Question

How to replace _asm nop instructions in 64-bit. compiles and works in 32-bit.

Answer 1

I believe that you can use the __nop intrinsic function. This should compile into the appropriate machine instruction for the processor that you are building.

http://msdn.microsoft.com/en-us/library/aa983381(VS.80).aspx

UPDATE:

Here is an example that I just created with VS2008. It compiles for both Win32 and x64 configurations:

#include "stdafx.h"
#include <intrin.h>

int _tmain(int argc, _TCHAR* argv[])
{
    // The intrinsic function below will compile to a NOP machine instruction.
    // This is the same as _asm nop in the 32-bit compiler.
    __nop();
    return 0;
}

In case you are wondering, I disassembled the above example and the x64 output was:

wmain    proc near
         db    66h
         nop
         nop
         xor   eax, eax
         retn
wmain    endp

The Win32 output was:

_main    proc near
        nop
        xor    eax, eax
        retn
_main   endp