Nios® II Processor Reference Guide

ID 683836
Date 8/28/2023
Document Table of Contents

7.4.4. Function Prologues

The Nios II C/C++ compiler generates function prologues that allocate the stack frame of a function for storage of stack temporaries and outgoing arguments. In addition, each prologue is responsible for saving the state of the calling function. This entails saving certain registers on the stack. These registers, the callee-saved registers, are listed in Nios II ABI Register Usage Table in the Register Usage section. A function prologue is required to save a callee-saved register only if the function uses the register.

Given the function prologue algorithm, when doing a back trace, a debugger can disassemble instructions and reconstruct the processor state of the calling function.

Note: An even better way to find out what the prologue has done is to use information stored in the DWARF-2 debugging fields of the executable and linkable format (.elf) file.

The instructions found in a Nios II function prologue perform the following tasks:

  • Adjust the stack pointer (to allocate the frame)
  • Store registers to the frame
  • Set the frame pointer to the location of the saved frame pointer

A function prologue

/* Adjust the stack pointer */
addi    sp, sp, -16   /* make a 16-byte frame */

/* Store registers to the frame */
stw     ra, 12(sp)    /* store the return address */
stw     fp, 8(sp)     /* store the frame pointer*/
stw     r16, 4(sp)    /* store callee-saved register */
stw     r17, 0(sp)    /* store callee-saved register */

/* Set the new frame pointer */
addi    fp, sp, 8