Using Assembly from Microsoft Visual C++ 2010

Dr. Orion Lawlor, 2011-10-12

Step 1: Write C++ Code

First, hit File - New Project to make a standard C++ console application:

Next, remove the Microsoft junk, like the procompiled header.

Next, write some test C++ code. It helps to run at this point, to make sure everything works.

Step 2: Write assembly code

Now write an assembly language source file with these critical pieces:

• Give the file the .S extension, the standard extension for assembly code.
• Start the file with "section .text", so the code is executable.
• Make the function "global", so the linker can see it.
• Make the function name begin with an underscore--for some reason Windows compilers stick an underscore on the front of every function name.

Visual doesn't know how to compile the .S file by default, so it ignores it: "Does not participate in build." Change this to "Custom Build Tool": Now you need to download and install NASM (or YASM, etc.) You then give Visual Studio the Command Line needed to run your assembler, just like at the DOS prompt:

	"\Program Files\nasm\nasm" -f win32  asmstuff.S

Build the project, and you should be able to call assembly functions from your C++, and vice versa!  Don't forget extern "C" from C++!

Simple C++ code:

#include <iostream>
extern "C" int foo(void); // written in assembly!

int main() {
	std::cout<<"Foo returns "<<foo()<<"\n";
	system("pause");
	return 0;
}

section .text ; makes this executable
global _foo ; makes this visible to linker
_foo:
	mov eax,7 ; just return a constant
	ret