---
title: "__m64"
description: "Learn more about: __m64"
ms.date: "11/04/2016"
f1_keywords: ["__m64_cpp"]
helpviewer_keywords: ["__m64 keyword [C++]"]
---
# `__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](keywords-cpp.md)\
[Built-in types](fundamental-types-cpp.md)\
[Data Type Ranges](data-type-ranges.md)