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Date: Sat, 10 May 2008 10:05:33 +0200
From: Georg Bauhaus <see.reply.to@maps.futureapps.de>
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Subject: Re: How to check a Float for NaN
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Date: 2008-05-10T10:05:33+02:00
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Jerry wrote:
> On May 9, 12:49 pm, a...@anon.org (anon) wrote:
>>   --
>>   -- Nan is define to be greater than value of hex 16#FF000000#
>>   --
>>
>>   function isNan ( N : Float ) return Boolean is
>>
>>      T : Long_Long_Integer := Long_Long_Integer ( N ) ;
>>
>>     begin
>>       if T > 16#FF000000# then
>>         return True ;
>>       else
>>         return False ;
>>       end if ;
>>     end ;
>>

> No offense to anon, but does this work? Does anyone know of a
> reference for this definition of a NaN?
> 
> Also, wondering how one might detect an Inf.
> 
> It seems that there is enough information in this thread to begin
> writing a little package to deal with some of these things.

I think you will need IEEE floats, not just Ada Floats.
(Some compilers have corresponding types, or implement their
Floats accordingly.) Consulting a text book again, I find that
IEEE defines representations for NaN (both quiet and signalling NaN),
Inf, ...

IIUC, for testing for NaN, basically

- ignore the sign bit
- expect all exponent bits set to 1
- expect the fraction part to be /= 0

This would explain why there are cases when an unchecked_conversion
of an IEEE FPT word to some "scalar word" will allow you to look
at the bits and decide whether it can represents a NaN.

Showing my ignorance I wonder, though, what happens when a fpt register
is effectively unchecked_converted into a CPU register suitable
for a long...