What is the stack pointer in the ARM Cortex-M4?

The stack pointer in the ARM Cortex-M4 is a register that points to the top of the stack memory. It indicates where the next push or pop operation will take place. The stack grows downwards in memory, so the stack pointer holds the address of the last stacked item. It is decremented when pushing to the stack, and incremented when popping from the stack.

Stack Memory Organization in Cortex-M4

In Cortex-M4, the stack is located in the SRAM memory region. By default, the processor initializes the main stack pointer (MSP) to the top of the SRAM region on reset. The stack capacity depends on the size of the SRAM in a particular Cortex-M4 implementation.

The stack grows downward from the high memory addresses to the low memory addresses. For example, if SRAM is present at addresses 0x2000 0000 – 0x2000 FFFF in a 1MB SRAM Cortex-M4 device, the initial MSP will point to 0x2000 FFFF. As data is pushed to the stack, the SP decrements and moves closer to 0x2000 0000. If the stack grows beyond the SRAM boundary, it will overwrite other data present in the memory map.

In addition to the MSP, Cortex-M4 also contains the process stack pointer (PSP). This allows context switching between threads and processes. The PSP operates similarly to MSP, pointing to a separate stack region for a process or thread.

Stack Pointer Usage in Cortex-M4

The stack pointer is used automatically by the processor when pushing or popping data to/from the stack. Some common cases where SP is used:

• Pushing/popping registers during exception or interrupt handling
• Pushing arguments to functions and popping return values
• Allocating memory for local variables in functions
• Storing return addresses for function calls

When a function is called, the return address is pushed to the stack. Space is also allocated on the stack for local variables. Registers R0-R3, R12, LR, PC, and xPSR are pushed in exception handlers. All these operations involve decrementing SP by 4 bytes per word pushed.

When data is popped from the stack, SP increments by 4 bytes. Local variables are effectively popped when a function returns. Registers and return addresses are restored by popping in exception handlers. SP will eventually be restored to its original value once a function exits and all pushes/pops have been balanced out.

Modifying the Stack Pointer

Directly modifying the stack pointer allows manual manipulation of the stack. Some cases where SP may be modified:

• To allocate a larger stack for complex functions
• Creating a custom stack frame layout for a function
• Switching to a different stack (from MSP to PSP)

The stack pointer is modified using operations like subtraction or addition with an offset value. For example: SUB sp, sp, #16 ; Allocate 16 bytes on stack ADD sp, sp, #8 ; Deallocate 8 bytes

When allocating stack space, the SP should be subtracted and decremented. The space is deallocated by adding back to SP. Modifying SP directly allows full control over the stack usage.

Stack Pointer Register in Cortex-M4

The stack pointer is available in the Cortex-M4 as two registers:

• MSP – Main stack pointer, used by default out of reset
• PSP – Process stack pointer, can be used during context switch

Both MSP and PSP are banked register copies, meaning they exist once per exception mode. So there are five copies of MSP/PSP:

• MSP/PSP for Thread mode
• MSP/PSP for Handler mode
• MSP/PSP for Default FIQ mode
• MSP/PSP for Default IRQ mode
• MSP/PSP for System mode

This allows independent stack pointers per exception mode. At any time, MSP and PSP can be treated as standard ARM registers and manipulated directly using assembly or C code.

On exception entry, the processor automatically pushes registers and banked copies of SP to the stack pointed to by MSP. The exception handler then uses this stack. On exception exit, MSP is restored to switch back to the original stack.

Typical Stack Usage in Cortex-M4

A typical Cortex-M4 program utilizes the stack in the following ways:

• Main stack pointer MSP is initialized to the top of SRAM on reset
• Interrupt handlers use MSP and stack space is allocated automatically
• Functions allocate stack space for arguments and local variables
• Some static buffers and variables may also be allocated on stack
• MSP grows downwards until it reaches the bottom of SRAM
• PSP is not used unless doing context switching between threads

Thus for a simple single-threaded program, MSP is sufficient and used throughout. The Power-On Self-Test code initializes it, while interrupts and functions handle incrementing/decrementing it automatically.

Typical SP Register Handling Sequence

Here is a simplified overview of how SP gets updated on entering and exiting an exception handler:

1. Function Main() is running using MSP
2. Interrupt occurs, processor pushes registers etc. to current MSP
3. Handler mode selected and its MSP banked copy used
4. ISR handler runs using Handler MSP, can push/pop freely
5. ISR handler exits with special instructions
6. Previous MSP restored from stack, Main() resumes
7. Main() continues to use original MSP until next interrupt

The handler thus has its own stack space separate from Main()’s stack usage. This prevents corruption between interrupt handling and main program execution.

Initializing the Stack Pointer

There are a few ways the Cortex-M4 stack pointer can be initialized at startup:

• Automatically – Bootloader code initializes SP to end of RAM
• Software – Manually set SP register in startup code
• Linker Script – .stack section sets memory region for SP

The vector table also contains a default value for the initial SP. The standard startup code loops to copy this into the live SP register. So the vector table SP and linker .stack values must match.

To change the SP initialization, either the vector table or linker file must be edited. The bootloader or System Initialization (SYS) routine can also optionally modify SP. So there are a few different places that control initial SP value.

Stack Overflow Detection

Since the Cortex-M4 stack is located in a fixed SRAM region, overflow is possible if stack usage exceeds space. Some ways to detect stack overflow:

• Check SP value during debugging forGetting close to end of region
• Use MPU to generate exception on SP overflow
• Fill end of stack region with known value like 0xAA, check on corruption
• Use linker script to allocate stack size and raise error if exceeded

Once an overflow occurs, the stack will silently overwrite other variables. This causes crashes or corruption that is difficult to debug. So overflow detection is very useful during development.

Using the Process Stack Pointer

PSP provides a way to maintain separate stack usage when multiple threads or processes run on a Cortex-M4:

• Each thread has its own context with unique PSP and stack
• Scheduler switches between threads by changing PSP
• Enter handler using MSP, switch to thread’s PSP

This requires smart context switching logic to save/restore PSP for each thread. PSP approach enables good encapsulation between threads but adds complexity to design.

Typical SP Values During Execution

On a Cortex-M4 with 1MB SRAM, here are some typical SP values that may be seen:

• Reset – SP = 0x2000 FFFF (Top of 1MB SRAM)
• Main code running – SP = 0x2000 FF00 (Decremented for main stack)
• ISR routine entered – SP = 0x2000 FE00 (ISR uses separate stack)
• Complex function entered – SP = 0x2000 FC00 (More main stack usage)
• PSP for Thread 1 – 0x2000 BFFF (Separate process stack)

So SP will start at the top of SRAM and decrement as the program executes. Exact values depend on usage, but it always starts high and moves downwards on stack push.

Typical Failures Related to Stack Pointer

Some common problems seen with SP usage:

• Overwriting SRAM contents due to stack overflow
• Corrupted return addresses leading to crash on function return
• Stack/heap collision when they grow into each other
• Incorrect exception handling due to wrong SP usage
• Failure to save/restore SP during context switch

Lack of free stack space is usually indicated by random crashes or strange behavior. Tools like debuggers or profilers help identify stack usage and potential issues.

Debugging Stack Pointer Issues

Debugging stack issues requires tracking SP usage along with the program flow. Some tips for debugging SP related problems:

• View stack usage in debugger frames/windows
• Set watchpoint on SP to break when it changes unexpectedly
• Look for SP modification not matched by push/pop
• Check for exception entries/exits not saving SP
• Match callgraph to stack usage timeline in profiler

Software tools that reconstruct stack usage are very helpful. The callgraph and SP value over time reveal issues. Browser testing can uncover stack errors quickly.

Summary

The stack pointer register is essential to any ARM Cortex-M4 program’s execution. It manages all pushes and pops to the stack, handling function calls, interrupt processing, and more. Monitoring SP usage and preventing stack overflow are key to avoiding crashes or undefined behavior. With robust SP handling, the Cortex-M4 stack can be leveraged efficiently for complex programs.