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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: How to get high-precision floating point--MPFR
Date: Fri, 19 Oct 2012 11:45:41 +0200
Organization: cbb software GmbH
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Date: 2012-10-19T11:45:41+02:00
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On Fri, 19 Oct 2012 01:57:19 -0700 (PDT), Jerry wrote:

> And why is the Ada type using less
> mantissa bits than the hardware?

AFAIK x86/x86-64 do not have hardware 128-bit floats (IEEE or not). See 

http://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format

Which also says that gcc's long double is merely 80-bit extended precision.
That, I presume, corresponds to the GNAT's Long_Long_Float.

You could compare attributes of Long_Long_Float and
Interfaces.C.long_double, T'Machine_Mantissa (A.5.3), in particular, it
should be 64 for 80-bit extended precision.

Why GNAT uses 128 bit instead of 80 actually used bit? Maybe, because of
alignment issues.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de