IEEE754 C 語言代碼

原創

2020-02-21 11:45

void ConvertDexToIEE754(float fpointer,unsigned char *a) //十進制轉化爲 IEEE745 小數
{
   int Flag=0;
    double   integer,decimal;    //整數，小數
    unsigned long   bininteger,bindecimal;   //二進制整數，二進制小數
    int   _power,i;

    if(fpointer<0)
   {
      fpointer=fpointer*(-1);
      Flag=1;
   }

    decimal = modf(fpointer,&integer);   //將整數位到存到 integer ，小數位返回到decimal
    if(decimal || integer)    //判斷 fpointer是否爲0
    {
     bindecimal = (unsigned long )(decimal * 0x800000);   //0x800000=2^23 。得到小數位二進制表現形式
     while((bindecimal & 0xff800000) > 0) //計算有沒有超過23位二進制數
     bindecimal >>= 1;
     if(integer > 0)
     {
      bininteger = (unsigned long )integer;
       for(i=0;i<32;i++)               //計算整數部分的2的冪指數，整數位轉化爲二進制後的位數，計算小數點移動的位數
       {
          if(bininteger&0x1)
         _power = i;
         bininteger >>= 0x1;
       }
   bininteger = (unsigned long )integer;
   bininteger &= ~(0x1 << _power); //去掉最高位的1
   if(_power >= 23) //如果冪指數>23 則捨棄小數位部分
   {
          bininteger >>= (_power-23);
          bindecimal = 127+_power;
          bininteger |= bindecimal << 23;
   }
   else
   {
          bininteger <<= (23 - _power);//將去掉最高位的整數二進制，移至第22爲
          bindecimal >>= _power;       //將小數部分左移1，
          bininteger |= bindecimal;    //二者向或得到1.x的x，
          bindecimal = 127+_power;     //指數，右移的小數點數+127
          bininteger |= bindecimal << 23; // 指數爲右移23爲變爲或上x。
   }
}
    else if(integer == 0)
    {
            bindecimal <<= 9; //將小數部分的二進制移至最高位31位
            _power = 0;
            bininteger = bindecimal;
            while(bininteger == ((bindecimal<<1)>>1)) //判斷小數位最高位是否爲1. 最高位爲0 ：真
            {
              _power++;
              bindecimal <<= 0x1;
              bininteger = bindecimal; //直到最高位爲1,退出循環
            }
            _power++;
            bindecimal <<= 0x1;    //將1.x的1去掉求x的值，
            bindecimal >>= 9;      //將小數位回到0-22位
            bininteger = bindecimal; //暫存到二進制整數中，
            bindecimal = 127-_power;
            bininteger |= bindecimal << 23; //將指數爲右移值23爲向或得到其值，
    }
     if(Flag==1)
      bininteger |= 0x80000000;

   i = 0;
   a[i++] =(unsigned char) ((bininteger >> 24) & 0xff);
   a[i++] = (unsigned char)((bininteger >> 16) & 0xff);
   a[i++] = (unsigned char)((bininteger >> 8 ) & 0xff);
   a[i++] =(unsigned char)( bininteger & 0xff);
    }
}

float ConvertIEE754ToDex(unsigned char *SpModRegister)
{
float x,fpointer;
unsigned long bininteger,bintmp;
int _power,i=0,s;

bintmp = SpModRegister[i++] << 8;
bintmp |= SpModRegister[i++];
bininteger = SpModRegister[i++] << 8;
bininteger |= SpModRegister[i++];
bininteger |= bintmp << 16;

if(bininteger == 0)
return 0.0;

bintmp = bininteger & 0x7FFFFF;
x = (double)bintmp / 0x800000;
bintmp = bininteger >> 23;
_power = bintmp & 0xff;

bintmp = bininteger & 0x80000000;
s = (bintmp) ? 1 : 0 ;

bintmp =(unsigned long ) pow(2,fabs(_power-127)) ;
if(_power >= 127)
fpointer = (1+x) * bintmp ;
else
fpointer = (1+x) / bintmp ;
//bintmp = (fpointer * 10000 + 5)/10;
//fpointer = (double)bintmp / 1000;

if(s)
   fpointer *= -1;

return (fpointer);
}