MetallicPriest - 6 months ago 59

C Question

I have a program, which is running on two processors, one of which does not have floating point support. So, I need to perform floating point calculations using fixed point in that processor. For that purpose, I will be using a floating point emulation library.

I need to first extract the signs, mantissas and exponents of floating point numbers on the processor which do support floating point. So, my question is how can I get the sign, mantissa and exponent of a single precision floating point number.

Following the format from this figure,

That is what I've done so far, but except sign, neither mantissa and exponent are correct. I think, I'm missing something.

`void getSME( int& s, int& m, int& e, float number )`

{

unsigned int* ptr = (unsigned int*)&number;

s = *ptr >> 31;

e = *ptr & 0x7f800000;

e >>= 23;

m = *ptr & 0x007fffff;

}

Answer

I think it is better to use unions to do the casts, it is clearer.

```
#include <stdio.h>
typedef union {
float f;
struct {
unsigned int mantisa : 23;
unsigned int exponent : 8;
unsigned int sign : 1;
} parts;
} double_cast;
int main() {
double_cast d1;
d1.f = 0.15625;
printf("sign = %x\n",d1.parts.sign);
printf("exponent = %x\n",d1.parts.exponent);
printf("mantisa = %x\n",d1.parts.mantisa);
return 0;
}
```

Example based on http://en.wikipedia.org/wiki/Single_precision