DOS Files: Magnetic Memory in .NET

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DOS Files: Magnetic Memory
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Very simply, DOS files are memory banks stored on a magnetic coating rather than inside silicon chips A DOS file contains some number of bytes, stored in a specific order One major difference from RAM memory is that DOS files stored on disk are sequential-access memory banks A disk (be it floppy or hard) is a circular platform coated with magnetic plastic of some sort (Here, magnetic plastic is simply a polymer in which iron oxide particles or something similar is embedded) In a floppy disk drive, the platform is a flexible disk of tough plastic; in a hard disk, the platform is a rigid platter of aluminum metal Data is stored as little magnetic disturbances on the plastic coating in a fashion similar to that used in audio cassettes and VCRs A sensor called a read/write head sits very close beside the rotating platform and waits for the data to pass by A simplified illustration of a rotating disk device is shown in Figure 41 The area of the disk is divided into concentric circles called tracks The tracks are further divided radially into sectors A sector (typically containing
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Figure 41: Rotating disk storage 512 bytes) is the smallest unit of storage that can be read or written at one time A DOS disk file consists of one or more sectors containing the file's data The read/write head is mounted on a sliding shaft that is controlled by a solenoid mechanism The solenoid can move the head horizontally to position the head over a specific track (In Figure 41, the head is positioned over track 2 counting from 0, remember!) However, once the head is over a particular track, it has to count sectors until the sector it needs passes beneath it The tracks can be accessed at random, just like bytes in the computer's memory banks, but the sectors within a track must be accessed sequentially Perhaps the single most valuable service DOS provides is handling the headaches of distributing data onto empty sectors on a disk Programs can hand sectors of data to DOS, one at a time, and let DOS worry about where on the disk they can be placed Each sector has a number, and DOS keeps track of
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what sectors belong together as a file The first sector in a file might be stored on track 3, sector 9; the second sector might be stored on track 0, sector 4, and so on You don't have to worry about that When you ask for sector 0 of your file, DOS looks up its location in its private tables and goes directly to track 3, sector 9 and brings the sector's data back to you
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The data stored in a file are just binary bytes and can be anything at all Files like this, where there are no restrictions on the contents of a file, are called binary files, since they can legally contain any binary code Like all files, a binary file consists of some whole number of sectors, with each sector (typically) containing 512 bytes The least space any file on your disk occupies is 512 bytes; when you see the DOS DIR command tell you a file has 17 bytes it in, that's the count of how many bytes were stored in that file But like a walk-in closet with only one pair of shoes in it, the rest of the sector is still there, empty but occupying space on the disk A binary file has no structure, but is simply a long series of binary codes divided into numbered groups of 512 and stored out on disk in a scheme that for now is best left to DOS to understand Later on, you can study up on it, especially once you learn more about Linux, in which entire file systems can be loaded as though they were just more programs which, of course, they are
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