KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q160670: FPSCR is Not Being Saved Across Thread Context Switches

Article: Q160670
Product(s): Microsoft Windows NT
Version(s): 4.0
Operating System(s): 
Keyword(s): kbhw kbHardware
Last Modified: 09-AUG-2001

-------------------------------------------------------------------------------
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.