Microsoft KB Archive/148821

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Possible Data Loss with LBA and INT13 Extensions

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Q148821

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The information in this article applies to:


 * Microsoft Windows 95

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SYMPTOMS
After you perform one of the following actions, the contents of some Enhanced IDE (EIDE) hard disks may not seem to be correct:


 * Choosing "Restart the computer in MS-DOS mode" from the Shut Down menu.
 * Starting an MS-DOS-based program that is configured to run in MS-DOS mode.

Specifically, if there are multiple partitions on a large EIDE hard disk, the extended partitions (usually drive letters D and higher) seem to contain the contents of the primary partition (usually drive C), or to be corrupted. Or, new drives may appear that contain the contents of drives in the extended partition.

WARNING: In this situation, attempts to use disk-repair utilities such as ScanDisk to repair the apparent corruption will cause data corruption on the primary partition of the affected drive.

Any action that attempts to write data to the extended partitions when this symptom is observed may also result in data corruption.

Running the CHKDSK command when this symptom is observed may result in a "Probable non-DOS disk" message.

CAUSE
This situation is known to occur only when all of the following conditions exist:


 * The hard disk is a large (>504 MB) Enhanced IDE hard disk.
 * The hard disk is accessed using Logical Block Addressing (LBA).
 * The hard disk has been partitioned into multiple partitions with the Windows 95 version of FDISK, or another partitioning utility that creates MS-DOS extended partition types 0x0E and 0x0F.
 * The system BIOS supports INT13 extensions to access hard disk devices.


 * You have exited to MS-DOS mode from Windows 95 without rebooting the computer.

Under these circumstances, certain data structures maintained by the operating system for accessing these logical drives become corrupted in the transition from protected mode to real mode.

This does not occur when you boot the computer to a command prompt or from a floppy disk.

RESOLUTION
This issue is resolved by the following updated file for Windows 95:

  DISKTSD.VXD version 4.00.952 (dated 3/22/96) and later

Installation
To install the updated file, use the following steps:


 * 1) Download the Dsktsupd.exe file from one of the online services listed below to an empty folder.
 * 2) In My Computer or Windows Explorer, double-click the Dsktsupd.exe file you downloaded in step 1.
 * 3) Follow the instructions on the screen.
 * 4) After the installation process is finished, shut down and then restart your computer.

Instructions for Downloading
The following file is available for download from the Microsoft Download Center:

"Download.gifoad Dsktsupd.exe now]" For additional information about how to download Microsoft Support files, click the article number below to view the article in the Microsoft Knowledge Base:

"Q119591 How to Obtain Microsoft Support Files from Online Services" Microsoft used the most current virus detection software available on the date of posting to scan this file for viruses. Once posted, the file is housed on secure servers that prevent any unauthorized changes to the file.

MORE INFORMATION
For additional information about INT13H extensions, please see the following article in the Microsoft Knowledge Base:

"Q122052 Logical Block Addressing (LBA) Defined" For additional information about the use of large hard disks with MS-DOS and Windows 95, please see the following article in the Microsoft Knowledge Base:

"Q126855 Windows 95 Support for Large IDE Hard Disks"

Workaround
To work around this issue, in the event you do not have the updated DSKTSUPD.EXE file, use either of the following methods:

Method 1
Avoid exiting to MS-DOS mode from within Windows 95. To run programs that require MS-DOS mode, boot to a command prompt instead of Windows.

Method 2
Repartition the hard disk using one of the following methods.

WARNING: Repartitioning the hard disk will delete all data in the current partitions (logical disks). Make sure to back up your data before repartitioning the hard disk.

 Repartition the hard disk into a single primary partition.  Repartition the hard disk by starting FDISK with the following command line: "FDISK /X" This disables support for the LBA extended partition types 0x0E and 0x0F, and uses the older partition types 0x05 and 0x06 instead.  Repartition the hard disk with an older version of FDISK (prior to Windows 95) that does not support the LBA extended partition types. Repartition the hard disk using a third-party tool that does not use the LBA extended partition types.

Additional query words: XINT13 INT13h INT exit dos mirror

Keywords : kbfile diskmem win95 kbgraphxlinkcritical

Issue type :

Technology : kbWin95search kbWin95