MS-DOS Compatibility Mode in Windows 9x

Kenn's
Tech
Notes

20000316: MS-DOS Compatibility Mode in Windows 9x

March/2000
Tech Note #16

In Windows 9x, in the System control panel, the Performance tab should show 32-bit virtual memory and 32-bit file system when configured correctly. Occasionally, this window indicates that one or more drives is running in MS-DOS Compatibility mode. MS-DOS compatibility mode may be in use for either the file system or for virtual memory and you may get the error: "Compatibility Mode Paging reduces overall system performance." This problem can be caused by a variety of situations, such as: a legacy driver loaded in config.sys or system.ini (that should be replaced by a 32-bit driver), the hard disk controller is not recognized by Windows 9x or has been removed from Device Manager, there is a resource conflict between the hard disk controller and some other device, or the correct Windows 9x driver is missing or damaged.

Sometimes, however, after checking all the above scenarios and being quite confident that there are no conflicts and no incorrect or missing drivers, the message persists. This problem occurs sometimes when updating hard disk controller drivers or changing motherboards, when a new driver for the hard disk controller does not initialize properly. If the protected mode drivers for the hard disk controller fails to initialize (even once), Windows places an entry in the registry which will prevent proper 32-bit operation until you manually remove the entry.

If the System control panel indicates one or more drives is running in MS-DOS compatibility mode, and you wish to see if your system has this entry, run REGEDIT and look in:
HKEY_LOCAL_MACHINE\SYSTEM\CURRENTCONTROLSET\SERVICES\VXD\IOS

If there is an entry called "NOIDE", your system matches this scenario. In order for Windows to attempt to reinitialize the protected-mode IDE driver, you must remove the NOIDE entry.

Note that editing the registry is always dangerous, and that if this doesn't solve your problem, you may have missed a step in troubleshooting. For more detailed information regarding MS-DOS compatibility mode problems in Windows 9x, see Microsoft Technical Articles 151911 and 130179.

DISCLAIMER: This document is intended for the reference of computer support personnel within Winnipeg School Division No. 1. There is no warranty or liability if procedures recommended here have an adverse affect on any systems. Use them at your own risk. Any trademarks mentioned are the property of their owners, none of whom have certified any information provided here. Opinions expressed here are personal only and do not represent the policy of Winnipeg School Division No. 1 or any other organization anywhere.


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2001 June 9

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