OS CLASS NOTES

©2016
1.0 Operating System - Overview
An operating System (OS) is an intermediary between users and computer hardware. It provides
users an environment in which a user can execute programs conveniently and efficiently.

In technical terms, It is a software which manages hardware. An operating System controls the
allocation of resources and services such as memory, processors, devices and information.

Definition
An operating system is a program that acts as an interface between the user and the computer
hardware and controls the execution of all kinds of programs.

Following are some of important functions of an operating System.

 Memory Management
 Processor Management
 Device Management
 File Management
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  Security
 Control over system performance
 Job accounting
 Error detecting aids
 Coordination between other software and users

Memory Management
Memory management refers to management of Primary Memory or Main Memory. Main
memory is a large array of words or bytes where each word or byte has its own address.

Main memory provides a fast storage that can be access directly by the CPU. So for a program to
be executed, it must in the main memory. Operating System does the following activities for
memory management.

 Keeps tracks of primary memory i.e. what part of it are in use by whom, what part are not
in use.
 In multiprogramming, OS decides which process will get memory when and how much.
 Allocates the memory when the process requests it to do so.
 De-allocates the memory when the process no longer needs it or has been terminated.

Processor Management
In multiprogramming environment, OS decides which process gets the processor when and how
much time. This function is called process scheduling. Operating System does the following
activities for processor management.

 Keeps tracks of processor and status of process. Program responsible for this task is
known as traffic controller.
 Allocates the processor(CPU) to a process.
 De-allocates processor when processor is no longer required.

Device Management
OS manages device communication via their respective drivers. Operating System does the
following activities for device management.

 Keeps tracks of all devices. Program responsible for this task is known as the I/O
controller.
 Decides which process gets the device when and for how much time.
 Allocates the device in the efficient way.
 De-allocates devices.

File Management
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A file system is normally organized into directories for easy navigation and usage. These
directories may contain files and other directions. Operating System does the following activities
for file management.

 Keeps track of information, location, uses, status etc. The collective facilities are often
known as file system.
 Decides who gets the resources.
 Allocates the resources.
 De-allocates the resources.

Other Important Activities

Following are some of the important activities that Operating System does.

 Security -- By means of password and similar other techniques, preventing unauthorized

access to programs and data.
 Control over system performance -- Recording delays between request for a service
and response from the system.
 Job accounting -- Keeping track of time and resources used by various jobs and users.
 Error detecting aids -- Production of dumps, traces, error messages and other debugging
and error detecting aids.
 Coordination between other softwares and users -- Coordination and assignment of
compilers, interpreters, assemblers and other software to the various users of the
computer systems.

2.0 Types of Operating System

Operating systems are there from the very first computer generation. Operating systems keep
evolving over the period of time. Following are few of the important types of operating system
which are most commonly used.

Batch operating system

The users of batch operating system do not interact with the computer directly. Each user
prepares his job on an off-line device like punch cards and submits it to the computer operator.
To speed up processing, jobs with similar needs are batched together and run as a group. Thus,
the programmers left their programs with the operator. The operator then sorts programs into
batches with similar requirements.

The problems with Batch Systems are following.

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  Lack of interaction between the user and job.
 CPU is often idle, because the speeds of the mechanical I/O devices is slower than CPU.
 Difficult to provide the desired priority.

Time-sharing operating systems

Time sharing is a technique which enables many people, located at various terminals, to use a
particular computer system at the same time. Time-sharing or multitasking is a logical extension
of multiprogramming. Processor's time which is shared among multiple users simultaneously is
termed as time-sharing. The main difference between Multiprogrammed Batch Systems and
Time-Sharing Systems is that in case of Multiprogrammed batch systems, objective is to
maximize processor use, whereas in Time-Sharing Systems objective is to minimize response
time.

Multiple jobs are executed by the CPU by switching between them, but the switches occur so
frequently. Thus, the user can receives an immediate response. For example, in a transaction
processing, processor execute each user program in a short burst or quantum of computation.
That is if n users are present, each user can get time quantum. When the user submits the
command, the response time is in few seconds at most.

Operating system uses CPU scheduling and multiprogramming to provide each user with a small
portion of a time. Computer systems that were designed primarily as batch systems have been
modified to time-sharing systems.

Advantages of Timesharing operating systems are following

 Provide advantage of quick response.

 Avoids duplication of software.
 Reduces CPU idle time.

Disadvantages of Timesharing operating systems are following.

 Problem of reliability.
 Question of security and integrity of user programs and data.
 Problem of data communication.

Distributed operating System

Distributed systems use multiple central processors to serve multiple real time application and
multiple users. Data processing jobs are distributed among the processors accordingly to which
one can perform each job most efficiently.

The processors communicate with one another through various communication lines (such as
high-speed buses or telephone lines). These are referred as loosely coupled systems or distributed

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systems. Processors in a distributed system may vary in size and function. These processors are
referred as sites, nodes, computers and so on.

The advantages of distributed systems are following.

 With resource sharing facility user at one site may be able to use the resources available
at another.
 Speedup the exchange of data with one another via electronic mail.
 If one site fails in a distributed system, the remaining sites can potentially continue
operating.
 Better service to the customers.
 Reduction of the load on the host computer.
 Reduction of delays in data processing.

Network operating System

Network Operating System runs on a server and and provides server the capability to manage
data, users, groups, security, applications, and other networking functions. The primary purpose
of the network operating system is to allow shared file and printer access among multiple
computers in a network, typically a local area network (LAN), a private network or to other
networks. Examples of network operating systems are Microsoft Windows Server 2003,
Microsoft Windows Server 2008, UNIX, Linux, Mac OS X, Novell NetWare, and BSD.

The advantages of network operating systems are following.

 Centralized servers are highly stable.

 Security is server managed.
 Upgrades to new technologies and hardwares can be easily integrated into the system.
 Remote access to servers is possible from different locations and types of systems.

The disadvantages of network operating systems are following.

 High cost of buying and running a server.

 Dependency on a central location for most operations.
 Regular maintenance and updates are required.

Real Time operating System

Real time system is defines as a data processing system in which the time interval required to
process and respond to inputs is so small that it controls the environment. Real time processing is
always on line whereas on line system need not be real time. The time taken by the system to
respond to an input and display of required updated information is termed as response time. So in
this method response time is very less as compared to the online processing.

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Real-time systems are used when there are rigid time requirements on the operation of a
processor or the flow of data and real-time systems can be used as a control device in a dedicated
application. Real-time operating system has well-defined, fixed time constraints otherwise
system will fail.For example Scientific experiments, medical imaging systems, industrial control
systems, weapon systems, robots, and home-applicance controllers, Air traffic control system
etc.

There are two types of real-time operating systems.

Hard real-time systems

Hard real-time systems guarantee that critical tasks complete on time. In hard real-time systems
secondary storage is limited or missing with data stored in ROM. In these systems virtual
memory is almost never found.

Soft real-time systems

Soft real time systems are less restrictive. Critical real-time task gets priority over other tasks and
retains the priority until it completes. Soft real-time systems have limited utility than hard real-
time systems.For example, Multimedia, virtual reality, Advanced Scientific Projects like
undersea exploration and planetary rovers etc.

3.0 Operating System - Services

An Operating System provides services to both the users and to the programs.

 It provides programs, an environment to execute.

 It provides users, services to execute the programs in a convenient manner.

Following are few common services provided by operating systems.

 Program execution
 I/O operations
 File System manipulation
 Communication
 Error Detection
 Resource Allocation
 Protection

Program execution

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Operating system handles many kinds of activities from user programs to system programs like
printer spooler, name servers, file server etc. Each of these activities is encapsulated as a process.

A process includes the complete execution context (code to execute, data to manipulate,
registers, OS resources in use). Following are the major activities of an operating system with
respect to program management.

 Loads a program into memory.

 Executes the program.
 Handles program's execution.
 Provides a mechanism for process synchronization.
 Provides a mechanism for process communication.
 Provides a mechanism for deadlock handling.

I/O Operation
I/O subsystem comprised of I/O devices and their corresponding driver software. Drivers hides
the peculiarities of specific hardware devices from the user as the device driver knows the
peculiarities of the specific device.

Operating System manages the communication between user and device drivers. Following are
the major activities of an operating system with respect to I/O Operation.

 I/O operation means read or write operation with any file or any specific I/O device.
 Program may require any I/O device while running.
 Operating system provides the access to the required I/O device when required.

File system manipulation

A file represents a collection of related information. Computer can store files on the disk
(secondary storage), for long term storage purpose. Few examples of storage media are magnetic
tape, magnetic disk and optical disk drives like CD, DVD. Each of these media has its own
properties like speed, capacity, data transfer rate and data access methods.

A file system is normally organized into directories for easy navigation and usage. These
directories may contain files and other directions. Following are the major activities of an
operating system with respect to file management.

 Program needs to read a file or write a file.

 The operating system gives the permission to the program for operation on file.
 Permission varies from read-only, read-write, denied and so on.
 Operating System provides an interface to the user to create/delete files.
 Operating System provides an interface to the user to create/delete directories.
 Operating System provides an interface to create the backup of file system.

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Communication
In case of distributed systems which are a collection of processors that do not share memory,
peripheral devices, or a clock, operating system manages communications between processes.
Multiple processes with one another through communication lines in the network.

OS handles routing and connection strategies, and the problems of contention and security.
Following are the major activities of an operating system with respect to communication.

 Two processes often require data to be transferred between them.

 The both processes can be on the one computer or on different computer but are
connected through computer network.
 Communication may be implemented by two methods either by Shared Memory or by
Message Passing.

Error handling
Error can occur anytime and anywhere. Error may occur in CPU, in I/O devices or in the
memory hardware. Following are the major activities of an operating system with respect to error
handling.

 OS constantly remains aware of possible errors.

 OS takes the appropriate action to ensure correct and consistent computing.

Resource Management
In case of multi-user or multi-tasking environment, resources such as main memory, CPU cycles
and files storage are to be allocated to each user or job. Following are the major activities of an
operating system with respect to resource management.

 OS manages all kind of resources using schedulers.

 CPU scheduling algorithms are used for better utilization of CPU.

Protection
Considering a computer systems having multiple users the concurrent execution of multiple
processes, then the various processes must be protected from each another's activities.

Protection refers to mechanism or a way to control the access of programs, processes, or users to
the resources defined by a computer systems. Following are the major activities of an operating
system with respect to protection.

 OS ensures that all access to system resources is controlled.

 OS ensures that external I/O devices are protected from invalid access attempts.

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 OS provides authentication feature for each user by means of a password.

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4.0 Operating System - Properties
Following are few of very important tasks that Operating System handles

Batch processing
Batch processing is a technique in which Operating System collects one programs and data
together in a batch before processing starts. Operating system does the following activities
related to batch processing.

 OS defines a job which has predefined sequence of commands, programs and data as a
single unit.
 OS keeps a number a jobs in memory and executes them without any manual
information.
 Jobs are processed in the order of submission i.e first come first served fashion.
 When job completes its execution, its memory is released and the output for the job gets
copied into an output spool for later printing or processing.

Advantages

 Batch processing takes much of the work of the operator to the computer.
 Increased performance as a new job get started as soon as the previous job finished
without any manual intervention.

Disadvantages

 Difficult to debug program.

 A job could enter an infinite loop.
 Due to lack of protection scheme, one batch job can affect pending jobs.

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Multitasking
Multitasking refers to term where multiple jobs are executed by the CPU simultaneously by
switching between them.Switches occur so frequently that the users may interact with each
program while it is running. Operating system does the following activities related to
multitasking.

 The user gives instructions to the operating system or to a program directly, and receives
an immediate response.
 Operating System handles multitasking in the way that it can handle multiple operations /
executes multiple programs at a time.
 Multitasking Operating Systems are also known as Time-sharing systems.
 These Operating Systems were developed to provide interactive use of a computer system
at a reasonable cost.
 A time-shared operating system uses concept of CPU scheduling and multiprogramming
to provide each user with a small portion of a time-shared CPU.
 Each user has at least one separate program in memory.

 A program that is loaded into memory and is executing is commonly referred to as a

process.
 When a process executes, it typically executes for only a very short time before it either
finishes or needs to perform I/O.
 Since interactive I/O typically runs at people speeds, it may take a long time to
completed. During this time a CPU can be utilized by another process.
 Operating system allows the users to share the computer simultaneously. Since each
action or command in a time-shared system tends to be short, only a little CPU time is
needed for each user.
 As the system switches CPU rapidly from one user/program to the next, each user is
given the impression that he/she has his/her own CPU, whereas actually one CPU is
being shared among many users.

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Multiprogramming
When two or more programs are residing in memory at the same time, then sharing the processor
is referred to the multiprogramming. Multiprogramming assumes a single shared processor.
Multiprogramming increases CPU utilization by organizing jobs so that the CPU always has one
to execute.

Following figure shows the memory layout for a multiprogramming system.

Operating system does the following activities related to multiprogramming.

 The operating system keeps several jobs in memory at a time.

 This set of jobs is a subset of the jobs kept in the job pool.
 The operating system picks and begins to execute one of the job in the memory.
 Multiprogramming operating system monitors the state of all active programs and system
resources using memory management programs to ensures that the CPU is never idle
unless there are no jobs

Advantages

 High and efficient CPU utilization.

 User feels that many programs are allotted CPU almost simultaneously.

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Disadvantages

 CPU scheduling is required.

 To accommodate many jobs in memory, memory management is required.

Interactivity
Interactivity refers that a User is capable to interact with computer system. Operating system
does the following activities related to interactivity.

 OS provides user an interface to interact with system.

 OS managers input devices to take inputs from the user. For example, keyboard.
 OS manages output devices to show outputs to the user. For example, Monitor.
 OS Response time needs to be short since the user submits and waits for the result.

Real Time System

Real time systems represents are usually dedicated, embedded systems. Operating system does
the following activities related to real time system activity.

 In such systems, Operating Systems typically read from and react to sensor data.
 The Operating system must guarantee response to events within fixed periods of time to
ensure correct performance.

Distributed Environment
Distributed environment refers to multiple independent CPUs or processors in a computer
system. Operating system does the following activities related to distributed environment.

 OS Distributes computation logics among several physical processors.

 The processors do not share memory or a clock.
 Instead, each processor has its own local memory.
 OS manages the communications between the processors. They communicate with each
other through various communication lines.

Spooling
Spooling is an acronym for simultaneous peripheral operations on line. Spooling refers to putting
data of various I/O jobs in a buffer. This buffer is a special area in memory or hard disk which is
accessible to I/O devices. Operating system does the following activites related to distributed
environment.

 OS handles I/O device data spooling as devices have different data access rates.

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  OS maintains the spooling buffer which provides a waiting station where data can rest
while the slower device catches up.
 OS maintains parallel computation because of spooling process as a computer can
perform I/O in parallel fashion. It becomes possible to have the computer read data from
a tape, write data to disk and to write out to a tape printer while it is doing its computing
task.

Advantages

 The spooling operation uses a disk as a very large buffer.

 Spooling is capable of overlapping I/O operation for one job with processor operations
for another job.

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5.0 Operating System - Processes
Process
A process is a program in execution. The execution of a process must progress in a sequential
fashion. Definition of process is following.

 A process is defined as an entity which represents the basic unit of work to be

implemented in the system.

Components of process are following.

S.N. Component & Description

Object Program
1
Code to be executed.
Data
2
Data to be used for executing the program.
Resources
3
While executing the program, it may require some resources.
Status
Verifies the status of the process execution.A process can run to completion only when all
4
requested resources have been allocated to the process. Two or more processes could be
executing the same program, each using their own data and resources.

Program
A program by itself is not a process. It is a static entity made up of program statement while
process is a dynamic entity. Program contains the instructions to be executed by processor.

A program takes a space at single place in main memory and continues to stay there. A program
does not perform any action by itself.

Process States
As a process executes, it changes state. The state of a process is defined as the current activity of
the process.

Process can have one of the following five states at a time.

S.N. State & Description

New
1
The process is being created.

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 Ready
2 The process is waiting to be assigned to a processor. Ready processes are waiting to have
the processor allocated to them by the operating system so that they can run.
Running
3
Process instructions are being executed (i.e. The process that is currently being executed).
Waiting
4
The process is waiting for some event to occur (such as the completion of an I/O operation).
Terminated
5
The process has finished execution.

Process Control Block, PCB

Each process is represented in the operating system by a process control block (PCB) also called
a task control block. PCB is the data structure used by the operating system. Operating system
groups all information that needs about particular process.

PCB contains many pieces of information associated with a specific process which are described
below.

S.N. Information & Description

Pointer
1 Pointer points to another process control block. Pointer is used for maintaining the
scheduling list.
Process State
2
Process state may be new, ready, running, waiting and so on.
Program Counter
3 Program Counter indicates the address of the next instruction to be executed for this
process.
4 CPU registers

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 CPU registers include general purpose register, stack pointers, index registers and
accumulators etc. number of register and type of register totally depends upon the computer
architecture.
Memory management information
This information may include the value of base and limit registers, the page tables, or the
5
segment tables depending on the memory system used by the operating system.This
information is useful for deallocating the memory when the process terminates.
Accounting information
6 This information includes the amount of CPU and real time used, time limits, job or process
numbers, account numbers etc.

Process control block includes CPU scheduling, I/O resource management, file management
information etc.. The PCB serves as the repository for any information which can vary from
process to process. Loader/linker sets flags and registers when a process is created. If that
process get suspended, the contents of the registers are saved on a stack and the pointer to the
particular stack frame is stored in the PCB. By this technique, the hardware state can be restored
so that the process can be scheduled to run again.

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6.0 Operating System - Process Scheduling
Definition
The process scheduling is the activity of the process manager that handles the removal of the
running process from the CPU and the selection of another process on the basis of a particular
strategy.

Process scheduling is an essential part of a Multiprogramming operating system. Such operating

systems allow more than one process to be loaded into the executable memory at a time and
loaded process shares the CPU using time multiplexing.

Scheduling Queues
Scheduling queues refers to queues of processes or devices. When the process enters into the
system, then this process is put into a job queue. This queue consists of all processes in the
system. The operating system also maintains other queues such as device queue. Device queue is
a queue for which multiple processes are waiting for a particular I/O device. Each device has its
own device queue.

This figure shows the queuing diagram of process scheduling.

 Queue is represented by rectangular box.

 The circles represent the resources that serve the queues.
 The arrows indicate the process flow in the system.

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Queues are of two types

 Ready queue
 Device queue

A newly arrived process is put in the ready queue. Processes waits in ready queue for allocating
the CPU. Once the CPU is assigned to a process, then that process will execute. While executing
the process, any one of the following events can occur.

 The process could issue an I/O request and then it would be placed in an I/O queue.
 The process could create new sub process and will wait for its termination.
 The process could be removed forcibly from the CPU, as a result of interrupt and put
back in the ready queue.

Two State Process Model

Two state process model refers to running and non-running states which are described below.

S.N. State & Description

Running
1 When new process is created by Operating System that process enters into the system as in
the running state.

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 Not Running
Processes that are not running are kept in queue, waiting for their turn to execute. Each
entry in the queue is a pointer to a particular process. Queue is implemented by using linked
2
list. Use of dispatcher is as follows. When a process is interrupted, that process is
transferred in the waiting queue. If the process has completed or aborted, the process is
discarded. In either case, the dispatcher then selects a process from the queue to execute.

Schedulers
Schedulers are special system softwares which handles process scheduling in various ways.Their
main task is to select the jobs to be submitted into the system and to decide which process to run.
Schedulers are of three types

 Long Term Scheduler

 Short Term Scheduler
 Medium Term Scheduler

Long Term Scheduler

It is also called job scheduler. Long term scheduler determines which programs are admitted to
the system for processing. Job scheduler selects processes from the queue and loads them into
memory for execution. Process loads into the memory for CPU scheduling. The primary
objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and
processor bound. It also controls the degree of multiprogramming. If the degree of
multiprogramming is stable, then the average rate of process creation must be equal to the
average departure rate of processes leaving the system.

On some systems, the long term scheduler may not be available or minimal. Time-sharing
operating systems have no long term scheduler. When process changes the state from new to
ready, then there is use of long term scheduler.

Short Term Scheduler

It is also called CPU scheduler. Main objective is increasing system performance in accordance
with the chosen set of criteria. It is the change of ready state to running state of the process. CPU
scheduler selects process among the processes that are ready to execute and allocates CPU to one
of them.

Short term scheduler also known as dispatcher, execute most frequently and makes the fine
grained decision of which process to execute next. Short term scheduler is faster than long term
scheduler.

Medium Term Scheduler

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Medium term scheduling is part of the swapping. It removes the processes from the memory. It
reduces the degree of multiprogramming. The medium term scheduler is in-charge of handling
the swapped out-processes.

Running process may become suspended if it makes an I/O request. Suspended processes cannot
make any progress towards completion. In this condition, to remove the process from memory
and make space for other process, the suspended process is moved to the secondary storage. This
process is called swapping, and the process is said to be swapped out or rolled out. Swapping
may be necessary to improve the process mix.

Comparison between Scheduler

S.N. Long Term Scheduler Short Term Scheduler Medium Term Scheduler
It is a process swapping
1 It is a job scheduler It is a CPU scheduler
scheduler.
Speed is lesser than short term Speed is fastest among Speed is in between both short
2
scheduler other two and long term scheduler.
It provides lesser control
It controls the degree of It reduces the degree of
3 over degree of
multiprogramming multiprogramming.
multiprogramming
It is almost absent or minimal It is also minimal in time It is a part of Time sharing
4
in time sharing system sharing system systems.
It selects processes from pool It can re-introduce the process
It selects those processes
5 and loads them into memory into memory and execution can
which are ready to execute
for execution be continued.

Context Switch
A context switch is the mechanism to store and restore the state or context of a CPU in Process
Control block so that a process execution can be resumed from the same point at a later time.

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Using this technique a context switcher enables multiple processes to share a single CPU.
Context switching is an essential part of a multitasking operating system features.

When the scheduler switches the CPU from executing one process to execute another, the
context switcher saves the content of all processor registers for the process being removed from
the CPU, in its process descriptor. The context of a process is represented in the process control
block of a process.

Context switch time is pure overhead. Context switching can significantly affect performance as
modern computers have a lot of general and status registers to be saved. Content switching times
are highly dependent on hardware support. Context switch requires ( n + m ) bxK time units to
save the state of the processor with n general registers, assuming b are the store operations are
required to save n and m registers of two process control blocks and each store instruction
requires K time units.

Some hardware systems employ two or more sets of processor registers to reduce the amount of
context switching time. When the process is switched, the following information is stored.

 Program Counter
 Scheduling Information
 Base and limit register value
 Currently used register
 Changed State
 I/O State
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 Accounting

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7.0 Operating System Scheduling algorithms
We'll discuss four major scheduling algorithms here which are following

 First Come First Serve (FCFS) Scheduling

 Shortest-Job-First (SJF) Scheduling
 Priority Scheduling
 Round Robin(RR) Scheduling
 Multilevel Queue Scheduling

First Come First Serve (FCFS)

 Jobs are executed on first come, first serve basis.
 Easy to understand and implement.
 Poor in performance as average wait time is high.

Wait time of each process is following

Process Wait Time : Service Time - Arrival Time

P0 0-0=0
P1 5-1=4
P2 8-2=6
P3 16 - 3 = 13

Average Wait Time: (0+4+6+13) / 4 = 5.55

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Shortest Job First (SJF)
 Best approach to minimize waiting time.
 Impossible to implement
 Processer should know in advance how much time process will take.

Wait time of each process is following

Process Wait Time : Service Time - Arrival Time

P0 3-0=3
P1 0-0=0
P2 16 - 2 = 14
P3 8-3=5

Average Wait Time: (3+0+14+5) / 4 = 5.50

Priority Based Scheduling

 Each process is assigned a priority. Process with highest priority is to be executed first
and so on.
 Processes with same priority are executed on first come first serve basis.
 Priority can be decided based on memory requirements, time requirements or any other
resource requirement.

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Wait time of each process is following

Process Wait Time : Service Time - Arrival Time

P0 9-0=9
P1 6-1=5
P2 14 - 2 = 12
P3 0-0=0

Average Wait Time: (9+5+12+0) / 4 = 6.5

Round Robin Scheduling

 Each process is provided a fix time to execute called quantum.
 Once a process is executed for given time period. Process is preempted and other process
executes for given time period.
 Context switching is used to save states of preempted processes.

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Wait time of each process is following

Process Wait Time : Service Time - Arrival Time

P0 (0-0) + (12-3) = 9
P1 (3-1) = 2
P2 (6-2) + (14-9) + (20-17) = 12
P3 (9-3) + (17-12) = 11

Average Wait Time: (9+2+12+11) / 4 = 8.5

Multi Queue Scheduling

 Multiple queues are maintained for processes.
 Each queue can have its own scheduling algorithms.
 Priorities are assigned to each queue.

Page 27 of 63
8.0 Operating System - Multi-Threading
What is Thread?
A thread is a flow of execution through the process code, with its own program counter, system
registers and stack. A thread is also called a light weight process. Threads provide a way to
improve application performance through parallelism. Threads represent a software approach to
improving performance of operating system by reducing the overhead thread is equivalent to a
classical process.

Each thread belongs to exactly one process and no thread can exist outside a process. Each
thread represents a separate flow of control.Threads have been successfully used in
implementing network servers and web server. They also provide a suitable foundation for
parallel execution of applications on shared memory multiprocessors. Folowing figure shows the
working of the single and multithreaded processes.

Difference between Process and Thread

S.N. Process Thread

Thread is light weight taking lesser resources than a

1 Process is heavy weight or resource intensive.
process.

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 Process switching needs interaction with Thread switching does not need to interact with
1
operating system. operating system.

In multiple processing environments each

All threads can share same set of open files, child
1 process executes the same code but has its
processes.
own memory and file resources.

If one process is blocked then no other

While one thread is blocked and waiting, second
1 process can execute until the first process is
thread in the same task can run.
unblocked.

Multiple processes without using threads use

1 Multiple threaded processes use fewer resources.
more resources.

In multiple processes each process operates One thread can read, write or change another
1
independently of the others. thread's data.

Advantages of Thread
 Thread minimize context switching time.
 Use of threads provides concurrency within a process.
 Efficient communication.
 Economy- It is more economical to create and context switch threads.
 Utilization of multiprocessor architectures to a greater scale and efficiency.

Types of Thread
Threads are implemented in following two ways

 User Level Threads -- User managed threads

 Kernel Level Threads -- Operating System managed threads acting on kernel, an
operating system core.

User Level Threads

In this case, application manages thread management kernel is not aware of the existence of
threads. The thread library contains code for creating and destroying threads, for passing
message and data between threads, for scheduling thread execution and for saving and restoring
thread contexts. The application begins with a single thread and begins running in that thread.

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Advantages

 Thread switching does not require Kernel mode privileges.

 User level thread can run on any operating system.
 Scheduling can be application specific in the user level thread.
 User level threads are fast to create and manage.

Disadvantages

 In a typical operating system, most system calls are blocking.

 Multithreaded application cannot take advantage of multiprocessing.

Kernel Level Threads

In this case, thread management done by the Kernel. There is no thread management code in the
application area. Kernel threads are supported directly by the operating system. Any application
can be programmed to be multithreaded. All of the threads within an application are supported
within a single process.

The Kernel maintains context information for the process as a whole and for individuals threads
within the process. Scheduling by the Kernel is done on a thread basis. The Kernel performs
thread creation, scheduling and management in Kernel space. Kernel threads are generally
slower to create and manage than the user threads.

Advantages

 Kernel can simultaneously schedule multiple threads from the same process on multiple
processes.

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  If one thread in a process is blocked, the Kernel can schedule another thread of the same
process.
 Kernel routines themselves can multithreaded.

Disadvantages

 Kernel threads are generally slower to create and manage than the user threads.
 Transfer of control from one thread to another within same process requires a mode
switch to the Kernel.

Multithreading Models
Some operating system provide a combined user level thread and Kernel level thread facility.
Solaris is a good example of this combined approach. In a combined system, multiple threads
within the same application can run in parallel on multiple processors and a blocking system call
need not block the entire process. Multithreading models are three types

 Many to many relationship.

 Many to one relationship.
 One to one relationship.

Many to Many Model

In this model, many user level threads multiplexes to the Kernel thread of smaller or equal
numbers. The number of Kernel threads may be specific to either a particular application or a
particular machine.

Following diagram shows the many to many model. In this model, developers can create as
many user threads as necessary and the corresponding Kernel threads can run in parallels on a
multiprocessor.

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Many to One Model
Many to one model maps many user level threads to one Kernel level thread. Thread
management is done in user space. When thread makes a blocking system call, the entire process
will be blocked. Only one thread can access the Kernel at a time,so multiple threads are unable to
run in parallel on multiprocessors.

If the user level thread libraries are implemented in the operating system in such a way that
system does not support them then Kernel threads use the many to one relationship modes.

One to One Model

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There is one to one relationship of user level thread to the kernel level thread.This model
provides more concurrency than the many to one model. It also another thread to run when a
thread makes a blocking system call. It support multiple thread to execute in parallel on
microprocessors.

Disadvantage of this model is that creating user thread requires the corresponding Kernel thread.
OS/2, windows NT and windows 2000 use one to one relationship model.

Difference between User Level & Kernel Level Thread

S.N. User Level Threads Kernel Level Thread

User level threads are faster to create and Kernel level threads are slower to create and
1
manage. manage.

Implementation is by a thread library at the Operating system supports creation of Kernel

2
user level. threads.

User level thread is generic and can run on Kernel level thread is specific to the operating
3
any operating system. system.

Multi-threaded application cannot take

4 Kernel routines themselves can be multithreaded.
advantage of multiprocessing.

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9.0 Operating System - Memory Management
Memory management is the functionality of an operating system which handles or manages
primary memory. Memory management keeps track of each and every memory location either it
is allocated to some process or it is free. It checks how much memory is to be allocated to
processes. It decides which process will get memory at what time. It tracks whenever some
memory gets freed or unallocated and correspondingly it updates the status.

Memory management provides protection by using two registers, a base register and a limit
register. The base register holds the smallest legal physical memory address and the limit register
specifies the size of the range. For example, if the base register holds 300000 and the limit
register is 1209000, then the program can legally access all addresses from 300000 through
411999.

Instructions and data to memory addresses can be done in following ways

 Compile time -- When it is known at compile time where the process will reside,
compile time binding is used to generate the absolute code.
 Load time -- When it is not known at compile time where the process will reside in
memory, then the compiler generates re-locatable code.
 Execution time -- If the process can be moved during its execution from one memory
segment to another, then binding must be delayed to be done at run time

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Dynamic Loading
In dynamic loading, a routine of a program is not loaded until it is called by the program. All
routines are kept on disk in a re-locatable load format. The main program is loaded into memory
and is executed. Other routines methods or modules are loaded on request. Dynamic loading
makes better memory space utilization and unused routines are never loaded.

Dynamic Linking
Linking is the process of collecting and combining various modules of code and data into a
executable file that can be loaded into memory and executed. Operating system can link system
level libraries to a program. When it combines the libraries at load time, the linking is called
static linking and when this linking is done at the time of execution, it is called as dynamic
linking.

In static linking, libraries linked at compile time, so program code size becomes bigger whereas
in dynamic linking libraries linked at execution time so program code size remains smaller.

Logical versus Physical Address Space

An address generated by the CPU is a logical address whereas address actually available on
memory unit is a physical address. Logical address is also known a Virtual address.

Virtual and physical addresses are the same in compile-time and load-time address-binding
schemes. Virtual and physical addresses differ in execution-time address-binding scheme.

The set of all logical addresses generated by a program is referred to as a logical address space.
The set of all physical addresses corresponding to these logical addresses is referred to as a
physical address space.

The run-time mapping from virtual to physical address is done by the memory management unit
(MMU) which is a hardware device. MMU uses following mechanism to convert virtual address
to physical address.

 The value in the base register is added to every address generated by a user process which
is treated as offset at the time it is sent to memory. For example, if the base register value
is 10000, then an attempt by the user to use address location 100 will be dynamically
reallocated to location 10100.
 The user program deals with virtual addresses; it never sees the real physical addresses.

Swapping
Swapping is a mechanism in which a process can be swapped temporarily out of main memory
to a backing store , and then brought back into memory for continued execution.

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Backing store is a usually a hard disk drive or any other secondary storage which fast in access
and large enough to accommodate copies of all memory images for all users. It must be capable
of providing direct access to these memory images.

Major time consuming part of swapping is transfer time. Total transfer time is directly
proportional to the amount of memory swapped. Let us assume that the user process is of size
100KB and the backing store is a standard hard disk with transfer rate of 1 MB per second. The
actual transfer of the 100K process to or from memory will take

100KB / 1000KB per second

= 1/10 second

= 100 milliseconds

Memory Allocation
Main memory usually has two partitions

 Low Memory -- Operating system resides in this memory.

 High Memory -- User processes then held in high memory.

Operating system uses the following memory allocation mechanism.

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S.N. Memory Allocation Description
In this type of allocation, relocation-register scheme is used to
protect user processes from each other, and from changing
Single-partition operating-system code and data. Relocation register contains value
1
allocation of smallest physical address whereas limit register contains range
of logical addresses. Each logical address must be less than the
limit register.
In this type of allocation, main memory is divided into a number of
fixed-sized partitions where each partition should contain only one
Multiple-partition
2 process. When a partition is free, a process is selected from the
allocation
input queue and is loaded into the free partition. When the process
terminates, the partition becomes available for another process.

Fragmentation
As processes are loaded and removed from memory, the free memory space is broken into little
pieces. It happens after sometimes that processes can not be allocated to memory blocks
considering their small size and memory blocks remains unused. This problem is known as
Fragmentation.

Fragmentation is of two types

S.N. Fragmentation Description

External Total memory space is enough to satisfy a request or to reside a process
1
fragmentation in it, but it is not contiguous so it can not be used.
Internal Memory block assigned to process is bigger. Some portion of memory
2
fragmentation is left unused as it can not be used by another process.

External fragmentation can be reduced by compaction or shuffle memory contents to place all
free memory together in one large block. To make compaction feasible, relocation should be
dynamic.

Paging
External fragmentation is avoided by using paging technique. Paging is a technique in which
physical memory is broken into blocks of the same size called pages (size is power of 2, between
512 bytes and 8192 bytes). When a process is to be executed, it's corresponding pages are loaded
into any available memory frames.

Logical address space of a process can be non-contiguous and a process is allocated physical
memory whenever the free memory frame is available. Operating system keeps track of all free
frames. Operating system needs n free frames to run a program of size n pages.

Address generated by CPU is divided into

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  Page number (p) -- page number is used as an index into a page table which contains
base address of each page in physical memory.
 Page offset (d) -- page offset is combined with base address to define the physical
memory address.

Following figure show the paging table architecture.

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Segmentation
Segmentation is a technique to break memory into logical pieces where each piece represents a
group of related information. For example ,data segments or code segment for each process, data
segment for operating system and so on. Segmentation can be implemented using or without
using paging.

Unlike paging, segment are having varying sizes and thus eliminates internal fragmentation.
External fragmentation still exists but to lesser extent.

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Address generated by CPU is divided into

 Segment number (s) -- segment number is used as an index into a segment table which
contains base address of each segment in physical memory and a limit of segment.
 Segment offset (o) -- segment offset is first checked against limit and then is combined
with base address to define the physical memory address.

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10.0 Operating System - Virtual Memory
Virtual memory is a technique that allows the execution of processes which are not completely
available in memory. The main visible advantage of this scheme is that programs can be larger
than physical memory. Virtual memory is the separation of user logical memory from physical
memory.

This separation allows an extremely large virtual memory to be provided for programmers when
only a smaller physical memory is available. Following are the situations, when entire program is
not required to be loaded fully in main memory.

 User written error handling routines are used only when an error occured in the data or
computation.
 Certain options and features of a program may be used rarely.
 Many tables are assigned a fixed amount of address space even though only a small
amount of the table is actually used.
 The ability to execute a program that is only partially in memory would counter many
benefits.
 Less number of I/O would be needed to load or swap each user program into memory.
 A program would no longer be constrained by the amount of physical memory that is
available.
 Each user program could take less physical memory, more programs could be run the
same time, with a corresponding increase in CPU utilization and throughput.

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Virtual memory is commonly implemented by demand paging. It can also be implemented in a
segmentation system. Demand segmentation can also be used to provide virtual memory.

Demand Paging
A demand paging system is quite similar to a paging system with swapping. When we want to
execute a process, we swap it into memory. Rather than swapping the entire process into
memory, however, we use a lazy swapper called pager.

When a process is to be swapped in, the pager guesses which pages will be used before the
process is swapped out again. Instead of swapping in a whole process, the pager brings only
those necessary pages into memory. Thus, it avoids reading into memory pages that will not be
used in anyway, decreasing the swap time and the amount of physical memory needed.

Hardware support is required to distinguish between those pages that are in memory and those
pages that are on the disk using the valid-invalid bit scheme. Where valid and invalid pages can
be checked by checking the bit. Marking a page will have no effect if the process never attempts
to access the page. While the process executes and accesses pages that are memory resident,
execution proceeds normally.

Access to a page marked invalid causes a page-fault trap. This trap is the result of the operating
system's failure to bring the desired page into memory. But page fault can be handled as
following

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Step Description

Check an internal table for this process, to determine whether the reference was a
Step 1
valid or it was an invalid memory access.

If the reference was invalid, terminate the process. If it was valid, but page have
Step 2
not yet brought in, page in the latter.

Step 3 Find a free frame.

Step 4 Schedule a disk operation to read the desired page into the newly allocated frame.

When the disk read is complete, modify the internal table kept with the process
Step 5
and the page table to indicate that the page is now in memory.

Restart the instruction that was interrupted by the illegal address trap. The process
can now access the page as though it had always been in memory. Therefore, the
Step 6
operating system reads the desired page into memory and restarts the process as
though the page had always been in memory.

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Advantages

Following are the advantages of Demand Paging

 Large virtual memory.

 More efficient use of memory.
 Unconstrained multiprogramming. There is no limit on degree of multiprogramming.

Disadvantages

Following are the disadvantages of Demand Paging

 Number of tables and amount of processor overhead for handling page interrupts are
greater than in the case of the simple paged management techniques.
 Due to the lack of an explicit constraints on a jobs address space size.

Page Replacement Algorithm

Page replacement algorithms are the techniques using which Operating System decides which
memory pages to swap out, write to disk when a page of memory needs to be allocated. Paging
happens whenever a page fault occurs and a free page cannot be used for allocation purpose
accounting to reason that pages are not available or the number of free pages is lower than
required pages.

When the page that was selected for replacement and was paged out, is referenced again then it
has to read in from disk, and this requires for I/O completion. This process determines the quality
of the page replacement algorithm: the lesser the time waiting for page-ins, the better is the
algorithm. A page replacement algorithm looks at the limited information about accessing the
pages provided by hardware, and tries to select which pages should be replaced to minimize the
total number of page misses, while balancing it with the costs of primary storage and processor
time of the algorithm itself. There are many different page replacement algorithms. We evaluate
an algorithm by running it on a particular string of memory reference and computing the number
of page faults.

Reference String
The string of memory references is called reference string. Reference strings are generated
artificially or by tracing a given system and recording the address of each memory reference. The
latter choice produces a large number of data, where we note two things.

 For a given page size we need to consider only the page number, not the entire address.
 If we have a reference to a page p, then any immediately following references to page p
will never cause a page fault. Page p will be in memory after the first reference; the
immediately following references will not fault.
 For example, consider the following sequence of addresses - 123,215,600,1234,76,96

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  If page size is 100 then the reference string is 1,2,6,12,0,0

First In First Out (FIFO) algorithm

 Oldest page in main memory is the one which will be selected for replacement.
 Easy to implement, keep a list, replace pages from the tail and add new pages at the head.

Optimal Page algorithm

 An optimal page-replacement algorithm has the lowest page-fault rate of all algorithms.
An optimal page-replacement algorithm exists, and has been called OPT or MIN.
 Replace the page that will not be used for the longest period of time . Use the time when
a page is to be used.

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Least Recently Used (LRU) algorithm
 Page which has not been used for the longest time in main memory is the one which will
be selected for replacement.
 Easy to implement, keep a list, replace pages by looking back into time.

Page Buffering algorithm

 To get process start quickly, keep a pool of free frames.
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  On page fault, select a page to be replaced.
 Write new page in the frame of free pool, mark the page table and restart the process.
 Now write the dirty page out of disk and place the frame holding replaced page in free
pool.

Least frequently Used(LFU) algorithm

 Page with the smallest count is the one which will be selected for replacement.
 This algorithm suffers from the situation in which a page is used heavily during the initial
phase of a process, but then is never used again.

Most frequently Used(MFU) algorithm

 This algorithm is based on the argument that the page with the smallest count was
probably just brought in and has yet to be used.

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11.0 Operating System - I/O Hardware
Overview
Computers operate on many kinds of devices. General types include storage devices (disks,
tapes), transmission devices (network cards, modems), and human-interface devices (screen,
keyboard, mouse). Other devices are more specialized. A device communicates with a computer
system by sending signals over a cable or even through the air.

The device communicates with the machine via a connection point termed a port (for example, a
serial port). If one or more devices use a common set of wires, the connection is called a bus.In
other terms, a bus is a set of wires and a rigidly defined protocol that specifies a set of messages
that can be sent on the wires.

Daisy chain
When device A has a cable that plugs into device B, and device B has a cable that plugs into
device C, and device C plugs into a port on the computer, this arrangement is called a daisy
chain. It usually operates as a bus.

Controller
A controller is a collection of electronics that can operate a port, a bus, or a device. A serial-port
controller is an example of a simple device controller. This is a single chip in the computer that
controls the signals on the wires of a serial port.

The SCSI bus controller is often implemented as a separate circuit board (a host adapter) that
plugs into the computer. It contains a processor, microcode, and some private memory to enable
it to process the SCSI protocol messages. Some devices have their own built-in controllers.

I/O port
An I/O port typically consists of four registers, called the status , control, data-in, and data-out
registers.

S.N. Register & Description

Status Register
The status register contains bits that can be read by the host. These bits indicate states such
1
as whether the current command has completed, whether a byte is available to be read from
the data-in register, and whether there has been a device error.
Control register
2 The control register can be written by the host to start a command or to change the mode of
a device. For instance, a certain bit in the control register of a serial port chooses between

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 full-duplex and half-duplex communication, another enables parity checking, a third bit sets
the word length to 7 or 8 bits, and other bits select one of the speeds supported by the serial
port.
Data-in register
3
The data-in register is read by the host to get input.
Data-out register
4
The data out register is written by the host to send output.

Polling
Polling is a process by which a host waits for controller response.It is a looping process, reading
the status register over and over until the busy bit of status register becomes clear. The controller
uses/sets the busy bit when it is busy working on a command, and clears the busy bit when it is
ready to accept the next command. The host signals its wish via the command-ready bit in the
command register. The host sets the command-ready bit when a command is available for the
controller to execute.

In the following example, the host writes output through a port, coordinating with the controller
by handshaking

 The host repeatedly reads the busy bit until that bit becomes clear.
 The host sets the write bit in the command register and writes a byte into the data-out
register.
 The host sets the command-ready bit.
 When the controller notices that the command-ready bit is set, it sets the busy bit.
 The controller reads the command register and sees the write command.
 It reads the data-out register to get the byte, and does the I/O to the device.
 The controller clears the command-ready bit, clears the error bit in the status register to
indicate that the device I/O succeeded, and clears the busy bit to indicate that it is
finished.

I/O devices
I/O Devices can be categorized into following category.

S.N. Category & Description

Human readable
1 Human Readable devices are suitable for communicating with the computer user. Examples
are printers, video display terminals, keyboard etc.
Machine readable
2 Machine Readable devices are suitable for communicating with electronic equipment.
Examples are disk and tape drives, sensors, controllers and actuators.
Communication
2
Communication devices are suitable for communicating with remote devices. Examples are

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 digital line drivers and modems.

Following are the differences between I/O Devices

S.N. Criteria & Description

Data rate
1
There may be differences of several orders of magnitude between the data transfer rates.
Application
2
Different devices have different use in the system.
Complexity of Control
3
A disk is much more complex whereas printer requires simple control interface.
Unit of transfer
4
Data may be transferred as a stream of bytes or characters or in larger blocks.
Data representation
5
Different data encoding schemes are used for different devices.
Error Conditions
6
The nature of errors differs widely from one device to another.

Direct Memory Access (DMA)

Many computers avoid burdening the main CPU with programmed I/O by offloading some of
this work to a special purpose processor. This type of processor is called, a Direct Memory
Access(DMA) controller. A special control unit is used to transfer block of data directly between
an external device and the main memory, without intervention by the processor. This approach is
called Direct Memory Access(DMA).

DMA can be used with either polling or interrupt software. DMA is particularly useful on
devices like disks, where many bytes of information can be transferred in single I/O operations.
When used with an interrupt, the CPU is notified only after the entire block of data has been
transferred. For each byte or word transferred, it must provide the memory address and all the
bus signals controlling the data transfer. Interaction with a device controller is managed through
a device driver.

Handshaking is a process between the DMA controller and the device controller. It is performed
via wires using terms DMA request and DMA acknowledge.

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Step Description
1 Device driver is instructed to transfer disk data to a buffer address X.
2 Device driver then instruct disk controller to transfer data to buffer.
3 Disk controller starts DMA transfer.
4 Disk controller sends each byte to DMA controller.
DMA controller transfers bytes to buffer, increases the memory address, decreases the
5
counter C until C becomes zero.
6 When C becomes zero, DMA interrupts CPU to signal transfer completion.

Device Controllers
A computer system contains a many types of I/O devices and their respective controllers

 network card
 graphics adapter
 disk controller
 DVD-ROM controller
 serial port
 USB
 sound card

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12.0 Operating System - I/O Softwares
Interrupts
The CPU hardware uses an interrupt request line wire which helps CPU to sense after executing
every instruction. When the CPU checks that a controller has put a signal on the interrupt request
line, the CPU saves a state, such as the current value of the instruction pointer, and jumps to the
interrupt handler routine at a fixed address. The interrupt handler part determines the cause of the
interrupt, performs the necessary processing and executes a interrupt instruction to return the
CPU to its execution state.

The basic mechanism of interrurpt enables the CPU to respond to an asynchronous event, such as
when a device controller become ready for service. Most CPUs have two interrupt request lines.

 non-maskable interrupt - Such kind of interrupts are reserved for events like
unrecoverable memory errors.
 maskable interrupt - Such interrupts can be switched off by the CPU before the
execution of critical instructions that must not be interrupted.

The interrupt mechanism accepts an address - a number that selects a specific interrupt handling
routine/function from a small set.In most architectures, this address is an offset stored in a table
called the interrupt vector table. This vector contains the memory addresses of specialized
interrupt handlers.

Application I/O Interface

Application I/O Interface represents the structuring techniques and interfaces for the operating
system to enable I/O devices to be treated in a standard, uniform way. The actual differences lies
kernel level modules called device drivers which are custom tailored to corresponding devices
but show one of the standard interfaces to applications. The purpose of the device-driver layer is
to hide the differences among device controllers from the I/O subsystem of the kernel, such as
the I/O system calls. Following are the characteristics of I/O interfaces with respected to devices.

 Character-stream / block - A character-stream device transfers bytes in one by one

fashion, whereas a block device transfers a complete unit of bytes.
 Sequential / random-access - A sequential device transfers data in a fixed order
determined by the device, random-access device can be instructed to seek position to any
of the available data storage locations.
 Synchronous / asynchronous - A synchronous device performs data transfers with
known response time where as an asynchronous device shows irregular or unpredictable
response time.
 Sharable / dedicated - A sharable device can be used concurrently by several processes
or threads but a dedicated device cannot be used.

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  Speed of operation - Device speeds may range from a few bytes per second to a few
gigabytes per second.
 Read-write, read only, or write only - Some devices perform both input and output, but
others support only one data direction that is read only.

Clocks
Clocks are also called timers. The clock software takes the form of a device driver though a
clock is neither a blocking device nor a character based device. The clock software is the clock
driver. The exact function of the clock driver may vary depending on operating system.
Generally, the functions of the clock driver include the following.

S.N. Task Description

The clock driver implements the time of day or
1 Maintaining the time of the day the real time clock function.It requires
incrementing a counter at each clock tick.
As a process is started, the scheduler initializes
the quantum counter in clock ticks for the
process. The clock driver decrements the
Preventing processes from running too quantum counter by 1, at every clock interrupt.
2
long When the counter gets to zero , clock driver
calls the scheduler to set up another process.
Thus clock driver helps in preventing processes
from running longer than time slice allowed.
Another function performed by clock driver is
3 Accounting for CPU usage doing CPU accounting. CPU accounting
implies telling how long the process has run.
Watchdog timers are the timers set by certain
parts of the system. For example, to use a
Providing watchdog timers for parts of the
4 floppy disk, the system must turn on the motor
system itself
and then wait about 500msec for it to comes up
to speed.

Kernel I/O Subsystem

Kernel I/O Subsystem is responsible to provide many services related to I/O. Following are some
of the services provided.

 Scheduling - Kernel schedules a set of I/O requests to determine a good order in which to
execute them. When an application issues a blocking I/O system call, the request is
placed on the queue for that device. The Kernel I/O scheduler rearranges the order of the
queue to improve the overall system efficiency and the average response time
experienced by the applications.

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  Buffering - Kernel I/O Subsystem maintains a memory area known as buffer that stores
data while they are transferred between two devices or between a device with an
application operation. Buffering is done to cope with a speed mismatch between the
producer and consumer of a data stream or to adapt between devices that have different
data transfer sizes.
 Caching - Kernel maintains cache memory which is region of fast memory that holds
copies of data. Access to the cached copy is more efficient than access to the original.
 Spooling and Device Reservation A spool is a buffer that holds output for a device, such
as a printer, that cannot accept interleaved data streams. The spooling system copies the
queued spool files to the printer one at a time. In some operating systems, spooling is
managed by a system daemon process. In other operating systems, it is handled by an in
kernel thread.
 Error Handling An operating system that uses protected memory can guard against
many kinds of hardware and application errors.

Device driver
Device driver is a program or routine developed for an I/O device. A device driver implements
I/O operations or behaviours on a specific class of devices. For example a system supports one or
a number of multiple brands of terminals, all slightly different terminals may have a single
terminal driver. In the layered structure of I/O system, device driver lies between interrupt
handler and device independent I/O software. The job of a device driver are following.

 To accept request from the device independent software above it.

 To see to it that the request is executed.

How a device driver handles a request is as follows: Suppose a request comes to read a block N.
If the driver is idle at the time a request arrives, it starts carrying out the request immediately.
Otherwise, if the driver is already busy with some other request, it places the new request in the
queue of pending requests.

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13.0 Operating System - File System
File
A file is a named collection of related information that is recorded on secondary storage such as
magnetic disks, magnetic tapes and optical disks.In general, a file is a sequence of bits, bytes,
lines or records whose meaning is defined by the files creator and user.

File Structure
File structure is a structure, which is according to a required format that operating system can
understand.

 A file has a certain defined structure according to its type.

 A text file is a sequence of characters organized into lines.
 A source file is a sequence of procedures and functions.
 An object file is a sequence of bytes organized into blocks that are understandable by the
machine.
 When operating system defines different file structures, it also contains the code to
support these file structure. Unix, MS-DOS support minimum number of file structure.

File Type
File type refers to the ability of the operating system to distinguish different types of file such as
text files source files and binary files etc. Many operating systems support many types of files.
Operating system like MS-DOS and UNIX have the following types of files:

Ordinary files

 These are the files that contain user information.

 These may have text, databases or executable program.
 The user can apply various operations on such files like add, modify, delete or even
remove the entire file.

Directory files

 These files contain list of file names and other information related to these files.

Special files:

 These files are also known as device files.

 These files represent physical device like disks, terminals, printers, networks, tape drive
etc.

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These files are of two types

 Character special files - data is handled character by character as in case of terminals or

printers.
 Block special files - data is handled in blocks as in the case of disks and tapes.

File Access Mechanisms

File access mechanism refers to the manner in which the records of a file may be accessed. There
are several ways to access files

 Sequential access
 Direct/Random access
 Indexed sequential access

Sequential access

A sequential access is that in which the records are accessed in some sequence i.e the
information in the file is processed in order, one record after the other. This access method is the
most primitive one. Example: Compilers usually access files in this fashion.

Direct/Random access

 Random access file organization provides, accessing the records directly.

 Each record has its own address on the file with by the help of which it can be directly
accessed for reading or writing.
 The records need not be in any sequence within the file and they need not be in adjacent
locations on the storage medium.

Indexed sequential access

 This mechanism is built up on base of sequential access.

 An index is created for each file which contains pointers to various blocks.
 Index is searched sequentially and its pointer is used to access the file directly.

Space Allocation
Files are allocated disk spaces by operating system. Operating systems deploy following three
main ways to allocate disk space to files.

 Contiguous Allocation
 Linked Allocation
 Indexed Allocation

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Contiguous Allocation

 Each file occupy a contiguous address space on disk.

 Assigned disk address is in linear order.
 Easy to implement.
 External fragmentation is a major issue with this type of allocation technique.

Linked Allocation

 Each file carries a list of links to disk blocks.

 Directory contains link / pointer to first block of a file.
 No external fragmentation
 Effectively used in sequential access file.
 Inefficient in case of direct access file.

Indexed Allocation

 Provides solutions to problems of contigous and linked allocation.

 A index block is created having all pointers to files.
 Each file has its own index block which stores the addresses of disk space occupied by
the file.
 Directory contains the addresses of index blocks of files.

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14.0 Operating System - Security
Security refers to providing a protection system to computer system resources such as CPU,
memory, disk, software programs and most importantly data/information stored in the computer
system. If a computer program is run by unauthorized user then he/she may cause severe damage
to computer or data stored in it. So a computer system must be protected against unauthorized
access, malicious access to system memory, viruses, worms etc. We're going to discuss
following topics in this article.

 Authentication
 One Time passwords
 Program Threats
 System Threats
 Computer Security Classifications

Authentication
Authentication refers to identifying the each user of the system and associating the executing
programs with those users. It is the responsibility of the Operating System to create a protection
system which ensures that a user who is running a particular program is authentic. Operating
Systems generally identifies/authenticates users using following three ways:

 Username / Password - User need to enter a registered username and password with
Operating system to login into the system.
 User card/key - User need to punch card in card slot, or enter key generated by key
generator in option provided by operating system to login into the system.
 User attribute - fingerprint/ eye retina pattern/ signature - User need to pass his/her
attribute via designated input device used by operating system to login into the system.

One Time passwords

One time passwords provides additional security along with normal authentication. In One-Time
Password system, a unique password is required every time user tries to login into the system.
Once a one-time password is used then it can not be used again. One time password are
implemented in various ways.

 Random numbers - Users are provided cards having numbers printed along with
corresponding alphabets. System asks for numbers corresponding to few alphabets
randomly chosen.
 Secret key - User are provided a hardware device which can create a secret id mapped
with user id. System asks for such secret id which is to be generated every time prior to
login.
 Network password - Some commercial applications send one time password to user on
registered mobile/ email which is required to be entered prior to login.

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Program Threats
Operating system's processes and kernel do the designated task as instructed. If a user program
made these process do malicious tasks then it is known as Program Threats. One of the common
example of program threat is a program installed in a computer which can store and send user
credentials via network to some hacker. Following is the list of some well known program
threats.

 Trojan Horse - Such program traps user login credentials and stores them to send to
malicious user who can later on login to computer and can access system resources.
 Trap Door - If a program which is designed to work as required, have a security hole in
its code and perform illegal action without knowledge of user then it is called to have a
trap door.
 Logic Bomb - Logic bomb is a situation when a program misbehaves only when certain
conditions met otherwise it works as a genuine program. It is harder to detect.
 Virus - Virus as name suggest can replicate themselves on computer system .They are
highly dangerous and can modify/delete user files, crash systems. A virus is generatlly a
small code embedded in a program. As user accesses the program, the virus starts getting
embedded in other files/ programs and can make system unusable for user.

System Threats
System threats refers to misuse of system services and network connections to put user in
trouble. System threats can be used to launch program threats on a complete network called as
program attack. System threats creates such an environment that operating system resources/
user files are mis-used. Following is the list of some well known system threats.

 Worm -Worm is a process which can choked down a system performance by using
system resources to extreme levels.A Worm process generates its multiple copies where
each copy uses system resources, prevents all other processes to get required resources.
Worms processes can even shut down an entire network.
 Port Scanning - Port scanning is a mechanism or means by which a hacker can detects
system vulnerabilities to make an attack on the system.
 Denial of Service - Denial of service attacks normally prevents user to make legitimate
use of the system. For example user may not be able to use internet if denial of service
attacks browser's content settings.

Computer Security Classifications

As per the U.S. Department of Defense Trusted Computer System's Evaluation Criteria there are
four security classifications in computer systems: A, B, C, and D. This is widely used
specifications to determine and model the security of systems and of security solutions.
Following is the brief description of each classfication.

S.N. Classification Description

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 Type
Highest Level. Uses formal design specifications and verification
1 Type A
techniques.Grants a high degree of assurance of process security.
Provides mandatory protection system. Have all the properties of a class
C2 system. Attaches a sensitivity label to each object.It is of three types.

 B1 - Maintains the security label of each object in the system.Label

is used for making decisions to access control.
2 Type B
 B2 - Extends the sensitivity labels to each system resource, such as
storage objects, supports covert channels and auditing of events.
 B3 - Allows creating lists or user groups for access-control to grant
access or revoke access to a given named object.

Provides protection and user accountability using audit capabilities. It is of

two types.

 C1 - Incorporates controls so that users can protect their private

3 Type C information and keep other users from accidentally reading /
deleting their data. UNIX versions are mostly Cl class.
 C2 - Adds an individual-level access control to the capabilities of a
Cl level system

Lowest level. Minimum protection. MS-DOS, Window 3.1 fall in this

4 Type D
category.

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15.0 Operating System - Linux
Linux is one of popular version of UNIX operating System. It is open source as its source code is
freely available. It is free to use. Linux was designed considering UNIX compatibility. It's
functionality list is quite similar to that of UNIX.

Components of Linux System

Linux Operating System has primarily three components

 Kernel - Kernel is the core part of Linux. It is responsible for all major activities of this
operating system. It is consists of various modules and it interacts directly with the
underlying hardware. Kernel provides the required abstraction to hide low level hardware
details to system or application programs.
 System Library - System libraries are special functions or programs using which
application programs or system utilities accesses Kernel's features. These libraries
implements most of the functionalities of the operating system and do not requires kernel
module's code access rights.
 System Utility - System Utility programs are responsible to do specialized, individual
level tasks.

Kernel Mode vs User Mode

Kernel component code executes in a special privileged mode called kernel mode with full
access to all resources of the computer. This code represents a single process, executes in single
address space and do not require any context switch and hence is very efficient and fast. Kernel

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runs each processes and provides system services to processes, provides protected access to
hardwares to processes.

Support code which is not required to run in kernel mode is in System Library. User programs
and other system programs works in User Mode which has no access to system hardwares and
kernel code. User programs/ utilities use System libraries to access Kernel functions to get
system's low level tasks.

Basic Features
Following are some of the important features of Linux Operating System.

 Portable - Portability means softwares can works on different types of hardwares in

same way.Linux kernel and application programs supports their installation on any kind
of hardware platform.
 Open Source - Linux source code is freely available and it is community based
development project. Multiple teams works in collaboration to enhance the capability of
Linux operating system and it is continuously evolving.
 Multi-User - Linux is a multiuser system means multiple users can access system
resources like memory/ ram/ application programs at same time.
 Multiprogramming - Linux is a multiprogramming system means multiple applications
can run at same time.
 Hierarchical File System - Linux provides a standard file structure in which system
files/ user files are arranged.
 Shell - Linux provides a special interpreter program which can be used to execute
commands of the operating system. It can be used to do various types of operations, call
application programs etc.
 Security - Linux provides user security using authentication features like password
protection/ controlled access to specific files/ encryption of data.

Architecture

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Linux System Architecture is consists of following layers

 Hardware layer - Hardware consists of all peripheral devices (RAM/ HDD/ CPU etc).
 Kernel - Core component of Operating System, interacts directly with hardware,
provides low level services to upper layer components.
 Shell - An interface to kernel, hiding complexity of kernel's functions from users. Takes
commands from user and executes kernel's functions.
 Utilities - Utility programs giving user most of the functionalities of an operating
systems.

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