1.File
A file is a named collection of related
information that is recorded on secondary storage such as magnetic disks,
magnetic tapes and optical disks.In general, a file is a sequence of bits,
bytes, lines or records whose meaning is defined by the files creator and user.
2.File Structure
File structure is a structure, which is
according to a required format that operating system can understand.
·
A file has a certain
defined structure according to its type.
·
A text file is a
sequence of characters organized into lines.
·
A source file is a
sequence of procedures and functions.
·
An object file is a
sequence of bytes organized into blocks that are understandable by the machine.
·
When operating system
defines different file structures, it also contains the code to support these
file structure. Unix, MS-DOS support minimum number of file structure.
3. File Type
File type refers to the ability of the operating
system to distinguish different types of file such as text files source files
and binary files etc. Many operating systems support many types of files.
Operating system like MS-DOS and UNIX have the following types of files:
ORDINARY FILES
·
These are the files that
contain user information.
·
These may have text,
databases or executable program.
·
The user can apply
various operations on such files like add, modify, delete or even remove the
entire file.
DIRECTORY FILES
·
These files contain list
of file names and other information related to these files.
Files are identified by their extensions.
Filename has two parts name and extension
separated by a “period” character.
4. File Access Mechanisms
File access mechanism refers to the manner in
which the records of a file may be accessed. There are several ways to access
files
·
Sequential access
·
Direct/Random access
·
Indexed sequential
access
SEQUENTIAL ACCESS
A sequential access is that in which the records are accessed in
some sequence i.e the information in the file is processed in order, one record
after the other. This access method is the most primitive one. Example:
Compilers usually access files in this fashion.
DIRECT/RANDOM ACCESS
·
Random access file
organization provides, accessing the records directly.
·
Each record has its own
address on the file with by the help of which it can be directly accessed for
reading or writing.
·
The records need not be
in any sequence within the file and they need not be in adjacent locations on
the storage medium.
INDEXED SEQUENTIAL ACCESS
·
This mechanism is built
up on base of sequential access.
·
An index is created for
each file which contains pointers to various blocks.
·
Index is searched
sequentially and its pointer is used to access the file directly.
5. Space Allocation
Files are allocated disk spaces by operating
system. Operating systems deploy following three main ways to allocate disk
space to files.
·
Contiguous Allocation
·
Linked Allocation
·
Indexed Allocation
CONTIGUOUS ALLOCATION
·
Each file occupy a
contiguous address space on disk.
·
Assigned disk address is
in linear order.
·
Easy to implement.
·
External fragmentation
is a major issue with this type of allocation technique.
LINKED ALLOCATION
·
Each file carries a list
of links to disk blocks.
·
Directory contains link
/ pointer to first block of a file.
·
No external
fragmentation
·
Effectively used in
sequential access file.
·
Inefficient in case of
direct access file.
INDEXED ALLOCATION
·
Provides solutions to
problems of contigous and linked allocation.
·
A index block is created
having all pointers to files.
·
Each file has its own
index block which stores the addresses of disk space occupied by the file.
·
Directory contains the
addresses of index blocks of files.
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