File Management In An Operating System
In this article I take a fleeting look at how the operating system(OS) deals with files.
Its a collection of data that typically 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
In addition, operations on single data elements within a file are supported by
File Control Blocks
File control blocks (FCB), sometimes referred to as file descriptors, are data structures that keep up 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
Location on secondary storage
Date and time or creation or last access
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 kind 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 refer to classifying the content of the file, such as a program, text, executable program or data.
In Windows operating systems, the kind is derived from the filename extension. Typical types and their extensions are
basic source program
c source program
Windows associates applications (programs) with specific types. For example, the default application that opens to course of action a kind.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 produced, free sectors are allocated to keep up 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 obtainable 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 complete, no more entries can be additional.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,
Beginning cluster number
Length in bytes
Creation date and last alternation right
permissions (an access control list)