J3/05-111r1 Date: 29 Jan 2005 To: J3 From: John Reid Subject: Signbit of NaN Here is a draft response for F03/0032, with thanks to Malcolm Cohen for his help. NUMBER: F03/0032 TITLE: Sign bit of IEEE NaN KEYWORDS: IEEE-754, NaN, sign bit, negative DEFECT TYPE: Interpretation STATUS: For consideration QUESTION: Do IEEE-754 NaNs have a sign bit? Can they be negative? Does a sign bit imply a value can be either positive or negative? Fortran 2003 in 14.10.2 (IEEE_COPY_SIGN) says that NaNs have a sign bit. But, 14.10.10 (IEEE_IS_NEGATIVE) says NaNs are not negative. This appears to be a contradiction between two parts of Fortran 2003. ANSWER: The representation of a NaN has a sign bit, and this is what is referred to in 14.10.2. However, that bit is not interpreted as a sign (see IEEE standard, 6.3 "This standard does not interpret the sign of a NaN"). Thus 14.10.10 correctly says that a NaN is never negative. EDITS: None. SUBMITTED BY: Fred Tydeman HISTORY: J3/05-111 m171 Submitted