Intel ASM Adding to EAX

Question

I'm fairly new to ASM. I'm running ARCH linux 64 bit and use the following commands to compiule and everything runs smoothly:

nasm -f elf32 -o file.o file.asm
ld -s -m elf_i386 -o file file.o

I am taking user input (as a decimal) and I simply want to double it. However, when I use:

add eax, eax

I get no ouput. I then tried:

add eax, 1

Which should add 1 to eax. However, this adds 1 to the memory address. I have 10 bytes reserved in my .bss section.

When I type "1234" it ouputs "234" (shifted +1 byte) as opposed to "1235"

.bss section:

    i: resd 
    j: resd 10

Full statement:

    mov eax, 3 ;syscall
    mov ebx, 2 ;syscall
    mov ecx, i ;input variable
    mov edx, 10 ;length of 'i'
    int 0x80 ;kernel call

    mov eax, i ;moving i into eax
    add eax, 0x1 ;adding 1 to eax
    mov [j], eax ;moving eax into j

Answer 1

What you are operating on is a string representation of the number.

You probably need to:

1. Convert the string to a number
2. Double the number
3. Convert the resulting number back to a string

So something like (untested code ahead):

mov esi,i ; get address of input string
; convert string to a number stored in ecx
xor ecx,ecx ; number is initially 0
loop_top:
mov bl,[esi] ; load next input byte
cmp bl,0 ; byte value 0 marks end of string
je loop_end
sub bl,'0' ; convert from ASCII digit to numeric byte
movzx ebx,bl ; convert bl to a dword
mov eax,10 ; multiply previous ecx by 10
mul ecx
mov ecx,eax
add ecx,ebx ; add next digit value
inc esi ; prepare to fetch next digit
jmp loop_top
loop_end:
; ecx now contains the number
add ecx,ecx ; double it
; to do: convert ecx back to a string value

Page 22 of this document contains a NASM implementation of the atoi function.