Path: utzoo!mnetor!uunet!husc6!mit-eddie!uw-beaver!cornell!rochester!PT.CS.CMU.EDU!K.GP.CS.CMU.EDU!lindsay
From: lindsay@K.GP.CS.CMU.EDU (Donald Lindsay)
Newsgroups: comp.arch
Subject: Re: Round-off
Message-ID: <614@PT.CS.CMU.EDU>
Date: 9 Jan 88 19:27:25 GMT
References: <189@mithras>
Sender: netnews@PT.CS.CMU.EDU
Organization: Carnegie-Mellon University, CS/RI
Lines: 14
Keywords: IBM HEX ???

In article <189@mithras> sims@stsci.EDU (Jim Sims) writes:
>I have been told and am seeking to confirm that the 309x (and others?) series
>from IBM does HEX roundoff. Is this for real?

Yes. The floating point format normalizes to the hex digit, not to the 
bit level. This means that a normalized number can still have one (or two
or three) leading zero bits.

Gene Amdahl has claimed that he did this in the hope of saving hardware.
Apparently the saving wasn't worth much. The downside is mostly apparent
to numeric analysts, who can't characterize accumulated roundoff as well
as they'd like.
-- 
	Don		lindsay@k.gp.cs.cmu.edu    CMU Computer Science