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Understanding How DOS Manages Information
DocumentID: 642877
Revision Date: 29-Feb-96 8:18:48 PM

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

Problem

Solutions: After a disk is organized into tracks and sectors by the DOS formatting process, DOS knows how to read from and write to the disk. After formatting, the operating system knows where to place the Read/Write head so that it reads information from a particular track. DOS reads and writes information to the disk in specific "clumps" of bytes called clusters. A byte is eight "0" and "1" signals. Typically, each character stored on a disk requires one byte of space, and a byte is referred to as one character of information.

Units Of Measure
1 = Strong magnetic signal
0 = Weak magnetic signal
Eight (8) 1's and 0's = One (1) byte

Low Density Disks
512 bytes = One sector
Two sectors = One cluster


High Density Disks
512 bytes = One sector
One sector = One cluster

Hard Disks
4, 8, or 16 sectors = One cluster

How Does DOS Manage Information?
Specific areas on a formatted disk hold particular kinds of information. The system area (explained below) contains the Boot record, the FAT (File Allocation Table), and the root directory. The entire system area is stored on track 0--the outermost track of the disk. Whenever DOS needs to know information about the files stored on a disk, DOS reads this information from the system area.

The Boot record is the section DOS reserves as the first sector of the first track of a disk. The boot record contains characteristics about the disk such as the version of DOS used to format the disk, the number of bytes per sector, and other information.


The File Allocation Table (FAT) contains information about disk storage. The FAT tells DOS which clusters on the disk are in use, which clusters are available for use, or which clusters are "bad" (unsuitable for magnetic transfer of binary information) and should not be used. When a file is written to disk, DOS consults the FAT to find out where to store the information. If a file is large and cannot fit in one contiguous space on the disk, DOS splits the file and uses the FAT to find the next available space. The result is a file that is stored in pieces on the disk, and is a fragmented or non-contiguous file.

The root directory is the third part of the system information which is the main directory (C:\) on a formatted hard disk. This space on a disk is allocated for storing information about the files in the root directory.

Answer:

Details:


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