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title: "__m64 | Microsoft Docs"
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# __m64
## Microsoft Specific  
 The `__m64` data type is for use with the MMX and 3DNow! intrinsics, and is defined in xmmintrin.h.  
  
```  
// 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 [!INCLUDE[vcprx64](../assembler/inline/includes/vcprx64_md.md)] 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)   
 [Fundamental Types](../cpp/fundamental-types-cpp.md)   
 [Data Type Ranges](../cpp/data-type-ranges.md)