5.1.1 Describe The Three Types of File Structure

 CHAPTER 5 FILE MANAGEMENT

Introduction

• Files are an abstraction mechanism

• A file consists of data on disk. Can regard this data as a sequence of bytes.

• A filename is an alias for this data. We can access the data by using the lename. So, we don't worry about:

• Where the data is on the disk (what cylinder,sector, etc)

• How the data is read in from disk to main memory

• The file system handles these details for us.

File Management

• File system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them.
• File systems may use a data storage device such as a hard disk or CD-ROM.
• Involve maintaining the physical location of the files.
• Provide access to data on a file server by acting as clients for a network protocol.
• It is distinguished from a directory service and registry.

5.1.1 Describe the three types of file structure. (CLO1)

a. Unstructured (byte sequence)

File Structure:

umstructured (byte sequence)

File Structure:Three kinds of files. (a) Byte sequence. (b) Record sequence. (c) Tree.

 a. Byte Sequence:

The file in Fig. (a) is just an unstructured sequence of bytes. In effect, the operating system does notknow or care what is in the file. All it sees are bytes. Any meaning must be imposed by user-level programs. Both UNIX and Windows 98 use this approach.

b. Record Sequence:

In this model, a file is a sequence of fixed-length records, each with some internal structure. Central tothe idea of a file being a sequence of records is the idea that the read operation returns one record andthe write operation overwrites or appends one record. As a historical note, when the 80-column punched card was king many (mainframe) operating systems based their file systems on files consistingof 80-character records, in effect, card images

c. Tree:

In this organization, a file consists of a tree of records, not necessarily all the same length, eachcontaining a key field in a fixed position in the record. The tree is sorted on the key field, to allow rapidsearching for a particular key.