How Does FAT Handle Data Structure?


Heather Bennett

In this article, we will explore how the File Allocation Table (FAT) handles data structure. FAT is a file system commonly used in various operating systems to organize and manage data on storage devices such as hard drives and flash drives.

Understanding FAT

FAT is a simple and straightforward file system that uses a table called the File Allocation Table to keep track of the allocation status of each cluster on a storage device. The cluster is the smallest unit of space in a FAT-based file system.

The Structure of FAT

The File Allocation Table consists of entries that represent each cluster on the storage device. These entries can have different values, indicating various states:

  • Free: When an entry has a value of 0, it means that the corresponding cluster is not allocated and available for storing data.
  • Reserved: Some special values are reserved for specific purposes, such as marking bad clusters or indicating the end of a file.
  • Allocated: An entry with a value greater than 0 indicates that the cluster is allocated and contains data.

The File Allocation Table itself is typically stored in a specific location within the storage device. It acts as an index that provides information about which clusters are allocated and which are free.

Data Storage with FAT

When you save a file on a storage device using FAT, the file system needs to find enough consecutive free clusters to accommodate the file’s size. It searches through the File Allocation Table looking for consecutive free entries that can fit the entire file.

Once it finds suitable clusters, it updates their corresponding entries in the File Allocation Table to mark them as allocated. The data of the file is then stored in these clusters.


One downside of the FAT file system is fragmentation. When you delete a file or modify its size, the clusters previously allocated to the file become free again. However, these clusters might not be contiguous and could be scattered across the storage device.

This fragmentation can cause slower read and write operations as the file system needs to jump between non-contiguous clusters to access the complete file. Defragmentation tools can help mitigate this issue by rearranging data to make it more contiguous.


The File Allocation Table (FAT) provides a simple and efficient way to organize data on storage devices. By using a table-based structure, FAT keeps track of which clusters are free or allocated. While it may suffer from fragmentation, FAT remains widely used due to its compatibility and simplicity.

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