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Monday, 19 December 2011

Operating Systems FILE SYSTEM FAT 32


FAT32
In order to overcome the volume size limit of FAT16, while still allowing DOS real-mode
code to handle the format without unnecessarily reducing the available conventional memory,
Microsoft decided to implement a newer generation of FAT, known as FAT32, with cluster counts
held in a 32-bit field, of which 28 bits are currently used.
In theory, this should support a total of approximately 268,435,438 (< 228) clusters, allowing
for drive sizes in the range of 2 terabytes. However, due to limitations in Microsoft's scandisk
utility, the FAT is not allowed to grow beyond 4,177,920 (< 222) clusters, placing the volume limit
at 124.55 gigabytes, unless "scandisk" is not needed.
FAT32 was introduced with Windows 95 OSR2, although reformatting was needed to use it,
and DriveSpace 3 (the version that came with Windows 95 OSR2 and Windows 98) neversupported it. Windows 98 introduced a utility to convert existing hard disks from FAT16 to FAT32
without loss of data. In the NT line, support for FAT32 arrived in Windows 2000.
Windows 2000 and Windows XP can read and write to FAT32 filesystems of any size, but
the format program on these platforms can only create FAT32 filesystems up to 32 GB. Thompson
and Thompson (2003) write[4] that "Bizarrely, Microsoft states that this behavior is by design."
Microsoft's knowledge base article 184006[3] indeed confirms the limitation and the by design
statement, but gives no rationale or explanation. Peter Norton's opinion[5] is that "Microsoft has
intentionally crippled the FAT32 file system."
The maximum possible size for a file on a FAT32 volume is 4 GiB minus 1 B (232-1 bytes).
For most users, this has become the most nagging limit of FAT32 as of 2005, since video capture
and editing applications can easily exceed this limit, as can the system swap file.

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