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RE: Questions on data transfer and non-arithmetic handling rules



Thanks, Hassam.  That's what I've proposed.  -Chuck Stevens
 

Date: Sun, 1 May 2011 12:54:37 +0300
Subject: Re: Questions on data transfer and non-arithmetic handling rules
From: hfahmy@xxxxxxxxxxxxxxxxxxxxxxx
To: tydeman@xxxxxxxxx
CC: charles.stevens@xxxxxxxx; stds-754@xxxxxxxxxxxxxxxxx



2011/5/1 Fred J. Tydeman <tydeman@xxxxxxxxx>
On Sat, 30 Apr 2011 14:26:33 -0600 Charles Stevens wrote:
>
>1)  I do not find any rule offhand in IEEE Std 754-2008
>    Clause 5.4.2 or elsewhere that says what happens when
>    the "source" item is exactly the same format (and encoding)
>    as the "sink" item.  My personal assumption is that this
>    would be a "bit-for-bit" transfer, with no reformatting at
>    all, and that's what the COBOL rules specify.
>
>QUESTION:  Is there a rule that specifies this?

A very similar question:
Must "conversion" to same type of a signaling NaN trigger?

IEEE allows either way.
   Signal required logic:
   5.1 Overview:  General-computational operations -- might signal
   5.4 formatOf general-computational operations
   5.4.2 convertFormat() -- is one "conversion" operator
   7.2 Invalid operation -- a) any general-computational ... operation
       on a sNaN

   No signal allowed logic:
   5.1 Overview:  Quiet-computational operations -- do not signal
   5.5 Quiet-computational operations
   5.5.1 Sign bit operations -- ... signal no exception.
       copy(x) -- is the other "conversion" operator


To answer Charles' original question following the same path that Fred took, we get the following:

5.4.2 Conversion operations says: "These operations shall not propagate non-canonical results. "
so a  formatOf-convertFormat(source) from a source in one format to a destination in the same format will canonize any non-canonical input, i.e. it is not a bit for bit transfer.

On the other hand,
5.5.1 Sign bit operations says: "These operations may propagate non-canonical encodings."
so a copy(x) operation **may** be a bit for bit transfer.

I am not a COBOL writer but, in view of the above, I would propose that you can translate (COMPUTE B = A) to a convertFormat and translate (MOVE A TO B) to a copy(x) unless the A and B in the MOVE are not of the same format in which case you should use the convertFormat.

--
Hossam A. H. Fahmy
Associate Professor
Electronics and Communications Department
Cairo University
Egypt



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