#! perl -slw
use strict;

sub real8ToDouble {
	my( $real8 ) = shift;
	my $bits = reverse unpack 'b64', $real8;

	# IEEE 754 64-bit real (double) format
	# seeeeeee eeeeffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff
	#1-sign 11- mantissa 52-bits (+1 implicit) binary fraction
	my( $sign, $exp, $frac ) = $bits =~ m[(.)(.{11})(.{52})];

	my $expAsInt = unpack( 'v', pack 'b16', scalar reverse $exp ) - 1023;
	my $f = 1;
	my $n = 1;
	for my $bp ( split '', $frac ) {
		$n /= 2;
		$f += $n if $bp;
	}
	return ( $f * 2**$expAsInt ) * ( $sign ? -1 : 1 );
}

for ( 1 .. 20 ) {
	my( $s, $c, $m ) = (
		rand > 0.5 ? '+' : '-',
		rand,
		int( rand 600 ) -300
	);
	my $double = "$s${c}E$m";
	printf "%g : %g\n", $double, real8ToDouble( pack 'd', $double );
}

__END__
P:\test>452186.pl
-3.27454e+126 : -3.27454e+126
-6.33606e-135 : -6.33606e-135
-9.78943e-089 : -9.78943e-089
3.75458e-262 : 3.75458e-262
9.11469e-049 : 9.11469e-049
6.66809e-052 : 6.66809e-052
9.58191e+086 : 9.58191e+086
-7.59277e+284 : -7.59277e+284
3.93188e-021 : 3.93188e-021
-4.28009e-048 : -4.28009e-048
-5.32776e-173 : -5.32776e-173
5.55817e-147 : 5.55817e-147
-4.44031e+060 : -4.44031e+060
2.40295e+126 : 2.40295e+126
8.55194e+132 : 8.55194e+132
5.61005e+184 : 5.61005e+184
-9.57489e-107 : -9.57489e-107
8.71338e+130 : 8.71338e+130
5.66986e+227 : 5.66986e+227
3.41461e-116 : 3.41461e-116