Just as the most and least significant bytes of an integer are 0 and 3, the most-significant and least-significant bits of a double-precision floating-point number are 0 and 63. The beginning bit, and most significant bit, offsets of S, E, and F are 0, 1, and 12 respectively.
These offsets refer to the logical positions of the bits, not to their physical locations, which vary from medium to medium.
Consult the IEEE specifications about the encoding for signed zero, signed infinity (overflow), and de-normalized numbers (underflow) . According to IEEE specifications, the NaN (not a number) is system dependent and should not be used externally.