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[Gcl-devel] Re: [Maxima] Re: m68k gcl/maxima: Minor Floating point error

From: Steve Haflich
Subject: [Gcl-devel] Re: [Maxima] Re: m68k gcl/maxima: Minor Floating point errors
Date: Fri, 20 Sep 2002 15:42:55 -0700

   From: Camm Maguire <address@hidden>
   Greetings, and thanks for pointing this out.  Actually, I maintain a
   few numerical packages requiring this flag on m68k to pass the
   regression tests in the package (gcl/maxima, atlas, blas, lapack,...).
   I suppose the sacrifice of performance is the correct approach.
   Anyone actually using the 68k with a different opinion will likely
   sway my mind in the other direction :).  This would of course require
   sidestepping integrity tests in this case, as I won't have the time to
   maintain dual sets of expected results.

I actually think you have done the wise thing, but if any 68K fanatics
insist otherwise, I've just checked my venerable 68881/68882 manual.
There is a two-bit field in the FPCR Mode Control Byte that specifies
the rounding precision for any fp calculation, and setting it forces
IEEE-standard calculation.  The manual warns, however, that these are
intended for emulation of machines that only support these precisions,
and that there is a significant speed cost when the single- or
double-float rounding is turned on.  Also, OS's and libraries make it
nototiously difficult to manupulate FP control registers, and are even
more notoriously sloppy about preserving the values in control

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