Cortex-M0 Stack Pointer

The stack pointer on the ARM Cortex-M0 microcontroller is one of the key registers used for managing the stack memory region. The Cortex-M0, along with the entire Cortex-M series of microcontrollers, utilizes a stack-based architecture for function calls and interrupt handling. Understanding how the stack pointer operates is essential for effectively using the Cortex-M0 in embedded applications.

What is the Stack Pointer?

The stack pointer (SP) is one of the general purpose registers built into the Cortex-M0 CPU core. It contains a memory address that points to the current top of the stack. The stack is a region of memory used for temporary data storage during function calls and interrupt handling. When a function is called, the return address and local variables are pushed onto the stack. When an interrupt occurs, the CPU state is also pushed onto the stack. The stack pointer indicates where the next data should be written on the stack.

On the Cortex-M0, the stack pointer is 32 bits wide and points to a word-aligned address. This provides a stack capacity of up to 4GB, although in practical embedded applications the stack size is typically just a few kilobytes.

Stack Pointer Operation

The Cortex-M0 stack grows downwards from high memory addresses to low memory addresses. This means the stack pointer is decremented when data is pushed onto the stack, and incremented when data is popped off. For example, when a function is called the return address is pushed onto the stack by subtracting 4 from the stack pointer (each stack entry is one word = 4 bytes), then writing the return address to the new memory location pointed to by the stack pointer.

Some key operations involving the Cortex-M0 stack pointer:

• Push – Subtract 4 from stack pointer, write data to new stack pointer address
• Pop – Read data from stack pointer address, add 4 to stack pointer
• Function Call – Push return address onto stack, stack pointer is modified
• Function Return – Pop return address off stack, stack pointer is modified
• Interrupt – Push CPU state onto stack, stack pointer is modified
• Exception – Push exception information onto stack, stack pointer is modified

By carefully managing the stack pointer updates during these operations, the Cortex-M0 CPU can maintain separate stack areas for each process or thread. The current stack pointer value indicates which stack is active at any given time.

Cortex-M0 Stack Pointer Registers

There are two register related to the Cortex-M0 stack pointer:

• SP (R13) – This is the active stack pointer used by the CPU for current operations and accessing the current stack.
• MSP (R9) – This is the master stack pointer. It can be used to save the original stack pointer on context switches.

Typically the MSP is used to point to a master stack for privileged threads, while the SP points to the stack for unprivileged threads. The CONTROL register has a bit called SPSEL which selects whether MSP or SP is active.

Initializing the Stack Pointer

The stack pointer must be initialized before it is used, typically during the startup sequence of code execution. There are a few common ways the Cortex-M0 stack pointer may be initialized:

• Initialize SP or MSP directly by writing the address of the stack region to the register.
• Have the linker script assign the stack region address to SP or MSP.
• Push the initial value onto the stack during reset handler, popping it into SP or MSP.

It is common to initialize the MSP first, then switch to SP for application code. This helps separate privileged stacks from thread stacks.

Stack Overflow Protection

Since the Cortex-M0 stack grows downwards towards lower memory addresses, it is possible for the stack to overflow if too much data is pushed onto it. This could overwrite other data or code and cause a crash. To prevent this, there are some techniques that can detect or prevent stack overflows:

• Guard Region – A region of RAM just below the stack can trigger a hardware fault on write, detecting overflow.
• Stack Limit Register – The stack limit address can trigger a fault on overflow.
• Stack Canaries – Values placed between stack frames can detect overflow if overwritten.
• MPU Regions – The Memory Protection Unit can generate exceptions on stack overflow.

Additionally, care should be taken in software to not use more stack space than allocated, and to not corrupt the stack pointer value itself.

Using the Stack Pointer in Assembly

When coding in assembly language, the stack pointer is directly accessible for manipulation. This allows function calls, context switching, and manual stack allocation to be implemented in software.

Some examples of using the stack pointer in Cortex-M0 assembly code: // Push values onto stack SUB sp, #16 ; Subtract 16 from stack pointer STR r0, [sp, #0] ; Store register r0 to stack STR r1, [sp, #4] ; Store register r1 to stack STR r2, [sp, #8] ; Store register r2 to stack // Access stacked data LDR r3, [sp, #0] ; Load r3 from stack LDR r4, [sp, #4] ; Load r4 from stack // Pop values off stack ADD sp, #16 ; Add 16 to stack pointer // Function call stack usage PUSH {lr} ; Push link register (return address) BL func ; Call function POP {pc} ; Pop return address into program counter

The stacked data can be local variables, register values, return addresses, or any other temporary data. Direct manipulation of the stack pointer allows detailed control of the Cortex-M0 stack.

Typical Cortex-M0 Stack Contents

Here are some typical items that may be found on a Cortex-M0 stack during normal operation:

• Function return addresses
• Function parameters and local variables
• Register values and CPU state during interrupts
• Exception and interrupt handling information
• Stack canaries for tracking overflow
• Free space allocated with stack-based variables

Analyzing the contents of the stack can reveal useful information about the program execution flow, the current function call hierarchy, and the usage of stack space.

Interacting with the Stack via C Code

In C code, the stack pointer may not be directly accessible. However, the stack is still used implicitly for function calls, local variables, and other temporary data storage: void func(int a) { int b; // b stored on stack b = a + 1; // stack used for calculation … } int main() { func(42); // stack used for call … }

The compiler automatically generates code to manage the stack pointer and leverage the stack for these common operations. However, embedded C code can also benefit from explicitly using the stack in some cases for efficiency or control over memory usage.

Conclusion

In summary, the Cortex-M0 stack pointer is a key register that enables efficient embedded code execution via automatic stack allocation. Understanding how to initialize, manage, and utilize the SP/MSP registers and the stack memory region is an important skill for Cortex-M0 development. Careful usage of the stack allows minimizing memory usage, preventing overflows, and optimizing performance in embedded applications.