How to add two variables in NASM?

Question

Here is my code which takes in two variables and sums them and subtracts them. Then, it prints the output. I have removed some of the functions to keep it simple:

_start:

    call _printfirstmsg
    call _getvar1
    call _printsecondmsg
    call _getvar2
    call _addvar1var2
    call _printsum
    call _subvar1var2
    call _printdiff
    call _dispvar1
    call _dispvar2
    
    mov rax, 60
    mov rdi, 0
    syscall
    
_printsum:
    mov rax, 1
    mov rdi, 1
    mov rsi,addi
    mov rdx, 4
    syscall
    
    ret
    
    
 _printdiff:
    mov rax, 1
    mov rdi, 1
    mov rsi,subt
    mov rdx, 4
    
    
    syscall
    ret
    


_addvar1var2:
   mov rax, [var1]
   add rax, [var2]
   mov [addi], rax
   syscall
   ret

 
_subvar1var2:
  mov rbx, [var1]
  sub rbx, [var2]
  mov [subt], rbx
  syscall
  ret

When I run the function, instead of returning the values of _printsum and _printdiff, it shows some unknown characters as outputs. Please tell me where I am mistaken.

Sep Roland · Answer 1 · 2020-12-21T17:36:55.720

0

_addvar1var2:
  mov rax, [var1]
  add rax, [var2]
  mov [addi], rax
  syscall
  ret

What do you think that the syscall does in this addition code? RAX is the sum of those 2 numbers and not a function number. Similar problem in the subtraction code.

...it shows some unknown characters as outputs

Your addi and subt variables contain numbers. Before outputting them with a function that expects characters, you must first convert them into text characters.

Read about one way how to display a number
Do read the comments below that question so that you can correct the code.

edited Dec 21 '20 at 17:36

answered Dec 21 '20 at 17:07

Sep Roland

33,889
7
43
76

I wanted to end this function and return to start. – anonymous1234 Dec 21 '20 at 17:09
should I not be using syscall? – anonymous1234 Dec 21 '20 at 17:10
Makes no sense in that spot. It's a subroutine, so just return from it using `ret`. – Sep Roland Dec 21 '20 at 17:11
I tried it, and it is still showing the same output – anonymous1234 Dec 21 '20 at 17:15
Numbers need to get converted into text. – Sep Roland Dec 21 '20 at 17:16
@SCan you please elaborate that further? I am just a beginner trying to self-learn this. – anonymous1234 Dec 21 '20 at 17:23
Your *_printsum* code asks the syscall 1 to output 4 **characters** stored at the address *addi*, but at that address in memory there are no characters! What is stored there is the binary value of the sum that you've calculated. You need to write a conversion routine that translates e.g. the number 71295 into the string of characters "7", "1", "2", "9", and "5". Then that you can ask to output. – Sep Roland Dec 21 '20 at 17:29

How to add two variables in NASM?

1 Answers1