Sum 10 numbers and print result in NASM

Question

I have written following assembly code for adding 10 numbers.I am able to compile it and execute it but I am getting wrong result. I just wanted to know how do I print the value of total on scree.

section .data
num1: dw 10, 20, 30, 40, 50, 10, 20, 30, 40, 50
total: dw 0
msg :  db "sum=%d",10,0

section .text
    extern _printf
    global _main
_main:
    push ebp
    mov ebp,esp
    mov ebx,num1 ;point bx to first number
    mov ecx,10      ;load count of numbers in ecx
    mov eax,0       ;initialize sum to zero
loop:
    add eax,[ebx]
    add ebx,2
    sub ecx,1
    jnz loop
    mov [total],eax

    push total
    push msg
    call _printf

    pop ebp
    mov esp,ebp
    ret

solution

section .data
num1: dd 10, 20, 30, 40, 50, 10, 20, 30, 40, 50,300
total: dd 0
msg :  dd "sum=%d",10,0

    section .text
        extern _printf
        global _main
    _main:
        push ebp
        mov ebp,esp
        mov ebx,num1 ;point bx to first number
        mov ecx,11     ;load count of numbers in ecx
        mov eax,0       ;initialize sum to zero
    loop:
        add eax,[ebx]
        add ebx,4
        sub ecx,1
        jnz loop
        mov  [total],eax
        push dword [total]

        push msg
        call _printf
        mov esp,ebp
        pop ebp

        ret

Solution 1

I see a couple of issues here. First, you've got num1 and total declared as dw. dw may sound like it means "dword", but it means "data word". You want these to be dd - "data dword"... since that's how you're using them. (and add ebx, 4 not 2) If you really need to use word (16-bit) values in 32-bit code, it can be done, but is awkward.

The second problem I see is that push total before your call _printf pushes the address of total. You want the "[contents]" of memory here, so push dword [total]. (push msg is correct)

After this, you probably want add esp, 8 (I like to write it as add esp, 4 * 2 - two parameters of 4 bytes each). It is possible to "defer" this stack cleanup - mov esp, ebp will fix you up, but it needs to be done BEFORE the pop ebp!!!

... there may be more...

Solution 2

You are using 32-bit memory access to work with 16-bit values.

Each element of the array is 16 bits wide. However, since eax is 32 bits wide, the following treats the array as if each element was also 32 bits wide:

add eax,[ebx]

The following also isn't quite correct, for exactly the same reason:

mov [total],eax

I think there's also an issue whereby you're pushing the address of total before calling printf(), whereas you should probably be pushing its value.

Also, as pointed out by @JasonD, you need to clean up the stack after calling printf().

Finally, the

mov esp,ebp

instruction should be removed as all it does is corrupt your stack pointer.

Solution 3

dw defines 16-bit entities but add eax,[ebx] is adding 32-bit entities. Either change dw to dd or put WORD PTR before [ebx]. Also, as NPE points out, you need to change how you handle storing eax into total.

Comments

[NPE]

@JasonD: Oh yeah, I forgot about that. I'll add a note to the answer. Thanks.