Accessing stack pointer SP from C/C++ code

Hello Dinesh,

the stack pointer is typically held in register B15. However, the compiler is free to change the register assignments within a function for the benefit of optimization. So, you can probably not rely on B15 being the stack pointer at an arbitrary place within a more complex function.

To my knowledge, there is no way to access individual registers from C or C++ because the compiler manages the allocation of registers and assumes it can use any. So, the cleanest way to retrieve the stack pointer would be by way of a (C-callable) assembly function, which returns this value. However, here is a trick that might work to retrieve the B15 value from C:

- Implement a short sub function, and prevent it from being inlined.
- Exploit the following C calling convention: return value of a function is passed in register A4.
- Use inline assembly to make register accesses and retrieve B15. For a short function, it is assumed that B15 indeed contains the stack pointer.

unsigned int *returnSP( int dummy ) // register A4 is used for the first parameter passed to a C function, and also to pass the function's return value
{ // we can assume register A4 is available, i.e. not otherwise used by the compiler for this function implementation
   asm(" MV B15,A4"); // insert an assembly call - note: instruction starts with whitespace (otherwise it would be interpreted as label); the compiler will write out this assembly instruction, and not care about its meaning
   return dummy; // return value passed in register A4
}

In calling function:
myStackPointer = returnSP(0);

Thanks and best regards,
Andre.

P.S.: The above is true for the C6000 DSP family. I'm not sure whether that's the CPU that your question was about?