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How a file allocation table locates a file
A file occupies one or more clusters, which are fixed-size blocks the filesystem uses as its unit of allocation. Those clusters do not have to be adjacent on the disk. The FAT solves this by recording, for each cluster, either the number of the next cluster in the file or a special marker. Microsoft’s exFAT File System Specification describes the result as a cluster chain, and the concept is the same in the classic FAT documentation: a singly linked list of cluster indices.
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Microsoft’s exFAT File System Specification (section 4.1, current Microsoft Learn edition) defines it this way:
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Reading a file follows a fixed sequence:
- The filesystem reads the directory entry for the file. That entry records the first cluster of the file’s data.
- It reads the FAT entry for that cluster. The entry names the next cluster in the chain, or carries an end-of-chain marker.
- It repeats the lookup cluster by cluster until it reaches the end-of-chain marker.
- Each cluster visited is read from the data region, and the pieces are assembled in order into the file.
Directories are stored the same way. A directory is itself a file-like object whose contents are a list of entries, so its clusters are chained through the FAT too.
Reserved values: why an entry is not always a next-cluster number
It is tempting to picture every FAT entry as a plain pointer. Some values carry special meaning instead. The FAT uses reserved values to mark conditions such as a free cluster, a bad cluster, and the end of a chain. A filesystem that reads a free or bad marker as a real cluster number will misreport or misread the volume, which is why the specification treats these values as distinct states rather than addresses.
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FAT12, FAT16, and FAT32: what the names mean
The three classic FAT types are named for the nominal width of their FAT entries: 12, 16, and 32 bits. The name describes the entry size, not the capacity a user will see. Microsoft’s FAT general overview (version 1.03, dated December 6, 2000) notes that the type of an actual volume is determined from its cluster count under the format rules. A FAT32 label therefore does not by itself tell you how many files a volume can hold or how many bytes of user data it provides.
| Variant | Nominal FAT entry width | Root directory location | Notes |
|---|---|---|---|
| FAT12 | 12 bits | Separate root-directory region | Volume type is set by cluster count under the format rules, not by the name alone |
| FAT16 | 16 bits | Separate root-directory region | Same cluster-count rule applies |
| FAT32 | 32 bits | Stored in the data area | Has no fixed root-directory region; the overview states a 4 GB single-file size limit |
The root-directory and entry-width details come from Microsoft’s FAT general overview (version 1.03, December 6, 2000). That document describes the classic format and is historical reference material, not current guidance for specific devices.
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exFAT and the allocation bitmap
exFAT is a later member of the FAT family, and Microsoft identifies it as the successor to FAT32. The most important difference for the definition is how allocation state is recorded. FAT12, FAT16, and FAT32 use the FAT itself to record which clusters are free or in use. exFAT uses an allocation bitmap for that purpose. Cluster chains still exist in exFAT, and in relevant cases exFAT still uses FAT entries to represent them.
| Feature | FAT32 | exFAT |
|---|---|---|
| Records allocation state with | The FAT | An allocation bitmap |
| Cluster chains | Linked through FAT entries | Linked through FAT entries in relevant cases |
| Single-file size limit | 4 GB (stated in Microsoft’s “Tech Licensing Programs: exFAT and more,” accessed October 7, 2026) | Supports single files larger than 4 GB, per the same Microsoft comparison |
| Relationship | Predecessor | Identified by Microsoft as its successor |
The table is a format comparison. It does not show that every device or operating system can read every variant. Microsoft describes exFAT’s use with removable storage and cross-device interchange, but the practical support for a given device should be checked against that device’s own documentation.
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Where the FAT sits on a volume
Microsoft’s FAT overview describes a FAT volume as four regions, in order:
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- The FAT region, where the allocation table itself is stored.
- The root-directory region, which exists for FAT12 and FAT16 only.
- The file and directory data region, where the actual contents are stored.
Because FAT32 has no separate root-directory region, its root directory is stored in the data area like any other directory. Losing the FAT region means losing the chain information that ties clusters together, which is why the table is often the first structure a repair tool examines.
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Scope of this definition
This article defines the FAT and explains its mechanics. It does not establish which filesystem a particular drive uses, and it does not recommend a format for a particular device, workload, or operating system. Those decisions depend on the factors below.
- The largest single file you need to store. FAT32’s 4 GB limit rules it out for larger files; exFAT does not carry that limit.
- The volume size and cluster count, which determine the FAT type that a given format produces.
- Whether the drive is fixed internal storage or removable media, and which devices and operating systems must read it.
- Whether the source documentation is current. The FAT overview is a 2000 document; the exFAT specification is maintained on Microsoft Learn.
For a device-specific compatibility or formatting question, consult that device’s or operating system’s own documentation before relying on any general FAT rule.
In short: the file allocation table is the filesystem’s map of cluster chains, the FAT12, FAT16, and FAT32 names describe entry widths, and exFAT replaces the FAT’s allocation role with a bitmap while keeping cluster chains in use.
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