Whenever a file is saved on a hard disk, the operating system associates several attributes with it:

1. Name

Every file has a name used to identify and perform operations on it.
Example: a.txt, report.docx

2. Identifier

When a file is created, the operating system assigns a unique number to it internally.
This number is used by the OS to perform operations on the file.

3. Type

This specifies the kind of file.
Different file types include:

• Text file (.txt)
• PowerPoint file (.ppt)
• C++ file (.cpp)
• Word file (.docx)

File extension is used to indicate the file type.

4. Location

This shows the path where the file is stored on the hard disk.

Example:
C:\Program Files\Microsoft Word\a.docx

5. Size

This represents the size of the file in bytes, kilobytes (KB), megabytes (MB), etc.

Example:
a.txt → 10 KB

6. Protection

This defines access permissions for the file. There are three main types:

• Read (R) – allows reading the file
• Write (W) – allows modifying the file
• Execute (X) – allows running the file

These permissions are given to users based on user ID, time, and access rules.

7. Date

This records timestamps of the file:

• Creation time
• Last modified time
• Last accessed time

Example Summary

Different types of files are identified by their extensions. Each file type has a specific function.

1. Executable Files

• Extensions: .exe, .bin, .com
• Function: Ready-to-run machine language programs.

Example:
Double-clicking chrome.exe starts Google Chrome.

2. Object Files

• Extensions: .obj, .o
• Function: Compiled machine code, but not yet linked into an executable program.

Example:
After compiling program.c, the compiler creates program.o.

3. Source Code Files

• Extensions: .c, .cpp, .java
• Function: Files containing program source code written by programmers.

Example:
hello.cpp contains C++ code written by the user.

4. Text Files

• Extensions: .txt, .doc
• Function: Simple text documents or notes.

Example:
notes.txt contains written notes in Notepad.

5. Library Files

• Extensions: .lib, .dll
• Function: Contain reusable functions used by programs.

Example:
A .dll file may provide printing or graphics functions to software.

6. Print/View Files

• Extensions: .pdf, .zip, .rar
• Function: Files used for viewing, printing, or sharing compressed information.

Example:
report.pdf is used for reading and printing documents.

7. Archive Files

• Extensions: .arc, .zip, .tar
• Function: Group multiple files into one file, often compressed for storage or transfer.

Example:
project.zip contains multiple project files in one package.

8. Multimedia Files

• Extensions: .mpeg, .mp3, .mp4, .mov
• Function: Store audio, video, or both (audio-visual) data.

Example:
song.mp3 contains audio music
movie.mp4 contains video and audio

Quick Revision Table

A directory is used to organize and manage files in a computer system.

Key Points:

• A directory is a collection of files.
• It may also contain subdirectories.
• It stores file information such as:
  • Name
  • Size
  • Location
  • Access permissions
  • Date and time

Types of Directory Structures

There are four main types:

1. Single-Level Directory
2. Two-Level Directory
3. Tree-Structured Directory
4. Acyclic-Graph Directory

1. Single-Level Directory

Concept:

Only one directory (root directory) exists for all users. All files are stored in the same place.

Structure Diagram:

Root Directory
 ├── file1.txt
 ├── file2.cpp
 ├── file3.doc
 ├── file4.mp3

Advantages:

• Very simple to implement
• Easy to access files

Disadvantages:

• File naming conflict (same name cannot be used twice)
• No grouping of files (all files mixed together)
• Difficult to manage when number of files increases

2. Two-Level Directory

Concept:

Each user has their own separate directory under a root directory.

Structure Diagram:

Root Directory
 ├── User1
 │     ├── fileA.txt
 │     ├── fileB.cpp
 │
 ├── User2
 │     ├── fileA.txt
 │     ├── fileC.doc
 │
 ├── User3
       ├── fileX.mp3

Advantages:

• Same file names allowed for different users
• Better file organization
• Faster search within user directory

Disadvantages:

• No proper grouping within a user directory
• Limited sharing of files between users

3. Tree-Structured Directory

Concept:

A hierarchical structure where directories can contain subdirectories and files.

Structure Diagram:

Root
 ├── UserA
 │     ├── C_Files
 │     │      ├── file1.c
 │     │      ├── file2.c
 │     ├── Java_Files
 │            ├── file1.java
 │
 ├── UserB
       ├── Docs
              ├── fileA.doc

Advantages:

• Very well organized structure
• Easy file grouping
• Efficient searching using path

Example Path:

To access a file:

Root/UserA/C_Files/file1.c

Disadvantages:

• File sharing between different branches is difficult

4. Acyclic Graph Directory

Concept:

An extension of tree structure where files or directories can be shared between multiple directories, but cycles are not allowed.

Structure Diagram:

        Root
       /    \
    UserA   UserB
       \      /
        Shared_File

Explanation:

• Same file can be shared between multiple users
• No loops (acyclic structure)

Advantages:

• Efficient file sharing
• Saves storage space (no duplicate files)
• Flexible structure

Disadvantages:

• More complex to manage
• Requires link/reference handling
• Deletion becomes difficult (shared files issue)

When a file is stored, its data can be accessed in different ways.
There are three main access methods:

1. Sequential Access
2. Direct (Random) Access
3. Indexed Sequential Access

1. Sequential Access

In sequential access, data is read or written in a fixed order, one record after another.

To reach a record, you must pass through all previous records.

Example:

Magnetic Tape is the best example.

If you want record 50:

• You must read record 1 → 2 → 3 → ... → 50

Diagram:

Start → R1 → R2 → R3 → R4 → ... → R100 → End
                ↑ Current Position

Operations:

• Read next → reads next record
• Write next → writes next record
• Rewind → goes back to beginning

Advantages:

• Simple and easy to implement

Disadvantages:

• Very slow for searching specific data

2. Direct (Random) Access

In direct access, a file is made up of fixed-size records, and any record can be accessed directly without reading previous records.

Example:

Hard disk, SSD, CD

You can directly jump to record 10 without reading 1–9.

Diagram:

R1   R2   R3   R4   R5   R6   R7   R8
          ↑
       Jump directly to R4

Operations:

• Read n → read record number n
• Write n → write record number n
• Jump to n → move directly to record n

Advantages:

• Very fast access
• No need to read all previous data

Disadvantages:

• More complex than sequential access
• Requires fixed-size records

3. Indexed Sequential Access

This method is a combination of sequential + direct access using an index table.

The index stores:

• Key value
• Address (pointer) of record

How it works:

1. First search in the index
2. Find pointer
3. Go directly to the record

Diagram:

INDEX FILE              DATA FILE
---------              ------------
A  → 100              A  data
B  → 200              B  data
C  → 300              C  data
D  → 400              D  data
                      Z  data

If you search “Z”:

• First find Z in index
• Then jump directly to its location

Advantages:

• Faster than sequential access
• Easier searching using index
• Combines speed + order

Disadvantages:

• Extra memory needed for index
• Index maintenance is required

 Revision Table