Description of FAT32 File System

• Microsoft Windows 95
• Microsoft Windows 95 OEM Service Release 2

SUMMARY

This article describes the FAT32 file system included with Windows 95 OEM Service Release 2 (OSR2)

MORE INFORMATION

Windows 95 OSR2 provides an updated version of the File Allocation Table file system, called FAT32. This updated file system allows for a default cluster size as small as 4K, as well as support for EIDE hard disk sizes in excess of 2 GB.

Features

FAT32 provides the following enhancements over previous implementations of the FAT file system:

• Supports drives up to 2 terabytes in size.
• Uses space more efficiently.

  FAT32 uses smaller clusters (that is, 4K clusters for drives up to 8 GB in size), resulting in 10 to 15 percent more efficient use of disk space relative to large FAT16 drives.

• More robust.

  FAT32 has the ability to relocate the root directory and use the backup copy of the FAT instead of the default copy. In addition, the boot record on FAT32 drives has been expanded to include a backup of critical data structures. This means that FAT32 drives are less susceptible to a single point of failure than existing FAT16 volumes.

• More flexible.

  The root directory on a FAT32 drive is now an ordinary cluster chain, so it can be located anywhere on the drive. For this reason, the previous limitations on the number of root directory entries no longer exist. In addition, FAT mirroring can be disabled, allowing a copy of the FAT other than the first one to be active. These features allow for dynamic resizing of FAT32 partitions. Note, however, that while the FAT32 design allows for this capability, it will not be implemented by Microsoft in the initial release.

Compatibility Considerations

In order to maintain the greatest compatibility possible with existing programs, networks, and device drivers, FAT32 was implemented with as little change as possible to Windows 95's existing architecture, internal data structures, Application Programming Interfaces (APIs), and on-disk format. However, because 4 bytes are now required to store cluster values, many internal and on-disk data structures and published APIs have been revised or expanded. In some cases, existing APIs will not work on FAT32 drives. Most programs will be unaffected by these changes. Existing tools and drivers should continue to work on FAT32 drives. However, MS-DOS block device drivers (for example, ASPIDISK.SYS) and disk tools will need to be revised to support FAT32 drives.

All of Microsoft's bundled disk tools (Format, FDISK, Defrag, and MS-DOS- based and Windows-based ScanDisk) have been revised to work with FAT32. In addition, Microsoft is working with leading device driver and disk tool vendors to support them in revising their products to support FAT32.

NOTE: A FAT32 volume cannot be compressed using Microsoft DriveSpace or DriveSpace 3.

Performance

For most users, FAT32 will have a negligible performance impact. Some programs may see a slight performance gain from FAT32. In other programs, particularly those heavily dependent on large sequential read or write operations, FAT32 may result in a modest performance degradation.

Dual-Boot Personal Computers

At this time, Windows 95 OEM Service Release 2 is the only operating system capable of accessing FAT32 volumes. MS-DOS and the original version of Windows 95 do not recognize FAT32 partitions, and are unable to boot from a FAT32 volume.

Windows 95 OSR2 can still be booted to real mode (for example, to run a game) and can use FAT32 volumes.

NOTE: FAT32 volumes cannot be accessed properly if the computer is started using another operating system (for example, a Windows 95 or MS-DOS boot disk).

Creating FAT32 Drives

In OSR2, if you run the FDISK tool on a system with a drive over 512 MB, it asks whether to enable large disk support. If you answer Yes, any partition you create that is larger than 512 MB is marked as a FAT32 partition.

Support Boundaries

Microsoft will support the functionality of the FAT32 file system for error- free reading, and saving of files either in real mode or protect mode. We support the real- and protected-mode tools included with Windows 95. Microsoft will not provide support for programs that will not run on the FAT32 file system. For legacy programs that will not install on a FAT32 volume, or will not properly save files or read them, you will have to contact the manufacturer of the software package.

NOTE: Cluster sizes of less than 4K on FAT32 volumes are not supported by Microsoft.

For more information about Windows 95 OSR2, please see the following article in the Microsoft Knowledge Base:

   ARTICLE ID: Q155003
   TITLE     : Description of Windows 95 OEM Service Release 2