Path: utzoo!attcan!uunet!mcvax!nikhefh!i91
From: i91@nikhefh.hep.nl (Fons Rademakers)
Newsgroups: comp.sys.apollo
Subject: Problem with FPX on 580-T (for oj@apollo)
Message-ID: <494@nikhefh.hep.nl>
Date: 26 Jul 88 19:00:47 GMT
Reply-To: i91@nikhefh.hep.nl (Fons Rademakers)
Organization: Nikhef-H, Amsterdam (the Netherlands).
Lines: 114

This message is mainly intended for Oliver Jones @ Apollo. I mailed
him directly this message but I never got any reply. Maybe something
went wrong with the mail, who knows? Any comments form other qualified
persons are also welcome.

=========================== original message ====================
Hi Oliver,

   Thanks for your extensive reply concerning the Fpx problem. However...

Concerning the original values I gave
>      PA(1) = 100.00001
>      PA(2) = 100.00001
>      PA(3) = 100.00001
>      SAVXYZ = PA(1)*PA(2)*PA(3)
I understand clearly that this is arithmetically extreme and can be
subject to truncation when moving from register to memory.

Making the numbers a little less extreme, to bring them within 
IEEE floating point precision (+/- 5.96e-8), still results in
the same kind of problem.

Take the following numbers:

      PA(1) = 10.1      or even 1.1
      PA(2) = 10.1
      PA(3) = 10.1

Now PA(1)*PA(2)*PA(3) = 1030.301, well within any limits.

Still I get the following errors:

*------------------------------ RESULTS -------------------------------
* [86] ftn aap -save -indexl -zero -dba
* no errors, no warnings in AAP, Fortran version 9.66 1988/06/24 12:33:50 MET (Fri)
* [87] aap.bin
*  1 NOT OK
*                   -0.307197500000000E-04
*  2 OK
* [88] ftn aap -save -indexl -zero -dba -cpu fpx
* no errors, no warnings in AAP, Fortran version 9.66 1988/06/24 12:33:50 MET (Fri)
* [89] aap.bin
*  1 NOT OK
*                   -0.307197500000000E-04
*  2 OK
* [90] ftn aap -save -indexl -zero -cpu fpx
* no errors, no warnings in AAP, Fortran version 9.66 1988/06/24 12:33:50 MET (Fri)
* [91] aap.bin
*  1 OK
*  2 NOT OK
*                   -0.307197500000000E-04
* [92] ftn aap -save -indexl -zero
* no errors, no warnings in AAP, Fortran version 9.66 1988/06/24 12:33:50 MET (Fri)
* [93] aap.bin
*  1 OK
*  2 OK
* [94] ftn aap -save -indexl -zero -dbs
* no errors, no warnings in AAP, Fortran version 9.66 1988/06/24 12:33:50 MET (Fri)
* [95] aap.bin
*  1 OK
*  2 OK
*-----------------------------------------------------------------------

This error pattern (depending on the compiler options) is exactly as
it was in the extreme case. The error only changed from  -0.211181500000000E-01
to -0.307197500000000E-04.

To summarize, I think that there is still something wrong with the Fpx
(although [93] and [95] behave correctly).

In case you did not save the program I attach another copy here, so you can
try it out yourself.

Let me know what do you think of this. 
If there is a FPX problem, please, direct it to the people involved.                                   

Cheers, Fons Rademakers.
*-----------------------------------------------------
      PROGRAM AAP

      REAL PA(3)

      PA(1) = 10.1
      PA(2) = 10.1
      PA(3) = 10.1

      SAVXYZ = PA(1)*PA(2)*PA(3)

      IF (PA(1)*PA(2)*PA(3) .NE. SAVXYZ) THEN
         PRINT *, '1 NOT OK'
         PRINT 10000, PA(1)*PA(2)*PA(3) - SAVXYZ
      ELSE
         PRINT *, '1 OK'
      ENDIF

      SAVXY2 = PA(1)*PA(2)*PA(3)

      IF (SAVXY2 .NE. SAVXYZ) THEN
         PRINT *, '2 NOT OK'
         PRINT 10000, SAVXY2 - SAVXYZ
      ELSE
         PRINT *, '2 OK'
      ENDIF

10000 FORMAT(E40.15)

      END
*------------------------------------------------------

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