Market Bulletin
Windows Product Team, May 1996

The FAT32 File System


This market bulletin is intended to help customers understand Microsoft's FAT32 file system for Windows 95, which is due to start shipping with new PC's equipped with Windows 95 in the fall of 1996.

The existing File Allocation Table (FAT) file system was invented in 1977 as a way to store data on floppy disks for Microsoft Stand-alone Disk Basic. Although originally intended for floppy disks, FAT has since been modified to be a fast, and flexible system for managing data on both removable and fixed media.

A new generation of very large hard disks will soon be shipping, and the existing FAT data structures have finally reached the limit of their ability to support ever larger media. FAT currently can support a single disk volume up to 2 Gigabytes in size. FAT32 is an enhancement of the FAT file system that supports larger hard drives with improved disk space efficiency.

FEATURES

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


COMPATIBILITY CONSIDERATIONS

In order to maintain the greatest compatibility possible with existing applications, 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 and/or expanded. In some cases, existing APIs will fail on FAT32 drives. Most applications will be unaffected by these changes. Existing utilities and drivers should continue to work on FAT32 drives. However, MS-DOS block device drivers (e.g. ASPIDISK.SYS) and disk utilities for these will need to be revised to support FAT32 drives.

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

PERFORMANCE

For most users, FAT32 will have a negligible performance impact. Some applications may see a slight performance gain from FAT32. In other applications, particularly those heavily dependent on large sequential write operations, FAT32 may result in a modest performance degradation. The overall effect on raw disk performance is less than 5% however, and the overall impact on application performance as measured by WinStone is typically less than 1%.

DUAL-BOOT PERSONAL COMPUTERS

At this time, Windows 95 OEM Service Release 2 is the only operating system capable of accessing FAT32 volumes. Windows 3.1, MS-DOS and the original version of Windows 95 will not recognize FAT32 partitions, and thus they are unable to boot from a FAT32 volume. Microsoft's plans for supporting FAT32 in Windows NT are still being determined, but at this time, Windows NT is unable to access, or dual boot from, FAT32 volumes. At minimum, Microsoft will provide a utility to convert a FAT32 volume to an NTFS volume.

Customers who run Windows 95 in real mode (for example, to run a game) will be able to use FAT32 volumes, however.

CREATING FAT32 DRIVES

In OEM Service Release 2, if you run the FDISK utility on a system with a drive over 512MB, it will ask whether to enable large disk support. If you answer yes, any partition you create that's greater than 512MB will be marked as a FAT32 partition.

WHY NOT JUST ADD NTFS TO WINDOWS 95?

NTFS is an advanced file system, with support for many features not present in FAT32, including per-file compression, security and transactioning. It is not feasible to implement NTFS within the memory and compatibility constraints of the Windows 95 platform. Windows 95 still supports real-mode MS-DOS for booting and running some MS-DOS based games. Adding NTFS support to the MS-DOS kernel would have required a significant amount of MS-DOS memory, and thus would have precluded the use of many MS-DOS mode games and applications. Protect-mode only support for NTFS would not have allowed Windows to boot from an NTFS volume.

TECHNICAL IMPLEMENTATION

Because of the compatibility considerations described above, the implementation of FAT32 involved very little change to Windows 95. The major differences between FAT32 and earlier implementations of FAT are as follows:




© 1996 Microsoft Corporation
Microsoft, MS-DOS, Windows and Win32 are registered trademarks and Windows NT is a trademark of Microsoft Corporation.