Mixing C/C++ Code with x86 Assembly Code in Visual C++

When you need to perform low-level programming or speed demanding projects, you can consider mixing x86 ASM code within your C/C++ source code by using the __asm keyword. Using ASM in Visual C++ has advantages over using assembler such as the Microsoft assembler, such as:

• Variables can be used both inside the __asm block as well as outside the __asm block, as long as the variables are in scope.
• Secondly, you don't have to do assembling and linking in two different steps. Just use the build command with Visual C++.
• Lastly, you can conveniently make use of other windowing features.

The __asm keyword can be used in two ways:

__asm mov ax,bx
__asm add ax,cx

or

__asm {
	mov ax,bx
	add ax,cx

}

Here is an example of using C variables in the ASM code:

#include <windows.h>
#include <stdio.h>
#ifndef NULL
#define NULL 0
#endif
int WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
{
	short int i;
	char * txt;
	txt = (char *)malloc(256);
	i = 0;
	sprintf(txt,"i = %d",i);
	MessageBox(0,txt,"",MB_OK);

	__asm {
		pusha
		mov ax,i
		add ax,10
		mov i,ax
		popa
	}
	sprintf(txt,"i = %d",i);
	MessageBox(0,txt,"",MB_OK);
	return 0;
}