASK and __set_PRIMASK intrinsic functions allow the current interrupt state to be saved and restored around the critical 32-bit increment instruction. This guarantees the increment will complete atomically without an interrupt occurring midway.

Effects on Interrupt Latency

The completion of interrupted multi-cycle instructions does extend the interrupt latency by a few clock cycles. However, this is necessary to ensure correct program execution and data integrity.

The Cortex M3 interrupt handling architecture balances low interrupt latency with the ability to correctly handle atomicity in multi-cycle operations.

Debugging Multi-Cycle Atomic Operations

Debugging interrupt handling with multi-cycle atomic operations requires:

• Inspecting the disassembly around interrupts to identify multi-cycle instructions.
• Stepping through execution at the clock cycle level to observe how multi-cycle instructions are completed.
• Checking that memory and registers match expected values after interrupts.
• Verifying that critical sections are properly protected against interrupts.

Proper use of debugger features like breakpoints and single stepping is necessary to trace interrupt handling behavior.

Conclusion

The Cortex M3 architecture provides robust support for interrupt handling during multi-cycle instructions. Atomicity is guaranteed by allowing multi-cycle instructions to complete, saving context atomically, and disabling interrupts during critical sections.

Understanding these techniques allows developers to write Cortex M3 code that interacts correctly with interrupts and avoids subtle concurrency bugs. Proper interrupt handling continues to be a key challenge in embedded systems programming using ARM processors.