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Convert 64-bit double to 80-bit double in byte array form in visual studio
inline void ConvertDblToLongDbl(double dbl, unsigned char aCh[10])
{
__int64 ull= *(__int64*)(&dbl);
    *(unsigned short*)&aCh[8]= (unsigned short)((ull>>52&0x7FF+15360)|  //     exponent, from 11 bits to 15 bits
        ((ull&(__int64)1<<63)?0x8000:0));   // sign, the 16th bit
    ull= ull&0xFFFFFFFFFFFFF;
    *(__int64*)&aCh[0]= ull|0x8000000000000000;
}

void ConvertDoubleToLongDouble(double value, unsigned char result[10]) {
    __asm {
        fld value;
        mov ebx, result;
        fstp tbyte ptr [ebx];
    }
}