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Allocation Unit Definition
DocumentID: 659284
Revision Date: 29-Feb-96 8:29:48 PM

The information in this document applies to:
WordPerfect® 5.1 for DOS

Problem

Solutions: Allocation Unit:
An allocation unit is more commonly referred to as a cluster. A cluster can be a single sector (as with a single-sided 5-1/4 inch diskette). It can be two sectors long (as with the common 360K diskette), four (on the original AT 20M drive), or even eight (on the original 10M XT hard disk). Most hard drives today will have four sectors per allocation unit, unless special formatting software (other than DOS) has been used or if certain data compression utilities are used. Allocation units are listed when the DOS CHKDSK command is used.

When you first create a file on a brand new disk, all the sectors that contain the information in that file are in the same contiguous area. But, as you add and delete files on your disk and make existing files bigger and smaller, DOS ends up storing pieces of your files in clusters (allocation units) scattered over the entire disk surface. This kind of fragmentation slows everything down, since DOS has to churn through numerous read and write operations each time you load or save a file. It is also slowed down by the fact that the hard drive's read/write heads must move all over the place each time a file is read or written.

Answer:

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