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;
}

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.

Reference