It's even more complex than that, you have to take into account the architecture of the CPU as well. When I did this I found it most usefull to start from floats and get to the hex. I also did it via binary: 

$f2_5 = unpack("b64",pack("d",2.5));
print $f2_5 . "\n";
# Will return something like 00000000000000000000000000000000000000000
+00000000010000000000010

[download]

What I have done is to convert hex to binary then (possibly) shuffle the binary to a form suitable for this CPU and use pack/ unpack to assemble the float.

{
  my($endian,$binary2double_fun);

sub deduce_endian
  {
    my($v);

    # Deduce the machine's native FP format by encoding the 
    # number 2.5 as a bit string.  Once we know how to get
    # from doubles to bit strings we can deduce the function
    # for doing the reverse
    $v = unpack("b64",pack("d",2.5));
    if($v eq "00000000000000000000000000000000000000000000000000100000
+00000010")
      {
        # Intel or something like it
        $endian = "ieee-little";
        $binary2double_fun = sub 
          {
            # We have to reverse the bitstring
            my $bits = reverse(shift);

            # Then map to floats
            return(unpack("d",pack("b64",$bits)));
          };
      }
    elsif($v eq "01000000000001000000000000000000000000000000000000000
+00000000000")
      {
        # *NIX hardware
        $endian = "ieee-big";
        $binary2double_fun = sub 
          {
            # a combination of pack and unpack will do it
            my $bits = shift;
            return(unpack("d",pack("b64",$bits)));
          };
      }
    else
      {
        print STDERR "Not configured for this machine yet\n";
        exit(20);
      }
  }

sub hex2double
  {
    my($in,$out,$val) = @_;

    deduce_endian() if(!$endian);

    # The string we were given was a hex representation of 
    # the bits of an IEEE 64' double in network order.  
    # Convert to a string of 64 "0" and "1"s.  We cannot 
    # use pack here because then the order depends on the 
    # hardware
    $out = hex2binary($in);

    # Then map this to our double representation
    return &{$binary2double_fun}($out) if($binary2double_fun);

    # No mapping function defined!
    return 0.0;
  }
}

[download]

I have my own hex2binary function for various reasons that are not worth dwelling on. It sounds like you want one that takes into account the byte order of the input.

In reply to Re: Converting Hex to Float (and vise versa) by hawtin
in thread Converting Hex to Float (and vise versa) by unstable_geek

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