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From: alverson@decwrl.DEC.COM (Robert Alverson)
Newsgroups: net.micro.amiga
Subject: Re: Floating point libraries
Message-ID: <6093@decwrl.DEC.COM>
Date: Sat, 25-Oct-86 03:55:04 EST
Article-I.D.: decwrl.6093
Posted: Sat Oct 25 03:55:04 1986
Date-Received: Sun, 26-Oct-86 01:17:04 EST
References: <8610160630.AA06315@cory.Berkeley.EDU> <417@husc6.HARVARD.EDU> <432@wuphys.UUCP> <2019@videovax.UUCP>
Reply-To: alverson@decwrl.UUCP (Robert Alverson)
Organization: Digital Equipment Corporation
Lines: 21
Keywords: Fortran, Lattice, AmigaBasic
Summary: Read IEEE carefully

In article <2019@videovax.UUCP> stever@videovax.UUCP (Steven E. Rice, P.E.) writes:
>There may be more here than meets the eye!  It is my recollection that
>the IEEE Standard specifies that all operands are to be extended to the
>maximum precision of the implementation, the operation(s) performed, and
>the result(s) rounded to the final precision using the specified rounding
>mode.  Although Lattice seems to take it to an extreme (50% overhead
>to extend the operand and round it after seems awfully steep!), the fact
>that the Fortran double-precision takes more than twice as long as the
>single-precision indicates they are not following this procedure.

Hold on a sec! For a single operation, IEEE says that the operation is
to be performed as if it were with infinite precision.  However, only
a rounded result is stored.  Careful analysis will show that this can
be accomplished with only a few bits more than the result precision.
There is no need to always switch to double-extended!  I'm not sure
about intermediate results.  However, my feeling is that they will
only guarantee that operations be carried out at least at the result
precision--keeping intermediate results in double-extended format
would be a bonus.

Bob