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Article ID: 892520 - Last Review: January 16, 2015 - Revision: 4.0

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Assume the following scenario. In Microsoft Windows XP, you run a 16-bit application for which the following conditions are true:
  • The application is running in a Windows NT Virtual DOS Machine (NTVDM) environment.
  • The application accesses a COM port on the computer.
In this scenario, you notice that another 16-bit or 32-bit application cannot access the same COM port until you quit the first 16-bit application.


This problem occurs because the NTVDM CommsAutoClose setting does not work in Windows XP. Specifically, the code that handles the CommsAutoClose registry entry is not included in Windows XP.


Hotfix information

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: (
Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.


You must have Windows XP Service Pack 1 (SP1) or Windows XP Service Pack 2 (SP2) installed before you apply this hotfix.

Restart requirement

You do not have to restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any other hotfixes.

File Information

The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.
Windows XP Service Pack 1
   Date         Time   Version            Size    File name
   01-Dec-2004  06:03         92,224  Krnl386.exe
   01-Dec-2004  05:56                     33,840  Ntio.sys
   01-Dec-2004  05:56                     34,560  Ntio404.sys
   01-Dec-2004  05:56                     35,648  Ntio411.sys
   01-Dec-2004  05:56                     35,424  Ntio412.sys
   01-Dec-2004  05:56                     34,560  Ntio804.sys
   19-Jan-2005  03:37  5.1.2600.1620     396,800  Ntvdm.exe        
   04-Jun-2004  00:43  5.1.2600.1562     245,760  Wow32.dll  
Windows XP Service Pack 2
   Date         Time   Version            Size    File name
   19-Jan-2005  03:12  5.1.2600.2598     420,352  Ntvdm.exe    


Microsoft has confirmed that this is a bug in the Microsoft products that are listed in the "Applies to" section.

More information

For more information, click the following article number to view the article in the Microsoft Knowledge Base:
824684  ( ) Description of the standard terminology that is used to describe Microsoft software updates
For more information about the same problem in Microsoft Windows 2000, click the following article number to view the article in the Microsoft Knowledge Base:
301183  ( ) The NTVDM CommsAutoClose setting does not work in Windows 2000

Applies to
  • Microsoft Windows XP Professional
  • Microsoft Windows XP Home Edition
kbnosurvey kbarchive kbautohotfix kbhotfixserver kbqfe kbwinxpsp2fix kbwinxppresp2fix kbfix kbbug KB892520
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