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NTVDM Error Opening Comport

Added: 01/23/07
Article ID#: KIN0043

When installing and configuring a RDP serial interface, you may encounter a NTVDM hard error opening the comport.  Microsoft support recommends checking the settings of the Windows virtual DOS machine subsystem.  To view these settings, perform the following commands:

1. Open a command prompt.
2. If you cannot open a command prompt, there is a problem with the NTVDM subsystem.  Check the following items:
   1. Check the Config.nt and Autoexec.nt files in the SystemRoot%\System32 folder for non-standard settings.
      
      Config.nt
      dos=high,umb
      device=%SystemRoot%\System32\Himem.sys
      Files=20
      
      Autoexec.nt
      lh %SystemRoot%\System32\Mscdexnt.exe
      lh %SystemRoot%\System32\Redir
      lh %SystemRoot%\System32\Dosx
      lh %SystemRoot%\System32\Nw16 (only if CSNW is installed)
      lh %SystemRoot%\System32\Vwipxspx (only if CSNW is installed)
      
      ***REM any other lines in the Config.nt or Autoexec.nt.  Note that using REM statement in either of these two files to remark out entries may cause other programs to not function correctly.
      
3. If by changing the settings above the problem is not resolved, it may be necessary to stop programs from running at startup.  To determine if any programs are running at startup, review the following locations:
   1. %SystemRoot%\Profiles\user_name\Start menu\Programs
   2. %SystemRoot%\Profiles\Default user\Start menu\Programs
      
4. Examine the NTVDM system files in the %SystemRoot%\System32 folder for correct version by checking the size and date:
   • Ntio.sys
   • Ntdos.sys
   • Ntvdm.exe
   • Ntvdm.dll (Windows NT 3.1 only)
   • Redir.exe
     
5. The registry entries that are associated with the NTVDM subsystem are:
   
   • HKEY_LOCAL_MACHINE\system\CurrentControlSet\Control\Session Manager\Environment
     This key stores the environment variables from the Config.sys and Autoexec.bat files for use in Windows.
     
   • HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\VirtualDeviceDrivers
     This key stores the device drivers that are used in an NTVDM session.  Windows Setup creates these entries when a device driver is installed.

The information contained in this article comes directly from Microsoft knowledge base article # 314106.