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From: haynes@ucscc.UCSC.EDU.ucsc.edu (99700000)
Newsgroups: comp.arch
Subject: Re: base 10 float hardware
Message-ID: <1608@saturn.ucsc.edu>
Date: 15 Jan 88 05:58:52 GMT
References: <8801141342.AA15537@decwrl.dec.com>
Sender: usenet@saturn.ucsc.edu
Reply-To: haynes@ucscc.UCSC.EDU (Jim Haynes)
Organization: California State Home for the Weird
Lines: 25

But you can do some interesting things with the binary values
11-15.  I believe most of the recent decimal floating point hardware
has allowed variable-length mantissas.  The Fairchild SYMBOL architecture
used one of the values 11-15 to indicate "exactly".  I had about the
same time played mentally with the complement of this, using one of the
values 11-15 to indicate 'fuzz' and work out arithmetic rules for
handling fuzz so that it serves as an indicator of precision loss.

That is, if you have approximate data you can input a fuzz digit
following the last good digit.  Or if the hardware has to round off
or truncate it can furnish a fuzz digit following the last good
digit.  The fuzz digits participate in arithmetic just like good
digits, except the result of doing any operation on fuzz is fuzz
(except multiplying fuzz by zero).  Everything a fuzz digit touches
turns to fuzz.

Another idea we were discussing years ago in Harry Huskey's class
was to represent decimal data in base 100 in 7 bits, or in base 1000
in 10 bits.  Both of these are more efficient than base 10 in 4 bits;
but at the time the hardware requirements were pretty formidable.
Maybe they wouldn't look so bad in today's technologies.

haynes@ucscc.ucsc.edu
haynes@ucscc.bitnet
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