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From: carter@masscomp.UUCP (Jeff Carter)
Newsgroups: net.arch
Subject: Re: IBM 360 float architecture problems
Message-ID: <741@masscomp.UUCP>
Date: Fri, 19-Jul-85 17:43:25 EDT
Article-I.D.: masscomp.741
Posted: Fri Jul 19 17:43:25 1985
Date-Received: Sat, 20-Jul-85 12:32:07 EDT
References: <36900010@ima.UUCP> <4006@alice.UUCP>
Reply-To: carter@masscomp.UUCP (Jeff Carter)
Organization: Masscomp - Westford, MA
Lines: 33
Summary: 

In article <4006@alice.UUCP> ark@alice.UUCP (Andrew Koenig) writes:
>John Levine argues in a rather lengthy article that the IBM floating
....etc..
>allowing numbers up to 7e75 rather than 1e38, but it's still a gain.
>
>And chopped arithmetic has its advantages too.  For instance, conversion
>from double to single cannot overflow, as it can on the VAX.

There seems to be a little confusion here concerning chopped
arithmetic/IBM format/VAX format for floating point.
The reason that you can't overflow from double -> single on some
hardware architectures relates to the width of the exponent field in
the floating point number. In the IEEE format, 8 bits are used
in single precision for the biased exponent, and 11 bits are used
in double precision. It does NOT matter if you round/chop/clear/set
the low order bits, the number will overflow if the exponent won't 
fit in the space allotted. Its been a long time since I used 
360/370 family machines, but I seem to remember that there 
are indeed more exponent bits in double precision than single.
( I could be wrong on the IBM details a lot of water's passed under
  the bridge since I last used an IBM, I prefer computers that
  accuarately reflect the calculations I intended to perform )
This means that the IBM *will* overflow on conversions, totally
unrelated to the rounding mode.
The last machine that I worked on that used the same number of bits
for exponent in both SP and DP was a DG MV/8000 (and I presume the 4, 6, & 10)
The 3 saved bits are used as part of the mantissa, resulting in much 
more accurate (but smaller dynamic range ) calculations.

Jeff Carter
MASSCOMP
1 Technology Park, Westford, MA 01886
UUCP: ....!{ihnp4|allegra|decvax}!masscomp!carter