Three floating point questions

[bearophile]

This program prints (dmd 2.048):
7ff4000000000000
7ffc000000000000

// program #1
import std.stdio: writeln, writefln;
import std.string: format;
void main() {
    string toHex(T)(T x) {
        string result;
        ubyte* ptr = cast(ubyte*)&x;
        foreach (i; 0 .. T.sizeof)
            result = format("%02x", ptr[i]) ~ result;
        return result;
    }
    writeln(toHex(double.init));
    writefln("%x", cast(ulong)double.init);
}

Do you know what cast(ulong) is doing here?

---------------------------

This program prints (dmd 2.048)::
-nan

// program #2
import std.stdio: writeln;
void main() {
    writeln(0.0 / 0.0);
}

Is it a bug of writeln?

---------------------------

Do you know why this isn't raising runtime errors?

// program #3
import std.math: FloatingPointControl;
import std.c.stdlib: atof;
void main() {
    double d3 = atof("3.0");
    double x; // nan
    FloatingPointControl fpc;
    fpc.enableExceptions(FloatingPointControl.severeExceptions);
    double r = x * d3;
}

Bye,
bearophile