Adding Numbers Example

This Tutorial is intended to respond a question about how to get what a function returns, and where to stored the return value of the function.

EAX The "Official" place for the return value of a function

As you may read in http://www.freebsd.org/doc/en/books/developers-handbook/x86-return-values.html

This function should return the number 5

int getNumber5(void){
__asm
{
mov eax,5
}
}

This is simple. We only copy 5 to eax register. And because eax is the "official" return register its the same as:

int getNumber5(void){
return (5);
}

Remember that in C/C++, and every language that I know, you may only return one value, or an address pointer. Usually 32 bits(4 bytes) in a 32 bit architecture, this is because of the size of EAX.

EAX is Not a Stack!

Please remember that EAX, is part of the general purpose registers.
Refer to http://rodrigosavage.blogspot.com/2010/07/hello-world-with-inline-asm.html#purposeReg

or to Intel documentation 253665.pdf, where they explain each register an how should you use it.

What intel says about each register:

• EAX — Accumulator for operands and results data
  
• EBX — Pointer to data in the DS segment
  
• ECX — Counter for string and loop operations
  
• EDX — I/O pointer
  
• ESI — Pointer to data in the segment pointed to by the DS register; source pointer for string operations
  
• EDI — Pointer to data (or destination) in the segment pointed to by the ES register; destination pointer for string operations
  
• ESP — Stack pointer (in the SS segment)
  
• EBP — Pointer to data on the stack (in the SS segment)
  

This means that EAX, is an Accumulator(http://en.wikipedia.org/wiki/Accumulator_(computing)).
And ESP, will point to the top of the stack that is manage by Intel push and pops instructions.

The next image was taken from Intel documantation.
It shows how are the registers and there names.

ADD — Add Instruction

Intel 253666.pdf documentation states that:
Adds the destination operand (first operand) and the source operand (second
operand) and then stores the result in the destination operand. The destination
operand can be a register or a memory location; the source operand can be an imme-
diate, a register, or a memory location. (However, two memory operands cannot be
used in one instruction.) When an immediate value is used as an operand, it is sign-
extended to the length of the destination operand format.

This means that the code

add EAX, number

will be the same as C/C++ like

EAX = EAX + number;

Just keep in mind that this is an integer addition.

Intel inline assembly function for adding two numbers

int addNumbers(int n1,int n2){
__asm
{
mov eax,n1
add eax,n2
}
}

This function only receives two parameters, that are identified by n1 and n2. We first copy n1 to EAX register
and later we add eax + n2, and save the result to eax(the return accumulator).

this would be the same as C/C++ like:

int addNumbers(int n1,int n2){
EAX=n1;
EAX = EAX+n2;
}

(keep in mind that you cannot not access eax directly or any other CPU register like this, inline assembly is necessary to do so).
Or at higher level:

int addNumbers(int n1,int n2){
return(n1+n2);
}

Final Code

#include <stdio.h>
#include <iostream>
using namespace std;

int addNumbers(int n1,int n2){
__asm
{
mov eax,n1
add eax,n2
}// In C/C++, what the function returns is usally stored in eax register

//return n1+n2;
}

void execQuestion2()
{

char format[] = "%u \n";
int n1 = 10;
int n2 = 11;
int result = addNumbers(n1, n2);

cout << n1 << " + " << n2 << " = ";

__asm {
// push n1 and n2 onto the stack
mov eax, n1
push eax
mov eax, n2
push eax
// call "addNumbers" method
call addNumbers
push eax // we put the result of addNumbers into the stack
// Question 2.4 - put formatting onto the stack
lea eax, format
push eax
// Question 2.5 - call "printf" method
call DWORD ptr printf
// Question 2.6 - empty the stack
pop ebx
pop ebx
pop ebx
pop ebx

}
}

int main()
{
execQuestion2() ;
getchar();
}

If you have any problem or question
pleas don't hesitate in commenting :)

I will gladly respond. No matter the question ^^