The stack pointer (SP) and program counter (PC) are important registers in the Cortex-M1 processor that control program execution flow. Directly modifying these registers allows developers fine-grained control over the stack and program flow. However, care must be taken as incorrect modifications can lead to crashes or undefined behavior.
Stack Pointer (SP)
The stack pointer register points to the top of the stack in memory. It is used to push and pop data to/from the stack. Modifying SP allows you to dynamically adjust the stack size and placement in memory.
Reading SP
To read the current value of SP, the register can simply be accessed like any other register in Cortex-M1: uint32_t sp_val; sp_val = SP; // Copy SP into sp_val
Modifying SP
SP can be directly written to in order to adjust the stack pointer to a new memory location: SP = NEW_STACK_LOCATION;
However, care must be taken when directly modifying SP:
- The new stack location pointed to by SP must be valid, allocated memory.
- Changing SP will invalidate any stack contents previously stored above the new SP location.
- Interrupts or exceptions may overwrite SP, so modifications may not be preserved.
- Modifying SP outside an exception handler or without interrupts disabled may cause race conditions.
Some common legitimate reasons to modify SP include:
- Setting up a separate stack for an RTOS thread.
- Switching to a different stack during exception handling.
- Dynamically adjusting stack size during runtime.
Saving and Restoring SP
When modifying SP, it is often necessary to later restore it to its original value. This can be done by reading SP and storing it before making changes: uint32_t orig_sp; // Save SP orig_sp = SP; // Modify SP SP = NEW_LOCATION; // Restore SP SP = orig_sp;
The PUSH and POP instructions can also be used to save and restore SP automatically.
Program Counter (PC)
The program counter register contains the current instruction address being executed by the processor. Modifying PC allows you to dynamically change program execution flow.
Reading PC
To read the current value of PC: uint32_t pc_val; pc_val = PC; // Copy PC to pc_val
Modifying PC
PC can be directly assigned to in order to jump to a new instruction address: PC = NEW_INSTRUCTION_ADDRESS;
This will cause the next instruction fetched to be from the new address. Some things to keep in mind:
- The new PC address must point to a valid instruction.
- Interrupts and exceptions will override PC changes.
- Modifying PC outside an exception handler may have unintended consequences.
Here are some examples of modifying PC correctly:
- Implementing a branch or jump instruction.
- Returning from an exception handler.
- Dynamically changing program flow based on run-time conditions.
Saving and Restoring PC
When modifying PC, you will often want to later restore it to its original value. This can be accomplished by first saving PC: uint32_t orig_pc; // Save PC orig_pc = PC; // Modify PC PC = NEW_ADDR; // Restore PC PC = orig_pc;
The PUSH and POP instructions can also be used to save and restore PC automatically.
Modifying SP and PC Safely
Here are some tips for safely modifying SP and PC in Cortex-M1:
- Make changes inside an exception handler or with interrupts disabled.
- Save original values before modifying and restore afterward.
- Validate memory addresses are valid before setting SP or PC.
- Use stack push/pop or store/load instructions to save state.
- Avoid asynchronous changes such as from interrupts.
- Use compiler intrinsics instead of direct register access when possible.
Example Code
Here is some example C code that demonstrates modifying SP and PC: // Save SP and PC uint32_t orig_sp = __get_SP(); uint32_t orig_pc = __get_PC(); // Disable interrupts to prevent asynchronous changes __disable_irq(); // Set Stack Pointer to new location __set_SP(NEW_STACK_LOCATION); // Set Program Counter to new instruction address __set_PC(NEW_INSTRUCTION_ADDRESS); // Do work… // Restore original SP and PC __set_SP(orig_sp); __set_PC(orig_pc); // Re-enable interrupts __enable_irq();
This shows the general pattern of saving state, setting new values, doing work, and restoring state when modifying SP and PC. The __get_* and __set_* intrinsics provide access to the registers while letting the compiler handle specifics.
Conclusion
The Cortex-M1 stack pointer and program counter registers control key aspects of program execution. Modifying these registers allows for dynamic stack and flow control, but care must be taken to do so correctly. Saving state, validating addresses, and using compiler intrinsics can help ensure modifications do not lead to crashes or undefined behavior.
