linux - NASM Assembly recursive fibonacci -


learning nasm assembly on 32-bit ubuntu.

i've been learning recursive functions. did factorial, here: understanding recursive factorial function in nasm assembly

watching code, thought maybe implement fibonacci well, using same code. here code, assuming parameter greater 0:

section .text global main main: ; ----------------------------------------------------------- ; main ; ----------------------------------------------------------- push    6 call    fibonacci mov     [ecx],eax add     byte [ecx],'0' mov     edx,1 call    print  ; ----------------------------------------------------------- ; exit ; ----------------------------------------------------------- mov eax,1 int 0x80  ; ----------------------------------------------------------- ; fibonacci recursivo: f(n) = f(n - 1) + f(n - 2) ; ----------------------------------------------------------- fibonacci:  push    ebp         ; retrieve parameter , put push    ebx         ; save previous parameter mov     ebp,esp     ; ebx register add     ebp,12      ; mov     ebx,[ebp]   ; ebx = param  cmp     ebx,1       ; check base case jle     base        ; base if (n <= 1)  dec     ebx     ; decrement ebx put in stack push    ebx     ; put (ebx - 1) in stack inc     ebx     ; restore ebx call    fibonacci   ; calculate fibonacci (ebx - 1) mov esi,eax     ; eax = (eax + ebx) pop ebx         ; retrieve ebx stack  sub ebx,2       ; decrement ebx put in stack push    ebx     ; put (ebx - 2) in stack add ebx,2       ; restore ebx call    fibonacci   ; calculate fibonacci (ebx - 2) mov edx,eax     ; eax = (eax + ebx) pop ebx         ; retrieve ebx stack  add esi,edx mov eax,esi  jmp end base:               ; base case mov eax,1           ; result 1  end:  pop     ebx         ; restore previous parameter pop     ebp         ; release ebp ret

it bit crude. calculate fibonacci (parameter - 1), again (parameter - 2), , add them , put them eax.

it doesn't work:

2 => 2 3 => 3 4 => 4 5 => 4

fortunately fixed segmentation fault errors, broke else doing that. don't see what's problem. can tell me why getting wrong values?

one particular observation that, reason, doing mov ecx,eax gave me segmentation fault error. that's why used esi instead. i'm not sure why, guess related.

whenever you're dealing recursion, have careful next layer in recursive chain state of current layer (e.g. register values). i'd suggest rewriting function follows:

fibonacci: push    ebp         ; retrieve parameter , put push    ebx         ; save previous parameter mov     ebp,esp     ; ebx register add     ebp,12      ; mov     ebx,[ebp]   ; ebx = param  cmp     ebx,1       ; check base case jle     base        ; base if (n <= 1)  lea ecx,[ebx-1] push ecx            ; push n-1 call    fibonacci   ; calculate fibonacci (ebx - 1) pop ecx             ; remove n-1 off stack  push eax            ; save result of fibonacci(n-1) lea ecx,[ebx-2] push ecx            ; push n-2 call    fibonacci   ; calculate fibonacci (ebx - 2) pop ecx             ; remove n-2 off stack pop ecx             ; ecx = fibonacci(n-1) add eax,ecx         ; eax = fibonacci(n-2) + fibonacci(n-1)  jmp end base:               ; base case mov eax,1           ; result 1  end: pop     ebx         ; restore previous parameter pop     ebp         ; release ebp ret

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