The stack pointer is one of the key registers in the ARM Cortex-M0 processor. It points to the top of the stack, which is a region of memory used to store temporary data during function calls and interrupts. Understanding how the stack pointer works is essential for programming and debugging Cortex-M0 based microcontrollers.
What is a Stack?
A stack is a last in, first out (LIFO) data structure used for storing temporary data. Some key properties of a stack are:
- Data is pushed/popped from the top of the stack
- The stack has a fixed size determined at compile time
- Pushing data onto a full stack typically causes a stack overflow
- Popping data from an empty stack causes a stack underflow
The stack grows downwards from high memory to low memory. Pushing adds data at the top and reduces the available stack space. Popping removes data from the top and increases available stack space.
Stack Pointer Register
The stack pointer (SP) is a 32-bit register that points to the top of the stack. It indicates where the next push or pop will take place. At startup, the SP is initialized to the end of the available stack space. As data is pushed/popped, the SP is decremented/incremented accordingly to track the current top of stack.
In Cortex-M0, the SP register is banked, meaning there are two separate copies:
- Main stack pointer (MSP) – used for main thread execution
- Process stack pointer (PSP) – used during exception handling
This allows independent stack usage for threads and exceptions. The CONTROL register is used to select between MSP and PSP.
Stack Operations
Common operations that manipulate the stack:
- Push – Writes data to the stack and decrements SP. Performed with PUSH and STM instructions.
- Pop – Reads data from the stack and increments SP. Done with POP and LDM instructions.
- Function Calls – On entry, arguments and return address are pushed. On exit, local variables are popped.
- Exceptions – Handler address, registers, and return address are pushed to stack. Popped on return.
- Interrupt Handling – Interrupt context is pushed onto the stack by hardware. Restored on return.
Keeping track of pushes and pops is necessary to monitor stack usage and avoid overflow.
Stack Overflow
The stack has a finite size determined at link time. Pushing too much data without popping can exceed the available stack space, corrupting other variables and leading to a hard fault exception. This is called stack overflow.
Common ways stack overflows occur:
- Recursive functions calling themselves without terminating
- Large stack allocations via arrays or structs
- Too many nested function calls
- Interrupt handlers pushing too much data
Ways to prevent stack overflow:
- Minimize stack usage in ISRs
- Reduce local variables and nested calls
- Allocate large data structures statically or on heap
- Use tail recursion optimization where possible
- Increase stack size in linker script
Enabling the stack overflow exception can help detect when overflow occurs.
Stack Underflow
Stack underflow happens when popping empty data off the stack using POP or LDM instructions. This will read invalid data from lower stack addresses. Underflow can occur due to:
- Popping too many times
- Corrupt stack pointer value
- POP/LDM inside interrupt handlers
- Forgetting to clean up stack after ISR
Ways to avoid stack underflow
- Initialize SP properly on startup
- Carefully match push and pop operations
- Disable interrupts during critical push/pop sequences
- Double check stack manipulation in ISRs
Like overflow, enabling the underflow exception can detect errors early.
Checking Stack Usage
During development, it is helpful to analyze stack usage to catch issues early. Some techniques include:
- Static analysis with linker map files
- Runtime stack monitoring with SP instrumentation
- Debugging and stepping through push/pop sequences
- Calculating worst-case stack usage manually
- Tool-assisted stack analysis in IDEs
Many IDEs or compilers can perform static analysis on code to estimate stack usage per function. Checking this against linker limits helps verify sufficient space.
Runtime monitoring involves sampling the SP periodically and determining max usage. This catches dynamic and interrupt driven usage.
Manual analysis involves adding up stack usage per function, interrupt, and data allocation based on architecture details.
Stack and Heap
The stack stores temporary data created by functions, interrupts, and exceptions. This data is managed automatically as functions execute.
The heap is used to allocate dynamic, persistent variables that must last longer than a single function. This requires explicit allocation and freeing.
Data suitable for the stack:
- Local function variables
- Function arguments
- Return addresses
- Register spill locations (context saving)
Data suitable for the heap:
- Global buffers
- Large data structures
- Persistent objects
- Dynamically sized data
Balancing stack vs heap usage properly ensures efficient memory utilization in embedded systems.
Summary
The Cortex-M0 stack pointer tracks the top of the stack region in memory. It is incremented/decremented automatically during pushes and pops to maintain the current stack level. Monitoring stack usage and overflow is an important part of embedded programming. Understanding the stack pointer helps write stable, robust embedded software on ARM Cortex-M0 processors.
