Linux assembly programming addition and multipication not getting output

[jovin555]

I am new to linux assembly language programming and am doing coding with basic addition and multiplication and getting wrong output.Following is my output


jovin@jovin-desktop ~/Documents/Linuxasmpgm/examples/tute4 $ ./runme
Give us a number:1
2nd number:2
3rd number:3
4th number:4
Sum of ints is 5
1x17 is 0
1-5x2is 0


The following is my code[main.cpp]


#include <iostream>

using namespace std;

extern "C" int AddInts(int a,int b,int c,int d);
extern "C" int MulBy17(int b);
extern "C" int Sub5x(int a,int b);

int main()
{
int a,b,c,d;
cout<<"Give us a number:";
cin>>a;
cout<<"2nd number:";
cin>>b;
cout<<"3rd number:";
cin>>c;
cout<<"4th number:";
cin>>d;

cout<<"Sum of ints is "<<AddInts(a,b,c,d)<<endl;
cout<<a<<"x17 is "<<MulBy17(b)<<endl;
cout<<a<<"-5x"<<b<<"is "<<Sub5x(a,b)<<endl;
cout<<endl;

return 0;
}

asm.asm


global AddInts
global MulBy17
global Sub5x

section .text
AddInts:
mov eax,edi
add eax,esi
add eax,ecx
add eax,edx
ret
MulBy17:
imul edi,17
mov eax,edi
ret
Sub5x:
imul esi,5
sub edi,esi
mov eax,edi
ret



makefile


runme: main.cpp asm.o
g++ main.cpp asm.o -o runme

asm.o: asm.asm
nasm -f elf asm.asm -o asm.o


Can anyone let me know what is wrong with my coding?

[DaviesX]

Why do you assume a, b, c, d will be in the register? They should be in the stack if using __std_call. Maybe you should try this if you are in 32 bits platform:



mov eax, [esp + 4]
add eax, [esp + 8]
add eax, [esp + 12]
add eax, [esp + 16]
ret

[jovin555]

Why do you assume a, b, c, d will be in the register? They should be in the stack if using __std_call. Maybe you should try this if you are in 32 bits platform:



mov eax, [esp + 4]
add eax, [esp + 8]
add eax, [esp + 12]
add eax, [esp + 16]
ret


I have tried as you mentioned and i got the correct output for the first case.but how can i solve the second and third case for multiplication and subtraction.?

I was watching a tutorial series and based on that i was doing the coding.The following is the link to the tutorial.Can you explain why in that tutorial the coding is the same as i have mentioned earlier and also getting the right output.The following is the link for the tutorial.

https://www.youtube.com/watch?v=nDj35pMLBQE

[DaviesX]

Great. I think this link will help: http://stackoverflow.com/questions/3699283/what-is-stack-frame-in-assembly (see the second answer).

When you call a function in C, the gcc will follow CDECL calling convention: parameters are pushed from right to left, so the leftmost parameter is the closest to esp while the rightmost parameter is the farthest.

The reason why the code in the video is different is because he was writing X64 assembly instead of X84. You can install a 64 bit Linux and code in X64 :)