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Microsoft Specific

The __m64 data type, for use with the MMX and 3DNow! intrinsics, is defined as follows:

// data_types__m64.cpp
typedef union __declspec(intrin_type) __declspec(align(8)) __m64
   unsigned __int64 m64_u64;
   float m64_f32[2];
   __int8 m64_i8[8];
   __int16 m64_i16[4];
   __int32 m64_i32[2];
   __int64 m64_i64;
   unsigned __int8 m64_u8[8];
   unsigned __int16 m64_u16[4];
   unsigned __int32 m64_u32[2];
} __m64;

int main()

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.

END Microsoft Specific

See Also

C++ Keywords | Fundamental Types | Data Type Ranges