# A common error occurring when compiling a 64\-bit application: error C4235, Assembler

Visual C\+\+ does not support 64\-bit inline assembler\.

That is why you get an error when trying to compile a code like this:

```cpp
void waitvrt(void)
{
  __asm {
      mov  dx,3dah
  VRT:
      in    al,dx
      test  al,8
      jnz    VRT
  NoVRT:
      in    al,dx
      test  al,8
      jz    NoVRT
  }
}
1>.\Third_party\Src\CreditsThread.cpp(111) :
error C4235: nonstandard extension used :
'__asm' keyword not supported on this architecture
```

If you still need to use assembler code, you may use a third\-party 64\-bit assembler, for example, MASM\.

But you will most likely need to rewrite the existing code in C/C\+\+\. The assembler code is likely to be obsolete and it is more reasonable to use modern functions provided by operating systems or C/C\+\+ constructs\. Using assembler for the purpose of optimization can rarely be justified because Visual C\+\+ compiler creates rather efficient code in most cases\. Also, remember that you may use intrinsic\-functions\.

Intrinsic\-functions are special system\-dependent functions performing such actions that cannot be performed at the level of C/C\+\+ code or that do it much more efficiently than other means\. On the whole, they allow you to get rid of inline\-assembler because it is often undesirable or impossible to use it\.

Programs can use intrinsic\-functions to create faster code because there are no expenses on calling common functions\. Of course, the size of the code will be a bit larger\. MSDN gives the list of the functions that can be replaced with their intrinsic\-versions\. For example, these are _memcpy_, _strcmp_, etc\.

Microsoft Visual C\+\+ compiler has a special option "/Oi" that allows you to replace the calls of some functions with their intrinsic\-versions automatically\.

Besides automatic replacement of common functions with intrinsic\-versions, you may use intrinsic\-functions in the code explicitly\. This is why it may be useful:

* As I have already said, inline assembler is not supported by Visual C\+\+ in the 64\-bit mode\. But intrinsic\-code is\.
* Intrinsic\-functions are simpler to use because you do not need to know registers or other similar low\-level constructs when dealing with them\.
* Intrinsic\-functions are updated in compilers\. But assembler code must be updated manually\.
* The embedded optimizer does not work with assembler code, so you need external linking of the module\. But this all does not concern intrinsic\-code\.
* Intrinsic\-code is easier to port than assembler code\.

Using intrinsic\-functions in automated mode \(with the help of the compiler switch\) allows you to get some per cent of performance gain at no cost, and "manual" introduction of intrinsic\-functions allows you to get even more\. That is why using them is absolutely justified\.

To learn more about intrinsic\-functions see the [Visual C\+\+ team blog](https://devblogs.microsoft.com/cppblog/new-intrinsic-support-in-visual-studio-2008/)\.