DOCUMENT:Q160670  09-AUG-2001  [winnt]
TITLE   :FPSCR is Not Being Saved Across Thread Context Switches
PRODUCT :Microsoft Windows NT
PROD/VER:4.0
OPER/SYS:
KEYWORDS:kbhw kbHardware

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - Microsoft Windows NT Workstation version 4.0 
 - Microsoft Windows NT Server version 4.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

The FPSCR (Floating Point Status and Control Register) is not being saved during
the thread context save/restore procedure. It may cause an invalid
floating-point exception mask.

NOTE: This problem happens on PowerPC-based computers only.

CAUSE
=====

When a process has multiple threads, the FPSCR register is not being saved
during the thread context save restore procedure. It is being treated as a per
process resource rather than a per thread resource.

For example:

The first thread in a process unmasks a floating point exception, for example
divide by zero, and then spawns another thread and waits for it to exit. The
other thread unmasks a different exception, for example underflow, and then
exits. When the first thread gets scheduled, the FPSCR register is set to
underflow being unmasked, but divide by zero being masked. Thus, when the first
thread executes, the FPSCR register is set to the state left by the second
thread.

STATUS
======

Microsoft has confirmed this to be a problem in Windows NT version 4.0. This
problem was corrected in the latest Microsoft Windows NT 4.0 U.S. Service Pack.
For information on obtaining the service pack, query on the following word in
the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K


Additional query words: prodnt ppc powerpc Dr Watson
======================================================================
Keywords          : kbhw kbHardware 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400
Version           : 4.0

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 2001.