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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: net.arch
Subject: Re: IBM 360 float architecture problems
Message-ID: <4027@alice.UUCP>
Date: Sat, 20-Jul-85 11:14:03 EDT
Article-I.D.: alice.4027
Posted: Sat Jul 20 11:14:03 1985
Date-Received: Mon, 22-Jul-85 06:22:57 EDT
References: <741@masscomp.UUCP>
Organization: Bell Labs, Murray Hill
Lines: 30

Jeff Carter tried to correct a point I made with too little detail:

> 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.

This may well be true, but is unrelated to the point I made.

Let me be more specific.  The VAX floating-point format is
identical in single and double precisions except for the
number of mantissa bits.  However, overflow is still possible
on double->single conversion because the VAX rounds rather
than chops.  All you need to do is convert some number
with the maximum possible exponent, all the high-order
mantissa bits (i. e.: the ones that correspond to short format)
turned on, and the next bit also turned on.  Then the rounding
that accompanies the conversion will cause an overflow.

Jeff goes on to say:

> 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 )

He is indeed wront; the 370 has 7 exponent bits in both single
and double.  Since the exponent represents a power of 16, the
largest magnitude is about 7.237e75.