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In this article I take a brief look at how the operating system(OS) deals with files.
Definition
Its a collection of data that normally is stored on a secondary storage device such as a hard disk or floppy diskette.
The operations performed on them
An OS must provide a number of operations associated with files so that users can safely store and retrieve data.
Typical operations are
Open
Close
Create
Copy
Rename
List
In addition, operations on single data elements within a file are supported by
Read
Write
Seek
File Control Blocks
File control blocks (FCB), sometimes referred to as file descriptors, are data structures that hold information about a file. When an OS needs to access a file, it creates an associated file control block to manage the file.
The structure of the file control block differs between operating systems, but most file control blocks include the following parts
Name
Location on secondary storage
Length
Date and time or creation or last access
Naming
Each OS uses a specific convention or practice for naming them.
MS-DOS Uses eight character names, a dot, then a three-character extension that denotes the type of file. Filenames are not case-sensitive.
UNIX Filenames can be up to 254 characters long and are case-sensitive.
Windows Filenames can be up to 255 characters long and are not case-sensitive.
Types
Types refer to classifying the content of the file, such as a program, text, executable program or data.
In Windows operating systems, the type is derived from the filename extension. Typical types and their extensions are
Extension
Type
.bas
basic source program
.c
c source program
.dll
system library
.doc
Word document
.exe
executable program
.txt
text
Windows associates applications (programs) with specific types. For example, the default application that opens to process a type.txt is the Notepad editor.
How an operating system keep track of files
The hard disk is comprised of a large number of sequentially numbered sectors. As files are created, free sectors are allocated to hold the its contents and marked as allocated.
To keep track of the sectors and whether they are allocated or free, and to which file they belong, the OS maintains a number of tables.
Root file system
When the OS is first installed, it creates a root file system on the disk that specifies how many sectors are available and how they will be allocated.
The root file system is a table of entries like a directory. In general, this is a fixed size, and once full, no more entries can be added.Each entry can be either a file or another directory table.
The Root file system entry
This is highly operating system specific, but an entry might look like,
Name
Beginning cluster number
Length in bytes
Type
Creation date and last modified right
permissions (an access control list)
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Source by James Kigwa
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