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Operating System Concepts. General Definition. An OS is a program which acts as an interface between computer system users and the computer hardware. It provides a user-friendly environment in which a user may easily develop and execute programs.

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General definition
General Definition

  • An OS is a program which acts as an interface between computer system users and the computer hardware.

  • It provides a user-friendly environment in which a user may easily develop and execute programs.

  • OS has 2 parts –Kernel and Shell. Kernel is responsible for interacting with Hardware and shell is responsible for interacting with user .Shell act as command interpreter.

  • OS hides the complexity of hardware from uninterested users.

  • Some examples of operating systems are UNIX,MS-DOS,MS-Windows, Windows/NT,OS/2, MacOS.

Computer system components
Computer System Components

  • Hardware – provides basic computing resources (CPU, Memory, I/O devices, Communication).

  • Operating System – Controls and coordinates the use of the hardware among various application programs for various users.

  • System & Application Programs – ways in which the system resources are used to solve computing problems of the users (Word processors, Compilers, Web browsers, Database systems, Video games).

  • Users – (People, Machines, other computers).

Functions of operating system
Functions of Operating System

  • Process Management: It takes care of creation and deletion of processes, scheduling.

  • Memory Management: It takes care of allocation and de-allocation of memory space to programs.

  • File Management: It takes care of issues such as storage, retrieval, naming, sharing and protection of files.

  • Security: It protects the resources and information of comp system against destruction and unauthorized access.

  • Command interpretation: It takes care of interpreting user commands and directing system resources to process the commands.

Process management
Process management

  • A process is a program in execution. It is a unit of work within the system.

  • Process needs resources to accomplish its task CPU, Memory, files, I/O devices

  • Process termination requires reclaim of any reusable resources.

    OS is responsible for the following activities in connection with process management:

  • Creating and deleting both user and system processes

  • Suspending and resuming processes

  • Providing mechanisms for process synchronization

  • Providing mechanisms for process communication

  • Providing mechanisms for deadlock handling

Memory management
Memory Management

  • All data in memory before and after processing

  • All instructions in memory in order to execute

  • Memory management determines what is in memory when optimizing CPU utilization and computer response to users

  • Memory management activities are:

  • Keeping track of which parts of memory are currently being used and by whom

  • Deciding which processes and data to move into and out of memory

  • Allocating and deallocating memory space as needed

Mass storage management
Mass-Storage Management

  • Usually disks used to store data that does not fit in main memory or data that must be kept for a “long” period of time.

  • OS activities

  • Free-space management

  • Storage allocation

  • Disk scheduling

  • Tertiary storage includes optical storage, magnetic tape

  • Varies between WORM (write-once, read-many-times) and RW (read-write)

Protection and security
Protection and Security

  • Protection–any mechanism for controlling access of processes or users to resources defined by the OS

  • Security–defense of the system against internal and external attacks even when protection is activated (viruses, identity theft)

    Systems generally first distinguish among users, to determine who can do what

  • User identities (user IDs, security IDs) include name and associated number, one per user

  • User ID then associated with all files, processes of that user to determine access control

  • Group identifier (group ID) allows set of users to be defined and controls managed, then also associated with each process, file

  • Privilege escalation allows user to change to effective ID with more rights

The Layers Of A System


Program Interface

User Programs

O.S. Interface


Hardware Interface/ Privileged Instructions


Layers of a computer system




Application Programs




System Programs

Operating System

Machine Language


Layers of a Computer System





Computer Hardware

Views of os
Views of OS

  • User View

  • Ease to use (Home PCs)

  • Efficient resource utilization (Work stations)

  • SYSTEM view of OS

  • OS is a resource allocator

    Manages all resources (CPU time, memory space, storage space, I/O and so on) Decides between conflicting requests for efficient and fair resource use

  • OS is a control program

    Controls execution of programs to prevent errors and improper use of the computer

Views of an operating system
Views of an Operating System

There are three classical views

  • Resource Manager – manages and allocates resources.

  • Resource Manager:

    • Manages and protects multiple computer resources: CPU, Processes, Internal/External memory, Tasks, Applications, Users,etc…

    • Handles and allocates resources to multiple users or multiple programs running at the same time and space (e.g., processor time, memory, I/O devices).

    • Decides between conflicting requests for efficient and fair resource use (e.g., maximize throughput, minimize response time).

  • Control program – controls the execution of user programs and operations of I/O devices.

  • Command Executer – Provides an environment for running user commands.

Machine Language


  • With the advantage of easier programming provided by the OS, the hardware, its machine language and the OS constitutes a new combination called as a virtual (extended) machine.

Operating System




Machine Language



  • In a more simplistic approach, in fact, OS itself is a program.

  • But it has a priority which application programs don’t have.

  • OS uses the kernel mode of the microprocessor, whereas other programs use the user mode.

  • The difference between two is that; all hardware instructions are valid in kernel mode, where some of them cannot be used in the user mode.

DOS program.


Introduction to dos
Introduction to DOS program.

  • What is DOS?

  • DOS stands for Disk Operating System. DOS controls the computer’s hardware and provides an environment for programs to run.

  • Why You Need DOS?.

  • DOS controls the flow of information between you and the computer (translator).

  • DOS allows you to store information on your computer.

  • DOS allows you to retrieve information stored on your computer.

Components of dos
Components of DOS program.

  • DOS is composed of a number of files.

  • These are IO.SYS, MSDOS.SYS and COMMAND.COM.

  • The disk which your computer uses to load and run DOS must contain these files and is called the System, or Startup, or Boot, disk.

Dos commands
DOS Commands program.

1. CD Change Directory Command :

  • Changes the current directoryEXAMPLES

  • C:\>cd \ - Changes current directory to the root directory.

  • C:\>cd .. - Goes up one directory. For example if you are within the C:\WINDOWS\COMMAND> directory this would take you to C:\WINDOWS>

    2. DIR (Directory) Command:

  • The DIRECTORY command lists the names and sizes of all files located on a particular disk.


  • C:\> dir a: - Shows directory of drive A

  • C:\> dir \agis - Shows files in a subdirectory on drive C (default)

  • C:\> dir /w - Shows directory in wide format, as opposed to a vertical listing.

3.Wildcard Characters ? * program.

  • A Wildcard Character is a character which has special meaning. In DOS wildcards are the * (the asterisk) and ? (the question mark ). The question mark represents any single character, and the asterisk a string of characters

    Example 1: Display only those files in the root of C whose name ends with the .COM extension

  • DIR C:\ *.COM

    Example 2: Display those files in the DOS directory which start with DISK, are followed by 4 characters, and have any extension.

  • DIR C:\DOS\DISK????.*

4. MKDIR (MD) Make Directory Command: program.

  • This command creates a new directory.


  • C:\> mkdir mine

  • Creates a directory called 'MINE'

    5.RMDIR/RD Remove Directory:

  • Removes (deletes) a directory.


  • C:\> RD C:\WINDOWS\PICTURES- Removes the PICTURES directory .

    6. REN Rename Command:

  • Renames a file/directory or files/directories.


  • C:\>ren c:\chope hope - Rename the directory chope to hope.

  • ren *.txt *.bak - Rename all text files to files with .bak extension.

7. program. COPY

  • Copies one or more files to another location.


  • copy *.* a: - This would copy all files in the current directory to the floppy disk in drive a:C:\>copy autoexec.bat c:\windows - Copies autoexec.bat from the current directory to the Windows directory of the C: drive.

    8. DELETE

  • Deletes one or more files.


  • C:\>del c:\windows\test.tmp - Deletes test.tmp from the C:\Windows directory (if it exists).C:\>del c:\windows\temp\*.* /P - Deletes all files in the C:\Windows\temp directory, and requires confirmation for each file.

9. PRINT - prints a text file program.

  • The PRINT command allows you to obtain a print-out of a text file.

    Example : Print two text files


    10. TREE - displays the directory structure

  • The TREE command enables the user to obtain a graphical view of the structure of directories and subdirectories on a disk.

    Example 1: Display the structure of the DOCS directory


    Example 2: Display the directory structure and files of C

  • TREE C: /F

Thanks…… program.