Submitted by:
JWALANA K V MAMATHA K
MINITHA E

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INTRODUCTION
A file is a collection of binary data. A file could represent a program, a document or in some cases part of the file system itself. In modern computing it is quite common for there to be several different storage devices attached to the same computer. A common data structure such as a file system allows the computer to access many different storage devices in the same way, for example, when you look at the contents of a hard drive or a CD you view it through the same interface even though they are completely different mediums with data mapped on them in completely different ways. Files can have very different data structures within them but can all be accessed by the same methods built into the file system. The arrangement of data within the file is then decided by the program creating it. The file systems also store a number of attributes for the files within it.
All files have a name by which they can be accessed by the user. In most modern file systems the name consists of three parts, its unique name, a period and an extension. For example the file 'bob.jpg' is uniquely identified by the first word ‘bob’; the extension jpg indicates that it is a jpeg image file. The file extension allows the operating system to decide what to do with the file if someone tries to open it. The operating system maintains a list of file extension associations. Should a user try to access 'bob.jpg' then it would most likely be opened in whatever the systems default image viewer is.
Some file systems also store data about which user created a file and at what time they created it. Although this is not essential to the running of the file system it is useful to the users of the system.
In order for a file system to function properly they need a number of defined operations for creating, opening and editing a file. Almost all file systems provide the same basic set of methods for manipulating files.
ACCESS METHOD
Access methods pertain to the sequence in which records will be written or read. The two choices are Sequential and Random.
The Sequential access method is simply to start reading or writing at the beginning of the file, and then continue through the file one record, after another, in sequence. If a transaction file is being processed against a master file sequentially, both files must have their records in the same sequence.
The Random access method provides for the reading or writing of unsequenced records, that is, in random order. The advantage of this method is the ability to go directly to the desired record without handling any other records ahead of it. To locate the desired record, its address must be supplied to the operating system before an attempt is made to read the record. The address may be determined either by calculating it with a mathematical algorithm, or formula, or by looking up the address in a table or index generated at the time the file was created. In either case, the address consists of the cylinder number, track number, and optionally a record number. A cylinder is defined as the surface area covered by all read-write heads in one position of the access mechanism.
DIRECTORY STRUCTURE
The data are organized mainly in 2 parts. First, the disks are split into one or more partitions(minidisk or volumes) typically each disk on a system contain at least one partition , which is a low level structure in which files and directories preserved, Second, each partition contain information about files with in it. This information is kept in entries in a device directory or volume table of contents (VTOC). This is simply called a directory and it records information such as name, location, size and type of all files on that partition.
A directory can be viewed as a simple table that translates file names into directory entry. We can insert entries, delete entries and search for a named entry and list all the entries in the directory.
Logical Structure of a Directory:
 Single level Directory:
It is the simplest directory structure. All the files are placed in the same directory
A single level directory has significant limitations. If the system has more than one user, when the number of files increased. Since all the files are in the same directory, each file should have unique name.
 Two-Level Directory:
A single level directory often needs to conclusion of file name between different users. The standard solution is to create separate directory for each user. In this structure each user has its own user file directory (UFD). Each UFD has a similar structure but list only the files of single user. Apart from this the system keeps a master file directory (MFD). Whenever a user job starts or a user logs in the system MFD is searched. The MFD is indexed by the user name or account number and each entry points to the UFD of that user.