Date:         Tue, 4 Dec 2001 09:31:42 -0800
Reply-To:     "William W. Viergever" <wwvierg@ATTGLOBAL.NET>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         "William W. Viergever" <wwvierg@ATTGLOBAL.NET>
Subject:      FTMH: SAS Notes for 04Dec2001: Similar equations yield different
              results on IEEE machines
Content-Type: multipart/alternative;

For the Mantissa-Heads:



>     006214 Similar equations yield different results on IEEE machines
>     SN     http://www.sas.com/service/techsup/unotes/SN/006/006214.html


SN-006214
Similar equations yield different results on IEEE machines


    On IEEE platforms (including Windows), floating point unit (FPU)
    registers offer greater precision than floating point memory
    locations.  It is not uncommon in C code for the same expression
    appearing in different parts of a program to produce slightly
    different results, because the compiler keeps intermediate results for
    a computation in the FPU registers.  This permits a higher degree of
    precision, since the floating point register has 12 extra bits for the
    mantissa and 4 extra bits for the exponent.

    For example:

      data _null_;
        p=1;
        p=p/100;
        q=1;
        q=q/100;
        r=p-q;
        put p= q= r=;
      run;

    produces:

    p=0.01  q=0.01  r=2.083701E-19

    The value of R is not exactly 0.  Internally, P is loaded in an FPU
    and divided by 100.  The result is written to memory, which causes the
    processor to convert the value from an 80-bit to a 64-bit floating
    point number.  Q is then loaded and divided by 100.  The result is
    again written to memory but an additional floating point store
    instruction is executed, causing the result in the FPU to not be
    discarded.  The subtraction is performed, which results in the slight
    difference in the generated result .  The difference would be zero if
    P was reloaded to an FPU prior to the subtraction.

    The workaround is to specify the undocumented option NOCODEGEN.

Product:Base SAS
Component:SAS data sets (OS)
Priority:N/A
Status:Usage Issue
Date:Mon, 5 Nov 2001

SystemVersion ReportedVersion Fixed
AIX/6000 7 TSP1
Compaq Tru64 UNIX 7 TSP1
HP-UX Operating Systems 7 TSP1
IBM OS/2 7 TSP1
Solaris 7 TSP1
Windows NT 7 TSP1
Windows 95 7 TSP1
Windows 98 7 TSP1
Unless otherwise stated above, no fixes are available for this issue.

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William W. Viergever          Voice  : (916) 483-8398
Viergever & Associates        Fax    : (916) 486-1488
Sacramento, CA 95825          E-mail : wwvierg@attglobal.net
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