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Unformatted text preview: ach application program must include its own code to interpret an input file into
the appropriate structure. This type of file structure is used in two of the most
popular operating systems, UNIX and MS-DOS.
Files as Sequence of Fixed-Length Records
As shown in Figure 14.16(b), in this case the operating system treats a file as a
sequence of fixed-length records. Each record has some internal structure defined
by the creator of the file. In this case all file access operations at operating system
level deal with the number of records to be read, written or skipped. This type of
file structure was commonly used in olden days operating systems. For example,
when the 80-column punched paper cards were popular as input media, and 132
column printers were popular as output device, many operating systems based
their file systems on input files consisting of 80-character records and output files
consisting of 132-charaeter records. The most recent operating system that used
this type of file structuring scheme was CP/M.
File Access Methods To use the information stored in a file, it must be accessed and read into computer
memory. The two commonly supported file access methods at operating system
level are sequential and random access. They are briefly described below.
Sequential Access Files.
Sequential access files are normally used with
sequential access storage media such as magnetic tape. Information stored in a
sequential access file can be accessed only sequentially. That is, a process can read
the bytes or records in the file in the order in which they are stored, starting at the
beginning. Reading of bytes or records randomly or out of order is not possible. A
sequential file can, however, be rewound and read as often as needed.
2. Random Access Files. Random access files are normally used with random
access storage media such as magnetic or optical disks. Unlike a sequential
access file, information stored in a random access file can be accessed randomly
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- Spring '14