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Pre-Course Study Pre-Course Study MaterialMaterial
OOperating perating SSystemsystems
Basic Principles of Operating Systems
An operating system is a collection of software that coordinates the working of the different components of the system and gets the user’s job done. The operating system provides the user with all the basic things necessary to do his job.
Is it enough to have microsoft word to write letters?
No.Surprised?
1. What if there is no interface to the key board?2. What if there is nothing that will control the output to VGA card?3. What if there is no interface to the human user so that he can
switch on “MS Word “ ?Yes the answers to all these questions is operating system.
Technical Definition: An OS is a collection of system software that coordinates between the hardware, provides a platform for softwares to run on and provides the user with an interface for command inputs.
EG. DOS, Linux, Unix, Windows, MAC, OS/2, Minix, and many more.
The governmental systems like Postal system, Railway System are analogous to Operating Systems.
What does an Operating What does an Operating System do?System do?
• An Operating System exploits the hardware resources of one or more processors to provide a set of services to system users.
• An Operating System also manages secondary memory and input/output devices on behalf of its users.
• Processor: Controls the operation of the computer and its Data Processing functions.. A single processor is often termed as the Central Processing Unit.
• Main memory: Stores data and programs. This memory is typically volatile. Contents of this memory are not retained even when the computer is shut down.
• I/O modules: Moves data between the computer and its external environment, such as disks, data communication equipments, and terminals.
• System bus: Provides for communication between processors, main memory, and I/O modules.
Structural Elements of a ComputerStructural Elements of a Computer
Schematic Diagram of a Computer Schematic Diagram of a Computer SystemSystem
INPUT OUTPUT
MEMORY
CONTROL UNIT
ALU
INPUT: Input devices (Keyboard, Mouse etc.) (RAW MATERIALS)OUTPUT: Output devices (VDU, Printers etc. ) (FINISHED PRODUCTS) MEMORY: Main memory (Volatile – commonly known as RAM) (GODOWN / INVENTORY)CONTROL UNIT: A device that generates Signals to command other units that what to do and what not to. (TOP MANAGEMENT)ALU: Arithmetic Logic Unit – The unit that performs the computations. (MACHINES)
Memory HierarchyMemory Hierarchy
Registers
Cache
Main Memory
Magnetic disk, CD-ROM, CD-RW
DVD-RW, DVD-RAM
Magnetic tape
GR
OW
ING
SP
EE
D A
ND
CO
ST
GR
OW
ING
SIZ
E A
ND
DA
TA
RE
TA
INA
BIL
ITY
Objectives of an OSObjectives of an OS• It is a program that
controls the execution of application programs
acts as an interface between applications and computer hardware
manages the resources associated with a computer
Objectives of an O.S. Convenience of use of the computer
Efficient use of the computing resources
Should be such that it can permit effective development, testing and
introduction of new system functions without affecting the service.
Thus the basic functions of an OS are –
1. Process Management – Managing the programs that are running.
2. Memory Management – Managing and rationing the memory between processes and data.
3. Storage Management – Managing the permanent Storage of data on disks or other media
4. I/O Management – Managing the input and output5. Device / Resource Management – Managing devices and
resources and allowing the users to share the resources6. Security and Protection – Securing the system against
possible unauthorized access to data or any other entity. Protecting the parts of the system against damage.
7. Booting the System and getting it ready to work.8. Data communications – Providing interface to connect to
other computers or allowing others to connect
Operating System as an InterfaceOperating System as an Interface
Application Programs
Utilities
Operating System
Computer Hardware
End user
Programmer
OS System Designer
Operating System ServicesOperating System Services
• Program Development – variety of services and utilities such as
Editors and debuggers. They are usually referred to as application program
development tools.
• Program Execution – involves a number of steps, such as loading
Instruction and data in the main memory, initialization of I/O devices and files,
and other resources to be allocated. OS handles the scheduling.
•Access to I/O devices – Each I/O device has its own set of
Instructions and control signals. O.S. hides these details to provide a
common interface to the user/programmer.
Operating System ServicesOperating System Services• Controlled access to files- nature of I/O device (disk drive, tape drive) and
structure of the data stored in the files in the storage medium. In the case of multiple users accessing the processor concurrently, proper protection mechanisms have to be used.
• System access- For shared or public systems, the O.S. controls access to the system as a whole and to specific system resources. Access system provides protection of resources, and data from unauthorized users, and resolves resource conflicts
• Error detection and response – Variety of errors may occur in computer system. Responses may range from program termination, retrial of the same operation, or reporting error to the application.
• Accounting – collecting usage statistics, CPU usages, monitor performance, etc.
What is the OS made of? What is the OS made of? The architecture shown below is applicable for The architecture shown below is applicable for
Unix and its family – Including LinuxUnix and its family – Including Linux
Hardware
Shell
Kernel and system software
Users
Other Applications
What the hell are these…..What the hell are these…..• User – The system representation of the human operator who
requests for services.• Application Software – Special software to help the user do
his task (E.g.. MS Word)• Shell – The program that interprets the commands or
requests given by the user and gets the job done by the kernel.
• Kernel – The core of the operating system. It uses the hardware to do the jobs required by the user or the system. It coordinates among the hardware and interfaces it with the above layers.
• System Software – Software that can access the hardware directly and generally provides various system services. (E.g.. The kernel itself, device drivers etc.).
• Hardware – The set of electronic devices that work together to ultimately do the job required by all the upper levels.
Why Unix or Linux?Why Unix or Linux?How the hell are they different from others?How the hell are they different from others?
Types of Operating Systems –Access based –1. Single User – Only one user can access the OS at a time. E.g. DOS,
Windows 9x2. Multi User – Multiple users can access the OS at the same time. E.g.
Unix, Windows NT, Linux
Processing Based1. Single processing – Only one process (job) has control of the whole
system at a time. E.g. DOS2. Multi Processing – Multiple processes (jobs) control defined domain
of their own. E.g. Windows (All versions), Unix, Linux
Environment Based1. Stand alone – Does not support connection from other systems. E.g.
DOS, Windows 9x, Workstation versions of Windows NT2. Networking – Supports connection from other systems. E.g. Unix,
Linux, Server versions of Windows NT
My God…My God…One OS! One Machine! One OS! One Machine!
Multiple jobs?Multiple jobs?
Techniques of Multiprocessing –• Multiprogramming• TimesharingNote that Multiuser is maintained using multiprocessing techniques – especially
timesharing technique.
Multiprogramming: When one process waits to get some job done which does not requires the CPU, instead of sitting idle the CPU picks up another process to work on.
Timesharing: The CPU time is equally divided among the processes in small slots. Small equal sized time slots are defined and allocated to the processes.
Closer look to the above techniquesCloser look to the above techniquesP1, P2, P3 are three different processesP1, P2, P3 are three different processes
Run Wait Run Wait
Run RunWait Wait
Run RunWait Wait
Wait
Wait
Run Run Run Run Run RunWait Wait
P1
P2
P3
P1, P2, P3
Any Advantage?
MultiprogrammingMultiprogramming
So what is timesharing?So what is timesharing?
P1
P2
P3
Normal Time interval
I/O Started by P1
P1 Completed
So which is better?So which is better?Multiprogramming or TimesharingMultiprogramming or Timesharing
• Multiprogramming maximizes CPU utilization
• Time-sharing minimizes user response rime
What about the Throughput?
ProcessesProcesses
A process is
• Normal: A program in state of execution program in state of execution
• Waste of words: Waste of words: An instance of a program running on a computerAn instance of a program running on a computer
• Fundoo…: Fundoo…: The entity that can be assigned to and executed on a The entity that can be assigned to and executed on a
processorprocessor
• More fundoo?... More fundoo?... A unit of activity characterized by a single A unit of activity characterized by a single
sequential thread of execution, a current state, and an associated sequential thread of execution, a current state, and an associated
set of system resourcesset of system resources
Parts of a ProcessParts of a Process• Components of a process
An executable program Associated data required by the program (variable, work space, buffer, etc.) Execution context: internal data used by O.S. to supervise a process = process state.
Main memory
context
data
Program
code
context
Program
code
data
Process list
Process Control Block Process Control Block A data structure by which the system identifies a process.A data structure by which the system identifies a process.It contains -It contains -• Identifier: A unique integer associated with a process• State : A currently executing process is in running state• Priority : Priority level relative to other processes• Program counter : Address of the next instruction of the program to be executed.• Memory pointers: pointers to the program code and data associated with the process, and any shared memory blocks • Context data: Data in the registers in the processor during process execution.• I/O status information: outstanding I/O requests, I/O devicesallocated to the process, a list of files is use by the process, etc.• Accounting information: amount of processor time, etc.
Other fundas…Other fundas…Resource ManagementResource Management
Responsibilities of the OS in Resource ManagementResponsibilities of the OS in Resource Management
Scheduling – The management of the queue of processes so as to make the decisions of when to execute which process.
O.S. manages the resources of a computer system and rations them among the processes.• Factors for scheduling policy:
Fairness: processes competing for a resource should be given fair chance of resource access Differential responsiveness: O.S. should make allocation and scheduling decisions to meet total set of requirements, and in a dynamic manner. E.g., if a process is waiting for an I/O device, O.S. may schedule that process as soon as the device is free. Efficiency: Maximize throughput, minimize response time, and accommodate as many concurrent users as possible.
Memory ManagementMemory Management
The OS manages the computer’s memory and allocates memory to processes and data.
Out of various memory management schemes the mot used nowadays is Virtual Memory. The fundas for Virtual Memory are given below…
1. A facility that allows programs to address memory from a logical viewpoint
2. Disregards the amount of physical memory actually available3. Meets the requirements of multiple user processes to reside
simultaneously in main memory without any interference4. (Demand) Paging system and Page faults5. Address of a word of a program = page number + offset within page6. Pages vs. Blocks mapping
……Physical memory is not alone now… !!!
It uses a fast disk (Backing store) to keep the parts of the processes not in use at the moment of time.The process's memory (also called logical memory) is divided into pages of equal size and kept in the backing store. The pages are brought in t the physical memory as and when required.The pages are kept in blocks created in the physical memory called frames. The sizes of frames and pages are equal. The allocation of block to a page depends upon the allocation policy used by the system.
Then how does it manage the disk?Then how does it manage the disk?Storage Management…Storage Management…
Auxiliary Memory (Secondary Memory/Non-Volatile Memory – Mainly Disks) are used to store data permanently. The unit of data storage on the Disk is called a File.A File is a logical structure that stores a chunk of data in the memory (specially auxiliary memory).A Directory is a logical group of files which is used to manage the manage the files on the disk.File Management –
The way the OS stores the files on the disk and keeps track of each of the files is called File Management.
File System – The logical arrangement of files and metadata (Data about data) on a disk is known as the file systemFile System – By File System we also mean the tree of directories and files that is logically created in the memory.
How does the tree look?How does the tree look?Shown below is typically a part of the Unix/Linux file systemShown below is typically a part of the Unix/Linux file system
home
/
dev etc usrbin
donkey monkey yankee bin sbin
america schedule
washington texas california
root
I am a directory
I am a file
The above diagram is only a part of the file system. The tree can be bigger and any number of nodes can exist under another.
/ /C: D:
Typical example of a Typical example of a DOS/Windows file system.DOS/Windows file system.
The names of common directories have been left to the reader to fill up.The names of common directories have been left to the reader to fill up.
Directory Content
/bin Common programs, shared by the system, the system administrator and the users.
/bootThe startup files and the kernel, vmlinuz. In recent distributions also grub data. Grub is the GRand Unified Boot loader and is an attempt to get rid of the many different boot-loaders we know today.
/dev Contains references to all the CPU peripheral hardware, which are represented as files with special properties.
/etc Most important system configuration files are in /etc, this directory contains data similar to those in the Control Panel in Windows
/home Home directories of the common users.
/initrd (on some distributions) Information for booting. Do not remove!
/lib Library files, includes files for all kinds of programs needed by the system and the users.
Directory Structure of Linux File Directory Structure of Linux File SystemSystem
Directory Content
/lost+found Every partition has a lost+found in its upper directory. Files that were saved during failures are here.
/misc For miscellaneous purposes.
/mnt Standard mount point for external file systems, e.g. a CD-ROM or a digital camera.
/net Standard mount point for entire remote file systems
/opt Typically contains extra and third party software.
/procA virtual file system containing information about system resources. More information about the meaning of the files in proc is obtained by entering the command man proc in a terminal window.
/root The administrative user's home directory. Mind the difference between /, the root directory and /root, the home directory of the root user.
/sbin Programs for use by the system and the system administrator.
/tmp Temporary space for use by the system.
/usr Programs, libraries, documentation etc. for all user-related programs.
/varStorage for all variable files and temporary files created by users, such as log files, the mail queue, the print spooler area, space for temporary storage of files downloaded from the Internet, or to keep an image of a CD before burning it.
……
Some important conceptsSome important conceptsData CommunicationData Communication
Data Communication is an important function of the Operating System. Communication always takes place between two processes. This is
because processes are the only active entity inside a computer system. The punch word in Data Communication is IPC (Inter Process Communication). IPC is the communication of data between two processes.
Different Techniques of IPC are 1. Message Passing – Data is sent from one process to another via
the kernel and other subsystems. 2. Shared Memory – A location of the memory is shared between
two processes – one process writes data to the location and another process reads the data.
Some technologies to Some technologies to implement the IPC conceptsimplement the IPC concepts
Pipes – A virtual data flow is established between two processes. The shared memory concept is implemented using pipes.Sockets – A logical connection point is opened for a process. The kernel an network subsystems take responsibility to efficiently manage the sending and receiving of data. The message passing concept is implemented in this way.Memory Mapped Files – A portion of the memory is mapped to a file. Whatever is written to the file gets written to the memory an vice versa. The shared memory concept is implemented in this way.
Security and ProtectionSecurity and ProtectionThe system may be vulnerable to attacksThe system may be vulnerable to attacks
More the functionalities in the system more the loop holes and more vulnerable to external or internal attacks
Are they the same things?No.Security is protecting a system against unauthorized attacks (Mostly external).Protection is protecting the components of the system from damage, blocking and unavailability of resource.
Who is going to attack?Who is going to attack?
There are various types of intrudersNaïve Users – These are kind of people who have a long nose and have nothing to do. They know less but keep on checking out places for other people’s data (Mails etc.).Skilled Users – These are people like inquisitive students who are more or less skilled and try out ways to access things they are not meant to just for fun or to learn.Purposeful Intruders – These people do nasty jobs on purpose – e.g. Hacking the Military Data of enemy nation, Hacking confidential data of the rival company, etc.
Some well known security threatsSome well known security threatsViruses – These are malicious codes that believe in infecting some data and spread. They generally come from infected removable storage media (floppies, CD’s) or recently they are also coming with mails on the internet. The viral code is generally executed by the user unknowingly. They reside as a part of some important program, file or metadata (Partition Table, Boot Sector etc.) and are activated whenever the innocent infected data is read or executed. When active it tries to search for other such files or data where it can copy itself. Trojans – These are sent to a person by an intruder on purpose. The program is sent with some innocent looking program or file and the user runs it unknowingly. The program when active acts as a server. The intruder can connect to it from the remote machine and command it to do malicious jobs in the recipient's computer.Worms – These consist of two parts one is called the loader and the other is the worm itself. The loader is rather innocent program which is sent to somebody on the network. This loader later on loads the worm onto this computer. Whenever the worm finds that the network is on it sends the loader to the connected machines and thus it spreads.
ContinuedContinued
Spoofing – This is a method to access a system one is not meant to access. The intruder takes form of a valid user or process and enters the system.Phising – This is a hacking method very much in use nowadays. The intruder creates a login screen similar to some well known site or system (Yahoo/ Google/ Unix or Linux login screen) and puts in online. Some way the fake login screen is run when the user needs to login. The user delivers the username and password without the knowledge of what is actually happening and this data is stored to a database accessible by the intruder.
Some ways to tackle intrudersSome ways to tackle intruders
Passwords – Good passwords – Mixture of numbers, letters of different case is a good security measure. Passwords ideally should not carry any personal information. These steps make it difficult to guess passwords.Periodically changing passwords – Some systems allow us to specify passwords and specify some period within which the password will have to be changed.Recursive Passwords – This is a technology in which each time a password is used, it is re-encrypted or changed automatically. The user is given a list of passwords for a period of time. If that list is secured, the user’s data is secured.
ContinuedContinuedEncryption – However good a password is, it is of no use without a good encryption algorithm protecting it. Encryption is a technique of generating a new text by applying some functions on the original text. The new text is called Cipher text. Some encryption algorithms allow the Cipher text to be Decrypted to the original text and some don’t.
EncryptionAlgorithm
Decryption Algorithm
PlaintextP
PlaintextP
Ciphertext C
A B
Encryption Key Decryption Key
ContinuedContinued
Antiviruses – These are programs which catch Viruses (Nowadays they detect all malicious programs – Trojans, Worms etc.) either when they are resident on the disk in some form or when trying to enter the system from some source.
Firewalls – Firewall blocks access of the system by any means from some specified external machines. So we create a firewall against some machine if we distrust it.However inspite of all these security measures, 100% security can never be ensured as any functionality in the system adds to the loopholes.
ProtectionProtection
The components in a system can also be vulnerable to internal malfunctioning. Just consider the situation that normal users are allowed to add new users. So any body can be bribed or made to create a new user which makes intrusion so easy. Let’s take another situation. Just suppose that a normal user can modify the configuration file that contains information for the network configuration. As he does not have the knowledge of the full network, his modifications may lead to a situation that the next day the network doesn’t work. Securing the components of the system against these kinds of problems is known as Protection.
Ways to ensure protectionWays to ensure protectionThe most widely used technique for ensuring protection is maintaining an access matrix. The components of the system are called entities. A matrix is maintained with the “entities to protect” on one axis and “entities to protect from” on the other. The cell corresponding to these two contain the access rights (Who can access what).
file1 CD Writer
user1 WriteReadexecute
Read
user2 read ReadWrite
That ends our discussion on Operating That ends our discussion on Operating SystemsSystems
Books suggested –Process Management , Memory Management, Storage Management – Fundamentals of
Operating Systems, Silbershatz, Galvin
Security, Encryption – Modern Operating Systems, Tanenbaum
Happy Studying!!!!