Value is defined to be
  0 if exponent=0
  (-1)**s  *  2**(exponent) * (1.mantissa)

see chapter 21 of the Language guide.

If you are writing a conversion program, consider writing the program
in TP and letting TP do the conversion...

var
  r : real;
  s : single;
...
  read(r);
  s := r;
  write(s);

Then you don't need to know the format yourself.