The upper extreme value of the exponent field is reserved for encoding non-numerical entities. According to Definition 5.2.2, an encoding for an explicit formal with exponent field is unsupported if its integer bit is 0. In all other cases an encoding with this exponent field is an infinity is its significand field is 0, and a NaN (Not a Number) otherwise. A NaN is further classified as an SNaN (signaling NaN) or a QNaN according the the MSB of its mantissa field:
(a) is an infinity for iff ;
(b) is a NaN for iff ;
(c) is an SNaN for iff is a NaN and ;
(c) is a QNaN for iff is a NaN and .
This function constructs an infinity with a given sign:
The following function converts an SNaN to a QNaN:
The following encoding, known as the real indefinite QNaN, has special significance in the handling of floating-point exceptions:
Infinities and NaNs will be discussed further in Part III.