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Q122323: WinNT 3.5 Software Update for the Pentium Floating Point Error

Article: Q122323
Product(s): Microsoft Windows NT
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 08-AUG-2001

3.50

WINDOWS

kbtool kbfix3.50.sp1

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The information in this article applies to:

- Microsoft Windows NT Workstation version 3.5 
- Microsoft Windows NT Server version 3.5 
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SUMMARY
=======

Microsoft has developed a utility (PENTNT.EXE) that, on Pentium-based systems,
checks whether the processor contains the floating point division error, reports
the current status of floating point, and allows disabling and re-enabling of
hardware floating point.

This software effectively turns off the math coprocessor by trapping the floating
point instructions. This makes the Pentium processor behave similarly to 486SX
or 386 processors that do not have a math coprocessor. The utility works on
Pentium and x86 systems.

Additionally, the solution is implemented through registry settings so that
system policy administrators can control the use of Pentium floating point
hardware on user systems.

STATUS
======

This utility is available in the latest U.S. Service Pack for Windows NT version
3.5. 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

This utility is included in Windows NT version 3.51.

MORE INFORMATION
================

Performance Considerations
--------------------------

Core operating system code (for window management, disk and file operations, and
application programming interfaces) in Windows NT does not perform floating
point calculations, and so will not be affected either by the potential floating
point hardware division errors, or by disabling hardware floating point.
However, choosing software emulation of floating point will result in lower
performance of floating point operations. Some utilities and applications such
as the Calculator application provided with Windows NT, and the OpenGL rendering
engine, may use floating point hardware when present. Most word processors,
databases, email, and so on, perform few or no floating point operations.

Math-intensive applications, including spreadsheets and technical applications,
typically perform floating point operations, and therefore will experience some
performance degradation. The OpenGL rendering engine, which is part of Windows
NT 3.5, and other 3D and CAD applications, are instances of this type of
application. The user of such applications will be able to choose between the
higher speed of hardware floating point, or slower floating point software
emulation libraries, which will not encounter the Pentium floating point
division error. Many applications contain their own floating point libraries to
use in this case. A few applications (such as Autocad) require floating point
hardware and will not run without it. Customers running these applications will
need to replace their Pentium processor in order to avoid the floating point
division error. The floating point software emulation's libraries are used on
systems with 486SX or 386 processors.

Running PENTNT.EXE
------------------

To start the PENTNT.EXE utility, at the MS-DOS prompt type:

  PENTNT [-C|-c] [-F|-f] [-O|-o] [-?|-H|-h]

The utility will test floating point division. If no error is found, a message
will appear recommending that you use floating point hardware. If a division
error is detected, a message appears notifying you of the error. You then have
several choices: to disable floating point hardware only if you are using a
Pentium processor with a division error, or disable floating point hardware
always. If you choose to disable or re-enable floating point emulation, you must
restart the computer for the changes to take effect.

The complete PENTNT command syntax is as follows:

  PENTNT [-C|-c] [-F|-f] [-O|-o] [-?|-H|-h]

where:

-C|-c Turns on conditional emulation. This means that floating point emulation
will be forced on if and only if the system detects the Pentium floating point
division error at start time. If you select this parameter, you must restart the
computer for the changes to take effect.

-F|-f Turns on forced emulation. This means that floating point hardware is
disabled and floating point emulation will always be forced on, regardless of
whether the system exhibits the Pentium division error. This parameter is useful
for testing software emulators and for working around floating point hardware
defects known to the operating system. If you select this parameter, you must
restart the computer for the changes to take effect.

-O|-o Turns off forced emulation and re-enables floating point hardware if it is
present. If you select this parameter, you must restart the computer for the
changes to take effect.

-?|-H|-h Displays help for the command.

Registry Entry
--------------

The Registry entry for the PENTNT utility is located in the Registry subtree
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SessionManager. The
following parameter is used by the PENTNT utility:

REG_DWORD
ForceNpxEmulation = 0 | 1 | 2

Specifies if the floating point emulation is turned on or off. If set to 0,
floating point emulation is not enabled and floating point hardware is enabled.
If set to 1, conditional floating point emulation is enabled. If set to 2,
unconditional emulation is enabled. By default, this parameter is set to 0.

Additional query words: prodnt fpu bug intel

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Keywords          :  
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNT350search kbWinNTW350 kbWinNTW350search kbWinNTSsearch kbWinNTS350 kbWinNTS350search

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