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CompTIA A+ Certification All-in-One Exam Guide

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file is larger than 4096 bytes, it will use as many clusters as needed to store

the file. The OS needs a method to fill one cluster, find another that’s unused,

and fill it, continuing to fill clusters until the file is completely stored. Once

the OS stores a file, it must remember which cluster holds the file, so it can

be retrieved later. If an OS stores a file smaller than 4096 bytes, the rest of

the cluster goes to waste. We accept this waste because most files are far

larger than 4096 bytes.

MS-DOS version 2.1 first supported hard drives using a data structure and

indexing system to keep track of stored data on the hard drive, and Microsoft

called this structure the file allocation table (FAT). Think of the FAT as

nothing more than a card catalog that keeps track of which clusters store the

various parts of a file. The official jargon term for a FAT is data structure,

but it is more like a two-column spreadsheet.

The left column (see Figure 9-16) gives each cluster a hexadecimal

number from 00000000 to FFFFFFFF. Each hexadecimal character

represents four binary numbers or 4 bits. Eight hex characters, therefore,

represent 32 bits. If you do the math (232), you’ll find that there are over four

billion clusters that can be tracked or indexed.

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