NASM addition program - linux

Question

I am a developer who uses high level languages, learning assembly language in my spare time. Please see the NASM program below:
section .data
section .bss
section .text
global main
main:
mov eax,21
mov ebx,9
add eax,ebx
mov ecx,eax
mov eax,4
mov ebx,1
mov edx,4
int 0x80
push ebp
mov ebp,esp
mov esp,ebp
pop ebp
ret
Here are the commands I use:
ian#ubuntu:~/Desktop/NASM/Program4$ nasm -f elf -o asm.o SystemCalls.asm
ian#ubuntu:~/Desktop/NASM/Program4$ gcc -o program asm.o
ian#ubuntu:~/Desktop/NASM/Program4$ ./program
I don't get any errors, however nothing is printed to the terminal. I used the following link to ensure the registers contained the correct values: http://docs.cs.up.ac.za/programming/asm/derick_tut/syscalls.html

Answer 1

You'll have to convert the integer value to a string to be able to print it with sys_write (syscall 4). The conversion could be done like this (untested):
; Converts the integer value in EAX to a string in
; decimal representation.
; Returns a pointer to the resulting string in EAX.
int_to_string:
mov byte [buffer+9],0 ; add a string terminator at the end of the buffer
lea esi,[buffer+9]
mov ebx,10 ; divisor
int_to_string_loop:
xor edx,edx ; clear edx prior to dividing edx:eax by ebx
div ebx ; EAX /= 10
add dl,'0' ; take the remainder of the division and convert it from 0..9 -> '0'..'9'
dec esi ; store it in the buffer
mov [esi],dl
test eax,eax
jnz int_to_string_loop ; repeat until EAX==0
mov eax,esi
ret
buffer: resb 10

Answer 2

programming in assembly requires a knowledge of ASCII codes and a some basic conversion routines. example: hexadecimal to decimal, decimal to hexadecimal are good routines to keep somewhere on some storage.
No registers can be printed as they are, you have to convert (a lot).
To be a bit more helpfull:
ASCII 0 prints nothing but some text editors (kate in kde linux) will show something on screen (a square or ...). In higher level language like C and C++ is it used to indicate NULL pointers and end of strings.
Usefull to calculate string lengths too.
10 is end of line. depending Linux or Windows there will be a carriage return (Linux) too or not (Windows/Dos).
13 is carriage return
1B is the ESC key (Linux users will now more about this)
255 is a hard return, I never knew why it is good for but it must have its purpose.
check http://www.asciitable.com/ for the entire list.

Answer 3

Convert the integer value to a string.
Here i have used macros pack and unpack to convert integers to string and macro unpack to do the vice-versa
%macro write 2
mov eax, 4
mov ebx, 1
mov ecx, %1
mov edx, %2
int 80h
%endmacro
%macro read 2
mov eax,3
mov ebx,0
mov ecx,%1
mov edx,%2
int 80h
%endmacro
%macro pack 3 ; 1-> string ,2->length ,3->variable
mov esi, %1
mov ebx,0
%%l1:
cmp byte [esi], 10
je %%exit
imul ebx,10
movzx edx,byte [esi]
sub edx,'0'
add ebx,edx
inc esi
jmp %%l1
%%exit:
mov [%3],ebx
%endmacro
%macro unpack 3 ; 1-> string ,2->length ,3->variable
mov esi, %1
mov ebx,0
movzx eax, byte[%3]
mov byte[%2],0
cmp eax, 0
jne %%l1
mov byte[%2],1
push eax
jmp %%exit2
%%l1:
mov ecx,10
mov edx,0
div ecx
add edx,'0'
push edx
inc byte[%2]
cmp eax, 0
je %%exit2
jmp %%l1
%%exit2:
movzx ecx,byte[%2]
%%l2:
pop edx
mov [esi],dl
inc esi
loop %%l2
%endmacro
section .data ; data section
msg1: db "First number : " ;
len1: equ $-msg1 ;
msg2: db "Second number : " ;
len2: equ $-msg2 ;
msg3: db "Sum : " ;
len3: equ $-msg3 ;
ln: db 10
lnl: equ $-ln
var1: resb 10
var2: resb 10
str1: resb 10
str2: resb 10
ans: resb 10
ansvar: resb 10
ansl: db ''
l1: db ''
l2: db ''
section.text ;code
global _start
_start:
write msg1,len1
read str1,10
pack str1,l1,var1
write msg2,len2
read str2,10
pack str2,l2,var2
mov al,[var1]
add al,[var2]
mov [ansvar],al
unpack ans,ansl,ansvar
write msg3,len3
write ans,10
write ln,lnl
mov ebx,0 ; exit code, 0=normal
mov eax,1 ; exit command to kernel
int 0x80 ; interrupt 80 hex, call kernel
To assembler, link and run:
nasm -f elf add.asm
ld -s -o add add.o
./add