6.1.2 C/C++ System Stack

The C/C++ compiler uses a stack to:

• Allocate local variables
• Pass arguments to functions
• Save register contents

The run-time stack grows from the high addresses to the low addresses. The compiler uses the R2 register to manage this stack. R2 is the stack pointer (SP), which points to the next unused location on the stack.

The linker sets the stack size, creates a global symbol, __STACK_SIZE, and assigns it a value equal to the stack size in bytes. The default stack size is 256 bytes. You can change the stack size at link time by using the --stack_size option with the linker command. For more information on the --stack_size option, see the linker description chapter in the PRU Assembly Language Tools User's Guide.

At system initialization, SP is set to a designated address for the top of the stack. This address is the first location past the end of the .stack section. Since the position of the stack depends on where the .stack section is allocated, the actual address of the stack is determined at link time.

The C/C++ environment automatically decrements SP at the entry to a function to reserve all the space necessary for the execution of that function. The stack pointer is incremented at the exit of the function to restore the stack to the state before the function was entered. If you interface assembly language routines to C/C++ programs, be sure to restore the stack pointer to the same state it was in before the function was entered.

For more information about using the stack pointer, see Section 6.3; for more information about the stack, see Section 6.4.

NOTE

Stack Overflow

The compiler provides no means to check for stack overflow during compilation or at run time. A stack overflow disrupts the run-time environment, causing your program to fail. Be sure to allow enough space for the stack to grow. You can use the --entry_hook option to add code to the beginning of each function to check for stack overflow; see Section 2.13.