try 2

gistfile1.txt

#include <stdio.h>
#include <string.h>

typedef struct {
    signed int exp:4;
    signed int val:4;
} minifloat;

float to_float(minifloat m){
    /* generate a product accumulator */
    float acc = 1.0;

    acc *= m.val;
    acc *= (m.exp >= 0) ? (1 << m.exp) : (1 / ((float) (1 << (-m.exp))));
    
    /* return the accumulator */
    return acc;
}

minifloat add_inv(minifloat m){
    minifloat t = m;
    t.val *= -1;
    return t;
}

minifloat add(minifloat m, n){
    /*first find the more significant floating point value*/
    signed int shift = m.exp - n.exp;
    minifloat res = (shift >= 0) ? m : n;
    if (shift >= 0)
        res.val += n.val / (1 << shift);
    else
        res.val += m.val / (1 << shift);
    return res;
}

main()
{
    minifloat m;
    m.exp = -3;
    m.val = 5;
    minifloat n = {2, 3}
    printf("%6.3f inverted is %6.3f\n", to_float(m), to_float(add_inv(m)));
    
}