Question

What’s the point of saving the old base pointer on the stack at the beginning of a function? I’m new to working with functions in assembly, but so far I have yet to see the point of doing this. It just gets pushed onto the stack and then popped off at the end, it doesn’t do anything. For example the following code works just fine without doing this:

.section .data
.section .text
.globl _start

.type add, @function
add:
    mov %rsp, %rbp
    mov 8(%rbp), %rax
    mov 16(%rbp), %rdi
    add %rax, %rdi

    mov %rbp, %rsp
    ret

_start:
    push $45
    push $36
    call add
    add  $16, %rsp

    mov $60, %rax
    syscall

I know that you could have simplified this even further by just using the stack pointer in this example, but I can see how that’s bad practice.

Answer 1

Every function using xBP to locate its parameters or local variables needs to set xBP to xSP at the very beginning.

By doing so, it destroys the previous value of xBP from the calling function and so, naturally, it should save and restore it by e.g. using push and pop.

If xBP isn’t used at all, it doesn’t need to be saved and restored.

Many compilers have an option to use xSP to locate function parameters and local variables. If that option is enabled, xBP may not need to be preserved (unless the calling convention requires its preservation).