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How to sum 2 numbers (in Gnu/linux assembly x80_64 & Nasm)?

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Ferran:
Hello everybody !

Suddenly a poor programmer forgot how to sum 2 numbers. He told me he was happy with i386 machines in 32 bits, but now in 64 bits he don't. He gave me this code if its possible we can help him.


--- Code: ---;-----------------------------------------------------------------
; sum.asm
; Learning to sum 2 numbers with 64 bit assembly
; (for GNU/Linux Assembly x80_64 Intel syntax & NASM compiler)
;
; Compile: nasm -f elf64 sum.asm
; Link: ld -s -o sum sum.o
; Run: ./sum
;
; Author: anonymous ashamed...
;------------------------------------------------------------------

segment .data
    num1: dd 12
    num2: dd 55
    lf: db " ", 10, 0
    res: dd "    ", 10, 0
    len_res equ $ - res

segment .text
global _start

_start:

.sum:
    mov rax, [num1]
    add rax, [num2]
    mov [res], rax   

.show_result:
    mov rax, 1
    mov rdi, 1
    mov rsi, res
    mov rdx, len_res
    syscall

.showLF:
    mov rax, 1
    mov rdi, 1
    mov rsi, lf
    mov rdx, 2
    syscall

.exit:
    xor edi, edi
    mov rax, 60
    syscall

--- End code ---

I appeal to the charity of the good people of this forum to help to this poor programmer.

Why don't works the code?

fredericopissarra:
Because you need to transform the binary value in a string...

Ferran:
I saw in a book in Assembly64 pdf http://www.egr.unlv.edu/~ed/assembly64.pdf one script that convert integer to ascii (itoa function), i mixed it with our script but it not prints nothing :(

this is the mix i did


--- Code: ---;-----------------------------------------------------------------
; sum.asm
; Learning to sum 2 numbers with 64 bit assembly
; (for GNU/Linux Assembly x80_64 Intel syntax & NASM compiler)
;
; Compile: nasm -f elf64 sum.asm
; Link: ld -s -o sum sum.o
; Run: ./sum
;
; Author: anonymous ashamed...
;------------------------------------------------------------------

segment .data
    num1 dd 1111111111
    num2 dd 2222222222
    res dd 0                    ; The number to print
    newline db 10

    ; Syscall information
    sys_exit equ 60
    sys_write equ 1

    ; Streams
    stdout equ 1

    ; Constants
    BUFFER_SIZE equ 10

section .bss
    numbuf resb BUFFER_SIZE     ; A buffer to store our string of numbers in

segment .text
global _start

_start:

.sum:
    mov rax, [num1]
    add rax, [num2]
    mov [res], rax

    mov rdi,[res]
    call itoa

 ; Write the string returned in rax out to stdout
    mov rdi,rax ; The string pointer is returned in rax - move it to rdi for the function call
    mov rsi,rcx
    call print

    ; Write the newline character to stdout
    mov rdi,newline
    mov rsi,1
    call print
   
   ; Exit
    mov rax,sys_exit
    mov rbx,0
    int 0x80

; Args: (rdi: char*, rsi: int)

print:   
    mov rax,sys_write
    mov rbx,stdout
    mov rcx,rdi
    mov rdx,rsi
    int 0x80
ret

itoa:
    push rbp
    mov rbp,rsp
    sub rsp,4    ; allocate 4 bytes for our local string length counter

    mov rax,rdi ; Move the passed in argument to rax
    lea rdi,[numbuf+10] ; load the end address of the buffer (past the very end)
    mov rcx,10     ; divisor
    mov [rbp-4],dword 0 ; rbp-4 will contain 4 bytes representing the length of the string - start at zero

.divloop:
    xor rdx,rdx ; Zero out rdx (where our remainder goes after idiv)
    idiv rcx    ; divide rax (the number) by 10 (the remainder is placed in rdx)
    add rdx,0x30    ; add 0x30 to the remainder so we get the correct ASCII value
    dec rdi        ; move the pointer backwards in the buffer
    mov byte [rdi],dl ; move the character into the buffer
    inc dword [rbp-4] ; increase the length
   
    cmp rax,0    ; was the result zero?
    jnz .divloop    ; no it wasn't, keep looping

    mov rax,rdi ; rdi now points to the beginning of the string - move it into rax
    mov rcx,[rbp-4]     ; rbp-4 contains the length - move it into rcx

leave              ; clean up our stack
ret

--- End code ---

I will follow searching the solution, but it seems difficult (for me). See you.

fredericopissarra:
it is wrong. But I already shown how to do it, in another post...

debs3759:
For starters, you are trying to print a number without converting it to an ASCII string first, as fredericopissarra already pointed out.

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