linux - Understanding recursive factorial function in NASM Assembly -


learning nasm assembly on 32-bit ubuntu. i'm trying learn recursive functions, starting factorial (note: here assuming parameter non-negative).

assuming have

push 3 call factorial

i want end 6 in eax.

here attempt:

section .text global main main: ; ----------------------------------------------------------- ; main ; ----------------------------------------------------------- push    3 call    factorial  ; ----------------------------------------------------------- ; exit ; ----------------------------------------------------------- mov eax,1 int 0x80  ; ----------------------------------------------------------- ; recursive factorial: n! = n * (n - 1)! ; ----------------------------------------------------------- factorial:  push    ebp         ; retrieve parameter , put mov     ebp,esp     ; ebx register add     ebp,8       ; mov     ebx,[ebp]   ; ebx = param  cmp     ebx,0       ; check base case je      base        ; base if (n == 0)  dec     ebx         ; decrement ebx put in stack push    ebx         ; put (ebx - 1) in stack inc     ebx         ; restore ebx call    factorial   ; calculate factorial (ebx - 1) mul     ebx         ; eax = (eax * ebx) = (ebx - 1)! * ebx pop     ebx         ; retrieve ebx stack  jmp end base:               ; base case mov     eax,1       ; result 1  end:  pop     ebp         ; release ebp ret

at least works base case, ha... other value push, returns 0. had suspicion maybe since eax 0, mul result in 0, explaining this. test, decided give eax value of 2, expecting non-zero value, kept resulting in 0.

can advice me on how recursive factorial function takes parameter stack? believe having seen examples, either not recursive or took parameters other places, or used bunch of variables (when think can done registers).

note factorial(n-1) overwrite factorial(n)'s value of ebx first thing does, thereby rendering inc ebx after push pointless. after you've reached base case you'll have situation ebx 0 when mul, , of course * 0 == 0.

the easiest fix change prologue to:

push    ebp         ; retrieve parameter , put push    ebx         ; save previous param mov     ebp,esp     ; ebx register add     ebp,12       ; mov     ebx,[ebp]   ; ebx = param

and epilogue to:

pop     ebx         ; restore previous param pop     ebp         ; release ebp ret

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