---
description: "Learn more about: __m64"
title: "__m64"
ms.date: "11/04/2016"
f1_keywords: ["__m64_cpp"]
helpviewer_keywords: ["__m64 keyword [C++]"]
ms.assetid: df0410e8-67c9-4954-88c8-fe2653575252
---
# __m64

**Microsoft Specific**

The **`__m64`** data type is for use with the MMX and 3DNow! intrinsics, and is defined in \<xmmintrin.h>.

```cpp
// data_types__m64.cpp
#include <xmmintrin.h>
int main()
{
   __m64 x;
}
```

## Remarks

You should not access the **`__m64`** fields directly. You can, however, see these types in the debugger. A variable of type **`__m64`** maps to the MM[0-7] registers.

Variables of type **_m64** are automatically aligned on 8-byte boundaries.

The **`__m64`** data type is not supported on x64 processors. Applications that use __m64 as part of MMX intrinsics must be rewritten to use equivalent SSE and SSE2 intrinsics.

**END Microsoft Specific**

## See also

[Keywords](../cpp/keywords-cpp.md)<br/>
[Built-in types](../cpp/fundamental-types-cpp.md)<br/>
[Data Type Ranges](../cpp/data-type-ranges.md)