MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

16-Bit C Compiler User’s Guide
4.9 THE C STACK USAGE
The C compiler uses the software stack to:
• Allocate automatic variables
• Pass arguments to functions
• Save the processor status in interrupt functions
• Save function return address
• Store temporary results
• Save registers across function calls
The run-time stack grows upward from lower addresses to higher addresses. The
compiler uses two working registers to manage the stack:
• W15 – This is the Stack Pointer (SP). It points to the top of stack which is defined
to be the first unused location on the stack.
• W14 – This is the Frame Pointer (FP). It points to the current function’s frame.
Each function, if required, creates a new frame at the top of the stack from which
automatic and temporary variables are allocated. The compiler option
-fomit-frame-pointer can be used to restrict the use of the FP.
FIGURE 4-1: STACK AND FRAME POINTERS
Stack grows
toward
greater
addresses
Function Frame
SP (W15)
FP (W14)
The C run-time startup modules (crt0.o and crt1.o in libpic30.a) initialize the
Stack Pointer W15 to point to the bottom of the stack and initialize the Stack Pointer
Limit register to point to the top of the stack. The stack grows up and if it should grow
beyond the value in the Stack Pointer Limit register, then a stack error trap will be taken.
The user may initialize the Stack Pointer Limit register to further restrict stack growth.
The following diagrams illustrate the steps involved in calling a function. Executing a
CALL or RCALL instruction pushes the return address onto the software stack. See
Figure 4-2.
DS51284H-page 70 © 2008 Microchip Technology Inc.