Question

We have to code in Assembly which is as you may know hard. Or professor didnt give us any information how to code or link properly. We had to learn it by ourselves. Tomorrow I have to hand in my work.
The aim is to print the n.th Fibonacci number to the console.

section .data
    fmt: db "fib=%d",10,0

section .text
extern _printf
global _main, fib

_main:
    mov eax, 10     ; e.g. n=10
    mov ebx, 1      ; we know f_0 and f_1
    sub eax, ebx    ; thats why n--
    push eax        ; push n
    push 0      ; f_0 = 0 
    push 1      ; f_1 = 1

    call fib

    push eax        ; in eax the result is stored
    push fmt
    call _printf

    mov ebx, 0
    mov eax, 1      ; exit(0)
    int 0x80    


fib:
    mov ecx, eax
    pop esi
    calc:
    pop ebx
    pop edx
    add ebx, edx
    mov eax, ebx
    push edx
    push ebx
    sub ecx, 1
    cmp ecx, 0
    jne calc
    push esi
    ret

After struggling with the linker I finally managed to assemble and link my program. But it doesn’t work – I get a segmentation fault everytime.

Edit:
corrected code – I get some text on the console, but sadly no correct number it always displays “fib=1” and again seg fault.

Answer 1

You have

push 0      ; f_0 = 0 
push 1      ; f_1 = 1

call fib

then

fib:
    mov ecx, eax
    calc:
    pop ebx
    pop edx

You can’t do that – the call instruction will push the return address on the stack, and that’s the first thing you’ll be popping off in fib.