The behaviour of the sign bit is completely specified in the x86 manuals. As that link says, the C standard got it wrong in a couple of places. The one thing which is implementation specific is that on Intel, an 80 bit load of a signalling NaN doesn't raise an exception, whereas it does on AMD. I don't know what Via does.
just a question: would it be a good idea to test this platform specific behavior in the phobos unit-tests? produce "an 80 bit load of a signalling NaN" check: intel: do not raise amd: do raise via: ... maybe only for showing that D is aware of this special cases?
