Operating Systems

OPERATING SYSTEMS.
SY B.Sc. I.T.
Author: Fahad Shaikh.

Operating Systems SY-IT
Unit - I
COMPONENTS OF COMPUTER SYSTEM
User 1 User2 User n
System and Application Program
Operating System
Computer Hardware
The above diagram gives a view of the components of a computer system, which can be roughly divided
into four components. The hardware, the operating system, the application program and the users.
The hardware consists of the CPU, memory and I/O devices which provide the basic computing
resources. The application program consists of programs such as word processors, compilers, and web
browsers. The operating system controls and co- ordinates the use of the hardware among the various
application programs for the various users. The Operating System provides the means for proper use of
the recourses in operation of the computer system.
The two basic goals of an operating system are convenience and efficiency
TYPES OF SYSTEMS:
1) Mainframe Systems:
Mainframe computer system where the first computers which were used to solve many
commercial and scientific applications. They are divided into three types.
i) Batch systems
ii) Multi programmed systems
iii) Time sharing systems
Fahad Shaikh (System Administrator) Page 2

i) Batch systems
Operating
system
User
program
Batch system consists of large machines in which the input devices were card readers and tape
drives. The output consists of line printers, tape drives and card punches. The user did not
interact directly with the computer system; the user prepared a job and submits it to the
computer operator. The job was usually in the form of punch cards.
The operating system in these early systems was very simple. Its major task was to
transfer control automatically from one job the next. The operating system is always resident in
the memory. In this execution environment, the CPU is often idle due to the speeds of the
peripheral devices. Hence the major disadvantage of this system was inefficient use of the CPU.
ii) Multi programmed systems
Operating system
Job 1
Job 2
Job 3
Job 4
Multi programming increases CPU utilization by organizing jobs so that the CPU always
has at least one job to execute. The operating system keeps many jobs in the memory at a time
and picks up a job to execute. In case the job has to wait for some task (such as I/O) the
operating system switches to execute another job. Hence the CPU is never idle.

Multi programming is the first instance where the operating system must make decisions
for the users. Hence multi programmed operating system are more sophisticated.
iii) Time sharing operating system /Multitasking system /Interactive system
A time shared operating system allows many users to share the computer simultaneously.
Since each action or command in a time share system is very short, only a little CPU time is
need for each user. As the system switches rapidly from one user to the next, each user is given
the impression that the entire computer system is dedicated to him even though it may be
shared among many users.
A time shared operating system uses CPU scheduling & multi programming to provide each
user with a small portion of time shared computer. Time sharing operating system are more
complex than multi programmed operating system because they need to provide protection, a
way to handle the file system, job synchronization, communication and deadlock free
environment
2) Desktop System:
Desktop systems during their initial stages did not had the feature to protect the operating
system from the user programs. Hence pc operating system where neither multiuser nor
multitasking. Since every user is having all the resources hence there is no question of efficiency
therefore the main goal of such system was maximizing user convenience and responsiveness.
With the advent of network this system needed protection hence this feature was also added.
3) Multiprocessor System:
Multiprocessor system (parallel system or tightly couple system) have more than one processor
in closed communicating sharing the computer bus, the clock as well as memory and peripheral
devices.
Multiprocessor systems have three main advantages:
I) Increased Throughput:- By increasing the number of processor more work is done in less time.
The speed up ratio with N processors is not almost equal to N due to overhead in keeping all
the work correctly also due to sharing of resources the speedup ratio decrease further
II) Economy: - Multiprocessor systems are more economic as compared to multiple single
processor system because they share peripheral, memory and power supplies

III) Increase Reliability: - If functions (if work) are properly distributed among several processors
then the failure of one processor will not hold the system but only slow down the system.
Suppose if we have 10 processors and if one fails, then each of the remaining would continue
the processing. The overall may degrade by 10%. The ability of a system to provide service
proportional to the level of surviving hardware is called graceful degradation. Systems designed
for graceful degradation are called fault tolerant.
Multiprocessor system can be realized in the following ways
 Tandem system :
This system uses both hardware and software duplication to ensure continuous operation even
in case of failures. The system consists of two identical processor each having its own local
memory the processor is connected by a bus. One processor is primary and other is backup.
Two copies of each processor (1 in primary in backup). At some fixed interval of time the
information of each process is copied from primary to backup. If a failure is detected, the
backup copies activated and restarted from the most recent check point.
The drawback of the system is at it is expensive.
 Symmetric multiprocessing system(SMP):
Memory
CPU …………… CPU …………………. CPU ………….. CPU
In symmetric multiprocessing each processor runs and identical copies of the operating system
and these copies communicate each other as needed. There is no master slaves relationship
between processor, all processor are peers. The benefit of this model is that many processors
can run simultaneously. The problem with this system would be that one processor (CPU) may
be sitting idle and other may be overloaded. This can be avoided if processors share certain

data structures. A multiprocessor system of this form will allow processes and resources to be
shared properly among various processors.
 Asymmetric multiprocessing:
MASTER
CPU
CPU . . . . . . CPU . . . . . CPU . . . . . CPU
SLAVES
In asymmetric multiprocessing each processing assigned a specific task. A master processor
controls the system. Other processors depend on the master for instruction or may have a
predefined task. This scheme defined a master slave relationship. The master processor
schedules and allocates work to the slave processors.
The difference between symmetric and asymmetric multiprocessing may be the result of either
hardware or software. Special hardware can differentiate the multiprocessor or the software
can be written to allow only one master and multiple slaves.
3) Distributed System:
Distributed systems depend on networking for their functionality, they are able to share
computational task and provide a rich set of features to users. Networks are of various types,
the type may depend by the protocols used, the distance between the nodes and the transport
media. TCP/IP is the most common network protocol a part from others. Most operating
system support TCP/IP.
A LAN exists within a room or a building. A VAN exists between cities and countries and so on.
The different transport media include copper wires, fiber optic, satellites and radios.
The following are the most common type of the system
1) Client server system
2) Peer to peer system

i) Client server system:
Client . . . . . . Client . . . . . . Client . . . . . . . Client
Network
SERVER
The above diagram gives the general structure of client server system in which we have a server
systems which satisfied request generated by client system.
Server systems can be broadly categorized as compute servers and file servers.
Compute server system provide an interface to which client can send request to perform an
action. In response to which they execute the action and send back result to the client.
File server system provide a file system interface where clients can create, update, read, and
delete files.
ii) Peer to peer system:
In this system the processors communicate with one and other through various communication
lines such as high speed buses or telephone lines. These systems are usually referred as loosely
coupled system.
The operating system designed for such a system is called network operating system which
provides features such as file sharing across the network, allowing features to allow different
process on different computer to exchange messages.
4) Real Time System:
A real time system has a well define fixed time constraints. Processing must be done within the
defined constraints otherwise the system will failed. A real time system functions correctly only
if it returns the correct result within the time limit. Real time systems are of two types.

i) Hard Real Time System
ii) Soft Real Time System
A hard real time system guarantees that critical task must be complete on time. In such a
system all delays must be bounded. The use of secondary storage should be extremely limited.
Also most advanced operating system features are absent in such system.
Soft real time systems are less restrictive than hard real time system. In this system a
critical real time task gets priority over other task. Soft real time can be easily achieved and can
be mixed with other type of system. These systems have limited applications as compared to
hard real time systems.
These systems are useful in multimedia, virtual reality and advanced scientific projects
5) Hand Held System:
Hand held systems include personal digital assistant (PDA) as well as cellular phone. These
devices have small size, less memory, slow processors and small screens.
Due to small amount of memory the operating system the applications must manage memory
efficiently. Faster processors are not included in these devices because fast processor would
require more power hence recharging and replacing the battery more frequently. Hence they
are designed so as to utilize the processor efficiently. Since the monitors of these devices are
very small, reading or browsing web pages becomes difficult.
6) Clustered System:
Clustered systems are composed of two or more individual systems coupled together.
Clustering is usually performed to provide high availability. A layer of clustered software runs
on the clustered nodes. Each node can monitor one or more of the other nodes. Each node can
monitor one or more of the other nodes. If a machine fails, the monitoring machine can take
ownership of its storage and restart the applications that were running on the failed machine.
The most common form of clustering are Asymmetric clustering and Symmetric clustering.
In Asymmetric clustering one machine is in hot standby mode which other is running the application.
The hot stand by a machine only monitors the active server. If that server fails, the hot standby host
(machine) becomes the active sever.

In symmetric clustering mode two or more host are running applications and are also
monitoring each other. These methods are more efficient as compared to asymmetric
clustering.
Unit - II
OPERATING SYSTEM STRUCTURE
Process Management:
 A process is a program in execution. A process needs certain resources, including CPU time,
memory, files, and I/O devices, to accomplish its task.
 The operating system is responsible for the following activities in connection with process
management.
I. Process creation and deletion
II. Process suspension and resumption
III. Provision of mechanisms for:
a) Process synchronization
b) Process communication on handling deadlock
A process is unit of work in a system. Such a system consists of collection of process, some of
which are operating system process and the rest are user process all these process execute
concurrently by multiplexing the CPU among theme.
Main Memory Management:
 Memory is large array of words or bytes, each with its own address. It is a repository of
quickly accessible data shared by the CPU and input output devices.
 Main memory is a volatile storage device. It loses its contents in case of system failure.
 The operating system is responsible for the following activities in connections with.

I. Keep track of which parts of memory are currently being used and by
whom.
II. Decide which processes to load when memory space become available.
III. Allocate and de-allocate memory space as needed.
File Management:
File management is one of the most visible components of the most visible components of an
operating system for convenient use of computer system the operating system provides a
uniform logical view of information storage. The operating system hides the physical properties
of its storage unit called as file.
 A file is a collection of related information defined by its creator. Commonly, files
represent programs (both source and object forms) and data.
 The operating system is responsible for the following activities in connection will file
management:
I. File creation and deletion.
II. Directory creation and deletion.
III. Support of primitives for manipulating files and directories.
IV. Mapping files onto secondary storage.
V. File backup on stable (nonvolatile) storage media.
Input output system management:
One of the purpose of the operating system is to hide the specification of the operating system
is to hide the specification of the hardware devices from the user only the device driver knows
the specification of the device to which it is assigned. In UNIX the specification of input output
devices are hidden from the bulk of operating system by input output sub system.
 The input output system consist of
I. A buffer caching system.
II. A general device driver interface.
III. Drivers for specific hardware device.
Secondary Storage Management:

 Since main memory (primary storage) is volatile and too small to accommodate all data
programs permanently, the computer system must provide secondary storage to back
up main memory.
 Most modern computer systems use disks as the principle on line storage medium, for
both programs and data.
 The operating system is responsible for the following activities in connection with disk
management.
I. Free space management.
II. Storage allocation.
III. Disk scheduling.
Command Interpreter System:
One of the most importance systems program for an operating systems is the command
interpreter which is the interface between the user and the operating system some operating
system includes the command interpreter in the kernel other operating system such as Unix
and Ms-Dos consider the command interpreter as a special program that is running when a job
is initiated.
When a new job is started in a batch system a program that reads and interprets control
statements is executed automatically. These programs are called as command line interpreter
or shell.
Many commands are giving to the operating system by control statements which deal with:
I. Process creation and management.
II. Input Output handling.
III. Secondary storage management
IV. Main memory management
V. File system access
VI. Protection
VII. Networking.
Operating System Services:
An operating system provides an environment for the execution of the programs it provides
certain services to program and to the user of those programs the services are

1. Program Execution: The system must be able to load a program into a memory and run
that program. The program must be able to end its execution either normally or
abnormally.
2. Input Output Operations: A running program may require input output. This input
output may involve a file or an input output device.
3. File System Manipulation: The programs need to create and delete file and also read
and write files.
4. Communication: One process need to exchange information to another process either
on the same computer or different computer systems, this is handled by the operating
system through shared memory or message passing.
5. Error detection: When a program is executing error may occur in the CPU, Memory,
Input Output devices or the user program for each type of error the operating system
should take proper action to ensure proper functioning of the system.
6. Resource Allocation: When multiple users are using a system it is the responsibility of
operating system to allocate or de-allocate the various resources of the system.
7. Accounting: Operating system keeps a track of use of the computer resources by each
user. This record may be use for accounting.
8. Protection: protection involves insuring that all access to system resources is controlled.
The system should also provide a security from outsiders.
System Calls:
System calls provide an interface between a process and the operating system. These calls are
generally available as assembly language instructions. A part from assembly language higher
level languages such as c, c++. Could replace the assembly language for writing the systems calls
systems calls are generated in the following way
Consider writing a simple program to read data from one file and to copy them to another file,
suppose the file names are obtained the program must open the input file and create the
output file. Each of these operation required a system calls when the program tries to open the
input file, it may find that no such file exits hence display s on error message through a system
call and the program terminates abnormally(another a system call).

Three general methods are used to pass parameters between a running program and the
operating system.
 Pass parameters in registers.
 Store the parameters in a table in memory, and the table address is passed as
parameters in a register.
 Push (store) the parameters onto the stack by the program, and pop off the stack by
operating system.
System calls can be grouped into five categories
1. Process control
2. File management
3. Device Management
4. Information maintenance
5. Communications
1. Process control: The following are the system calls with respect to process control
i. End, Abort: A running process may end normally or due to error condition the process
may be aborted.
ii. Load, Execute: A process may want to load and execute another program.
iii. Create process, Terminate process: In multiprogramming environment new processes
are created as well as terminated.
iv. Get process attribute, Set process attributes: When several process are executing we
can control its execution. This control requires the ability to determined and reset the
attribute of a process.
v. Wait for time: After creating new processes, the parent process may need to wait for
them (child process) to finish their execution.
vi. Wait event, Signal event: In case processors are sharing some data a particular process
may wait for certain amount of time or would wait for some specific event.
vii. Allocate memory and free memory: When a process is created or loaded it is allocated a
memory space. When the process completes its execution it is destroyed by the
operating system and its memory space is free.

2. File Management: The systems calls with respect to file management are
i. Create file, Delete file
ii. Open file, close file
iii. Read, write, reposition the file
iv. Get file attributes, set file attributes.
We need to crate and delete files for which the system calls are generated, after creating a file
we need open it and performed read or write operation or reposition it. Finally we need to
close the file each of these operations requires the system calls.
The various file attribute such as file name, file types, protection source and accounting
information, these attributes can be set by using two system call get file attributes, set file
attributes.
3. Device Management: The system calls related to device management are
i. Request device, Release device
ii. Read, write, reposition
iii. Get device attributes, Set device attribute
iv. Logically attached or Detached devices
A process may need some resources, if the resources are available, they can be granted
otherwise the process would fail. After getting the resource the process could use the resource
and finally released it.
We can also set device attribute and get device attributes through system call
4. Information maintenance: The various system calls related to information maintenance are
i. Get time or date, set time or date
ii. Get system data, set system data
iii. Get process, file, device attributes
iv. Set process, file, or device attributes.
We can get the current time and date and set it through system calls. Apart from it we can get
information regarding number of current user, operating system version, amount of free
memory space through system calls. We can even get the process attribute and set the process
attribute through system calls.

5. Communication: The various systems related to communications are
I. Create, delete communication connection
II. Send, receive messages
III. Transverse status information
IV. Attach or detach remote devices
There are two common models of communication
1) Message passing model
2) Shared memory model
Process A M Process A 1
Process B M shared memory 2
Process B
2 1
Kernel M kernel
Message passing shared memory
In message passing model information is exchange through and interposes communication
facility provide by the operating system. Before communication can take place a connection
must be opened. The name of the other communicator must also be known. After
identification, the identifiers are passed to general purpose open and closed calls provided by
the file system or open connection and close connection system calls depending upon the
system. The receiver process must give it permission for communication. Once the connection
is established they exchange messages by read message and write message system calls. The
close communication call communicates the communication model.

In shared memory module the processes used memory map system calls to gain access a region
of memory of other processes.
Processor may exchange information by reading and writing data in these shared areas. The
form of data and the locations are determined by these processor are not under operating
system controls.
Message passing is useful to be exchange. Shared memory allows maximum speed and
convenience of communication. However we need to deal with problems such as protection
and synchronization.
System Programs:
System programs provide a convenient environment for program development and execution.
They can be divided into following categories
1. File Management: These programs create, delete, copy, rename, print etc as well as
manipulate file and directories.
2. Status Information: Some programs provide the information of the system regarding
data, time, memory, number of users etc.
3. File Modification: several text editor may be available to create and modify the
contents of file stored on disk and tape.
4. Programming Language Support: compilers, assemblers and interpreters are provided
to the user with the operating system.
5. Program Loading and Execution: Once a program is assembled or compiled it must be
loaded into memory to be executed. The system may provide absolute loaders, re-locatable
loaders, and linkage editors and overlay loaders.
6. Communication: These programs provide the mechanism for creating virtual connection
among processes users and computer systems.

System structure:
1. Layered approach:
In layered approach they operating system is broken up into a number of layers, each build on
top of lower layers. The bottom layer is the hardware and the highest layer is the user interface.
A typical operating system layer may consist of data structures and a set of routines that can be
invoked by higher level layers.
The main advantage of layer approach is modularity. The layers are selected in such a way
that each layer uses functions and services of only lower level layers hence debugging become
must easier. The design and implementation of the systems are simplified when the system is
broken down into layer.
The major difficulty with layer approach involves the careful definition of the layers
because layer can use only those layers below it. Another difficulty of layered approach is that
they are less efficient than other.
2. Micro Kernel Approach: It was seen that as the Unix operating system expanded the
kernel become large and difficult to manage. Hence an approach called as microkernel
approach was used by modularizing the kernel. This method structures the operating
system by removing all non-essential components from the kernel and implementing

them as system and user level programs which result in smaller kernel. The main
function of the microkernel is to provide a communication facility between the client
programmed and various services which are running in user space. Communication is
provided by message passing.
The benefits of microkernel approach are that the operating system can be
easily extended. All new services are added to user space hence kernel need not be
modified. Since kernel is not modified while the microkernel is a smaller kernel. The
resulting operating system is easier to port from one hardware designed to another. The
microkernel also provides more security and reliability because most services are
running as user processes and not as kernel processes. If a service fails the rest of the
operating system remains intact.
3. System Design and Implementation:
i. Design Goals: The first problem in designing a system is to define the goals and
specification of the systems. The requirements can be divided into two basic
goals. From the user point of view the system should be easy (convenient) to use
easy to learn, reliable, safe and fast.
Form the designer point of view the system should be easy to design,
implement and maintained.
ii. Mechanisms and Policies: mechanisms determine how to do something while
policies will determine what will be done. Policies may change across places and
with respect to time. A general mechanism would be more desirable a change in
policy would that required redefinition of only certain parameter of the system.
iii. Implementation: After designing an operating system it must be implemented. It
can be implemented either by using assembly language or using by higher level
languages such as C or C++.
The advantages of using higher level languages are
 The code can be written faster and is more compact.
 It is easier to port from one hardware to other hardware for example Ms
Dos was written in assembly language hence is only available for the Intel
family of processor. While the UNIX operating system which was written
in c is available on different processors such as Intel, Motorola, ultra-spars
etc.

Unit - III
PROCESS MANAGEMENT
A process can be defined as a program in execution. Two essential elements of a
process are program code and a set of data associated with that code at any given
point in time, while the program is executing, it can be characterized by a number of
elements called process control block (PCB). The elements of PCB are
I. Identifier: every process has a unique identifier to differentiate it from other
processes.
II. State of the processor: It provides the information regarding the current state of
the process.
III. Priority: It gives the priority level of each process.
IV. Program Counter: It provides the address of the next instruction which is to be
executed.
V. Memory Pointers: These memory pointers point to the memory location
containing program code and data.
VI. Context Data: These are the data that are present in register of the processor
when the process is executing.
VII. Input Output Status Information: It includes pending input output request, input
output devices, which are assigned to the process etc.
VIII. Accounting Information: It includes the amount of processor time and clock time
used time limits and so on.

Process States:
Two State Process Model
The above diagram gives the simplest two state process models. A process is either being
executed by processor or not execute. In this model, a process may be in one of the two
states running or not running. When the operating system creates a new process and enters
that process into the system in the not running state. From time to time the currently
running process may be interrupted, the operating system will select to other process to run.
Hence a process may switched from running state to not running state while the other may
move from not running to running state.
Reasons for process creation
1) New batch job: The operating system is providing with a batch job control
stream. When the operating system is prepared to take on a new work, it will
read the next sequence of job controls command.

2) Interactive login: A user at a terminal logs on to the system.
3) Created by operating system to provide service: The operating system can
create a process on behalf of a user program to perform a function.
4) Created by existing process: A user program can create a number of processes
for the purpose of modularity.
Reasons for process termination
1. Normal completion.
2. Time limit exceeded.
3. Memory unavailable.
4. Bounds violation.
5. Protection error.
6. Arithmetic error.
7. Time over run.
8. Input output failure.
9. Invalid instruction.
10. Privileged instruction.
11. Data misuse.
12. Operating system intervention.
13. Parent termination.
14. Parent request.
Five State Process Model

The various states of the process in the given model are
1. New: A process has just been created and is not admitted to the pool(queue) of
executable process by the operating system .
2. Ready: The process is prepared to execute and is waiting for the processor.
3. Running: The process which is currently being executed.
4. Blocked: A process that cannot execute until some event occurs (such as input
output).
5. Exit: A process which is released by the operating system either because it is halted
or aborted due to some reason.
POSSIBLE TRANSITIONS:
The following are the possible transitions from one state to another
I. Null New: A new process is created to execute a program.
II. New Ready: The operating system moves a process from new state to
ready state in case memory space is available or there is a room for new
process so as to keep the number of process to constant.
III. Ready Running: A process jumps from read state to running state it
processor is running in the processor. The operating system selects a
particular process for the processor.
IV. Running Exit: The currently running process is terminated or aborted.
V. Running Ready: The most common reasons for this transitions are
a. A process exceeds its time limit.
b. Currently running process is preempted due to the arrival of high
priority process in the ready queue.
c. A process may itself release control of the processor.
VI. Running Blocked: A process is put in the blocked state if it request for
something for which it must wait. For example a process may request a
service from the operating system, operating system is not prepared to
service the request immediately are the process may wait for some input
output operation.
VII. Blocked Ready: A process in the blocked state is mode to the ready
state when the event for which it has been waiting occurs.

VIII. Ready Exit: A parent may terminate a child process at any time. Also if a
parent terminates all child process of the parent process terminate.
Process Description
In the above diagram we see that there are a number of processes each process needs
certain resources for its execution. Process p1 is running and has control of two input output
devices and occupying a part of main memory process p2 is also in the main memory but its
blocked and waiting for input output device. The process pn is swapped out and is
suspended.
The operating system controls the processes and manages the resources for the
processes using some control structures which are divided into four categories.

Memory tables are used to keep a track of both main memory and virtual memory. The
memory tables must include the following information.
i. The allocation of main memory to processes.
ii. The allocation of secondary memory to process.
iii. Any protection attributes of blocks of main and virtual memory.
iv. Any information needed to manage the virtual memory.
Input Output Tables: Input output tables are by the operating system to manage the input
output devices of the computer system. At any given time an input output device may be
available or not available. Hence the operating system must know the status of the input
output operation and also the location in the main memory where the transfer is carried
out.
File Tables: The operating system also maintains file table which provide information about
the existence of file, their location on the secondary memory, their current status and other
attributes.

Process Tables: The operating system must maintain process tables to manage processes
control them. In doing so the operating system must know where the process is located and
the attribute of the process. The various process attributes which is also called as process
control blocked is grouped into three categories.
1) Process identification.
2) Processor state information.
3) Process control information.
1. Process identification:
Identifiers: Numeric identifiers may be stored in the process control blocked which
include
i) Identifier of this process.
ii) Identifier of the parent process
iii) User identifier
2. Processor state information:
i) User visible registers: A user visible register is available to the user, there may be
about 8 to 32 of these registers.
ii) Control and status register: These registers are used to control the operation of
the processor which include program counter, condition codes and status
information.
iii) Stack pointers: Each process has one or more LIFO system stack associated with
it.
3. Process control information:
i) Scheduling and state information: These information is needed by the operating
system to performed scheduling. The information’s are process state, priority,
scheduling related information and event.
ii) Data structuring: A process may be linked to other process in a queue, ring or
some other structure.
iii) Inter-process communication: Various flag, signals and messages may be
associated with communication between two independent processes.
iv) Process privileges: Processes are granted privileges in terms of memory that may
be accessed and the types of instructions that may be executed.

v) Memory management: It includes pointers to segments and page tables.
vi) Resource ownership and utilization: Resources controlled by the process may be
indicated.
Operations and processes:
The processes in the system can execute concurrently and they must created and deleted
dynamically. Hence the operating system must provide a mechanism for process creation
and termination.
Process creation: A process may create several new processes through create process
system call. The creating process is called the parent process while the new processes called
the children of that processes. Each of these new processes may in turn create new
processes forming a tree of processes. When a process creates a new process, two
possibilities exist in terms of execution.
i) The parent continues to execute concurrently with its children.
ii) The parent waits until or all of its children have terminated.
There are two possibilities
(1) The child process is a duplicate of the parent process.
(2) The child process has a program loaded into it.
Process termination: A process terminates when it finishes executing its final statement and
ask the operating system to delete it by using exit system call. At that point, the process may
return the data to its parent process through wait system call. All the resources of the
process, including memory, files, input output buffers are de-allocated by the operating
system.
A parent may terminate its children for the following reasons.
1) The child has exceeded its usage of some of it resources which is allocated to it.
2) The task assigned to the child is no longer required.

3) The parent is exiting, in such a case the operating system does not allow a child to
continue if its parent terminates.
Co-operating Processes:
A process is co-operating process if it can affect or be affected by other processes executing
in the system. Co-operating processes has several advantages.
 Information sharing: Several users may be interested in same piece of information
we must provide an environment to allow concurrent access to these type of
resources.
 Computation speed up: We can break a task into smaller sub task, each of which will
be executing in parallel with the others.
 Modularity: We can construct the system by dividing the system functions into
separate process.
 Convenience: A user may have many task on which it can work at a time (editing,
printing and compiling).
Inter process communication (IPC):
Inter process communication facility is the means by which processes communicate by
among themselves. Inter process communication provides a mechanism to allow processes
to synchronize their action without sharing the same address space. Inter process
communication is particularly useful in distributed environment.
Inter process communication is best provided by a message passing system. The
function of a message system is to allow processes to communicate with one and another
without any shared memory. Communication among the user processes is achieved to
through passing of messages. An inter process communication facility provides at least two
operations send and receive.
If processes P and Q want to communicate they must send messages and receive
message from each other through a communication link.
There are several methods for logically implementing a link and send / receive operation

a) Direct communication OR Indirect communication:
Direct communication: With direct communication each process that wants to
communicate should name the receiver or sender of communication as follows
Send (P, message) - send a message to process P
Receive (Q, message) - receive a message from process Q
With direct communication exactly one link exist between each pair of processes.
Indirect communication: In indirect communication the messages are send and
received from mail boxes or ports. In which message can be placed or removed. Two
process can communicated only if they share a mail box. Communication is done in
the following way
Send (A, message) - send a message to mailbox A.
Receive (A, message) - receive a message to mailbox A.
In indirect communication a link may be associated with more than two processes.
More than 1 link may exist between each pair of communicating processes. The
mailbox may be owned either by a process or by the operating system.
b) Synchronization: Communication between process through message passing may be
either blocking or non-blocking (synchronous or asynchronous).
i. Blocking send: The sending process is blocked until the message is received
by the receiving process or by the mailbox.
ii. Non-Blocking send: The sending process sends the message and resumes
operation.
iii. Blocking receive: The receive block until a messages is available.
iv. Non-Blocking receive: The receiver gets receives either a valid message or a
null.
c) Buffering: buffering can be implemented in three ways
i. Zero capacity: The queue has maximum length zero. Hence the link cannot
have any messages waiting in it.

ii. Bounded capacity: The queue has finite length. If the link is fall the sender
must block until space is available in the queue.
iii. Unbounded capacity: The queue has almost infinite length hence the sender
never blocks.
Mutual exclusion using messages:
Count int n /* number of processes */
Void p (int)
{
Message msg;
While (true)
{
Receiver (box, msg); /* critical section */
Send (box, msg); /* remainder*/
}
}
Void main ()
{
Create mailbox (box);
Send (box, null);
Par begin (p (1), p (2), ... , p (n));
}
The above algorithm show how we can use message passing to get mutual exclusion. A set of
concurrent processes share a mailbox which can be use by all processes to send and receive.
The mail box is initialized to contain a single message which null content. A process wishing
to enter its critical section first attempts to receive a message. If the mailbox is empty then
the process is blocked.
Once a process gets the message, it performs it critical section and places the message back
into the mailbox. If more than one process perform the receive operation concurrently than
two cases arise.
 If there is a message, it is delivered to only one process and other are blocked.
 If the message queue is empty, all the processes are blocked, when the message is
available only one blocked process is activated and given the message.

THREADS:
A thread is a light weight process; it is a basic unit of CPU utilization and consists of thread
id, a program counter, a register set and a stack. All the threads belong to a process may
share code section, data section and other operating system resources. If a process has
multiple threads control it can do more tasks at a time.
Advantages of multithreading are
I. Responsiveness: Multithreading may allow a program to continue even if a part of it
is blocked for performing a lengthy operation.
II. Resource sharing: Threads share the memory and the resources of the process to
which they belong.
III. Economy: Creating, maintaining and switching a process is difficult as compared to
creating, maintaining and switching a thread.
E.g.: In Solaris creating a process is about 30 times slower than creating a thread.
IV. Utilization of multiprocessor: Architecture – The benefits of multithreading can be
greatly increased in a multiprocessor architecture, where each thread may run in
parallel on different processors.

Types of Threads:
1. User threads.
2. Kernel threads.
User Threads: User threads are supported above the kernel and are implemented at the
user level. All threads creation and scheduling are done in user space without kernel
intervention. Hence user level threads are fast to create and easy to manage. The drawback
with user level thread is that if kernel is single threaded, any user level thread performing a
blocking system call will cost the entire process to block.
Kernel Threads: Kernel threads are supported directly by the operating system. The kernel
performs threads creation, scheduling and management in the kernel space. Due to this
reasons they are generally slower to create and not easy to manage as compared to user
threads. Since the kernel is managing the threads, if a threads perform a blocking system
call, the kernel can schedule another thread for execution.
Multithreading Models:
The three common types of multithreading models.
1. Many to One.
2. One to One.
3. Many to Many.

Many to One:
The many to one model maps many user level threads to one kernel thread. Thread
management is done in user space, hence it is efficient but the disadvantage is that the
entire process will blocked if a thread makes a blocking system call. Since only one thread
can access the kernel at a time multiple threads are unable to run in parallel on
multiprocessors.
One to One:
One to One model maps each user thread to a kernel thread. It provides more concurrency
as compared to many to one model. In case a thread makes a blocking system calls only that
particular thread will be blocked while other still execute. In multiprocessor environment
multiple threads can run in parallel. The only drawback of this model is that creating a user
thread required creating the corresponding kernel threads.

Many to Many:
The many to many model multiplex many user level threads to a smaller or equal number of
kernel threads. The number of kernel threads may depend upon the particular application or
a particular machine. This model is better on the other two models. Developers can create as
many user threads as necessary and the corresponding kernel threads can run in parallel on
a multiprocessor.
Threading Issues:
I. The fork and exec system calls: If one thread in a program calls a fork system calls, the
new process may duplicate all threads or a new process may be a single threaded. If a
thread invokes the exec system call, the program specified in the entire process.
II. Cancellation: Thread cancellation is the task of terminating a thread before it has
completed. A thread which is to be cancelled is called as target thread. Thread
cancellation occurs in two different ways.
i) Asynchronous cancellation: One thread immediately terminates the target
thread.

ii) Differed cancellation: The target thread can periodically check if it should
terminate.
III. Signal handling: A signal is used to notify a process that a particular event has occurred.
Signal can be generated through following reasons.
 Generated by the occurrence of a particular event, whenever it is generated it
must be delivered to a process; once the signal is delivered it must be
handled. Signal can handled by two handlers.
I. A default signal handler.
II. User define signal handler.
In order to deliver the signal there are few options.
a) Deliver the signal to the thread to which the signal applies.
b) Deliver the signal to every thread in the process.
c) Deliver the signal to certain threads in the process.
d) Assign a specific thread to receive all signals for the process.
IV. Thread pools: The general idea behind a thread pool is to create a number of threads at
process start up and place them into a pool, where they sit and wait for work. The
benefits of thread pools are
i) We get fast service
ii) A thread pool limits the number of threads.
V. Thread specific data: Threads belonging to a process share the data of the process.
However each thread may need its own copy of certain data, such data is called thread
specific data.
CPU SCHEDULING
 Basic concepts.
 Scheduling criteria.
 Scheduling algorithm.
 Multiprogramming algorithm.
CPU Scheduler:

Whenever the CPU becomes idle, the operating system must select one of the processes in
the ready queue which is to be executed. The selection of the process is carried out by CPU
Scheduler (short term scheduler).
CPU scheduling decisions may take place under following four situations.
1. When a process switches from running state to waiting state.
2. When a process switches from running state to ready state.
3. When a process switches from waiting state to ready state.
4. When a process terminates.
Scheduling in case of 1 and 4 is called non-preemptive while under case 2 and 3
are called preemptive.
Dispatcher: Dispatcher module gives control of the CPU to process by the short term
scheduler, this involves:
i. Switching context.
ii. Switching to user mode.
iii. Jumping to the proper location in the user program to restart that program.
The dispatcher should be as fast as possible because it is used during every process
switch. The time it tackles for the dispatcher to stop one process and start another
process is known as dispatcher latency.
Scheduling criteria: The criteria which are used to compare the various scheduling
algorithms are
1. CPU utilization: It means CPU must be kept as busy as possible CPU utilization may
range from 0 – 100%.
2. Throughput: It gives the number of processes which are completed in a given unit
time.
3. Turnaround time: It is the sum of time a process spends waiting to get into the
memory. Waiting in the ready queue, executing and doing input output.
4. Waiting time: It is the amount of time a process has been waiting in the ready queue.
5. Response time: It is the amount of time it takes from when a request was submitted
until the first response is produced, not output (for time sharing environment).

Scheduling Algorithm
1) First Come First Serve Scheduling (FCFS): It is a purely non-preemptive algorithm. In this
scheme the process which requests the CPU first is allocated the CPU first. The
implementation of First come first serve policy is easily managed with a FIFO queue. The
code for FCFS scheduling is simple to write and understand.
The disadvantage of FCFS policy is that the average waiting time is quite high as
compared to other algorithms.
Eg: consider the following example with processes arriving in the order p1,p2,p3 and
the burst time in milliseconds.
The Gantt chart for the scheduling is:
P1 P2 P3
0 24 27 30
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: (0 + 24 + 27)/3 = 17
Convoy effect—In FCFS scheme if we have a one big process which is CPU bounded
and many small process which are input output bounded, it is seen that all other
small process wait for one big process to get the CPU. This results in lower CPU
utilization. This effect is called as convey effect FCFS algorithm is not suitable for time
sharing system because it is non-preemptive.
2) Shortest Job First (SJF) Scheduling: In this scheme, when the CPU is available it is
assigned to the process which has the smallest next CPU burst. If two processes has the
same next CPU burst, FCFS is used.
SJF algorithm is optional because it gives minimum average waiting time for a
given set of processes. The real difficulty with SJF algorithm is knowing the length of
the next CPU request.
SJF algorithm may be either preemptive or non-preemptive.
A preemptive SJF algorithm will preemptive the currently process if a new process
arrives in the ready queue having CPU burst time left for the current executing,
process. A non-preemptive SJF algorithm will allow the currently running process to
finish its CPU burst.

Eg: Non preemptive SJF
P1 P3 P2
P4
0 3 7 16
Eg: Preemptive SJF
Process Arrival Time
Burst Time
P0.0 7
1 P2.0 4
2 8 12
P 4.0 1
Process Arrival Time
Burst Time
P1 0.0 7
P2 2.0 4
P 4.0 1
P1 P2 P3
0 2 4 11
P2 P4
P1
5 7
16
3) Priority Scheduling: In this scheduling a priority is associated with each process and the
CPU is allocated to the process with higher priority. Priorities are generally some fixed
range of numbers. Priorities can be defined either internally or externally. Internally
defined priorities are use some measurable quantity to compute the priority of a process
(for example: time limits, memory, file etc).external priorities are said by criteria such as
importance of the process, the type of the process, departments sponsoring the process
etc.
Priority scheduling can be either preemptive or non-preemptive. When a process
arrives at the ready queue its priority is compared with the running process. In a
preemptive priority scheduling algorithm the running process will be preempted if
the newly arrived process is having higher priority. A non preemptive priority
scheduling algorithm will not preempt the running process.
The major drawback of priority scheduling algorithm is indefinite blocking
(starvation). A solution to the problem of starvation is aging. Aging is a technique of
increasing the priority of processes that wait in the system for long time.

Q. For the following set of processes find the average waiting time considering small
number to have higher priority.
Process Burst time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
P2 p5 p1 p3 p4
0 1 6 16 18 19
Waiting time of p1=6
Average waiting time = (6+0+16+18+1)/5= 8.2
4) Round Robin (RR): Round robin scheduling algorithm is designed specially for time
sharing system. It is a purely preemptive algorithm. In this scheme every process is
provided a time slice (time point of view) in case the process is unable to complete in the
given time slice, it is preempted and another process is executed. The ready queue is
treated as a circular queue. The CPU scheduler goes around the ready queue allocation
the CPU to each process for a time interval of one time slice. If there are n processes in
the ready queue and the time quantum is q, then each process gets 1/n of the CPU time
in chunks of at most q time units at once. No process waits more than (n-1)q time units.
Example of Round robin with time quantum= 20
Process Burst
Time
P1 53
P2 17
P 68
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 37 57 77 97 117 121134 154162

Multilevel Queue Scheduling:
A multilevel queue scheduling algorithm partition the ready queue into several separates
queue. The processes are permanently assigned to a particular queue. Its queue has its own
scheduling algorithm.
For example: The interactive processes queue may use round robin algorithm while batch
processes queue would use FCFS algorithm. A part from it there is a scheduling among a
queue which is implemented through preemptive priority scheduling algorithm. Each queue
has higher priority over low priority queue.
For example: Low process in the batch queue would run unless system process queue,
interactive process queue and interactive editing process queue are empty.
Multilevel Feedback Queue Scheduling:

Multilevel feedback queue scheduling allows a process to move between the queues.
Separate queues are formed on the basis of CPU burst time.
If a process uses too much CPU time, it is moved to a low priority queue. If a process waits
too long in a low priority queue it is moved to a higher priority queue.
For example: Consider the above diagram, a process entering the ready queue, it is put in
queue 0. A process in queue 0 is given in a time slice (quantum) of 8ms. If it does not finishes
within the given time, it is moved at the end of queue 1 and so on.
A multilevel feedback queue scheduling is defined by the following parameters.
 Number of queues.
 Scheduling algorithms for each queue.
 Method used to determine when to upgrade a process.
 Method used to determine when to demote a process.
 Method used to determine which queue a process will enter when that process
needs service.
PROCESS SYNCHRONIZATION
Definitions:
1. Critical section: A section of code within a process which cannot be executed while
one process is executing it is called as critical section.
2. Deadlock: A situation in which two or more processes are unable to proceed and
wait for each other is called as deadlock.
3. Live lock: A situation in which two or more processes continuously change their state
in response to changes in the other processes without doing any useful work.
4. Mutual exclusion: The requirement that only one process execute the critical
section.
5. Race condition: A situation in which processes read and write a shared data item and
the final result depend on the relative timing of their execution.
6. Starvation: A situation in which a run able process is not executed for an indefinite
period.

Principles of concurrency: concurrency has to deal with the following issues.
Communication among processes, sharing of resources, synchronization of the multiple
processes and allocation of the processor time.
Concurrency arises in three different contexts.
i. Multiple applications.
ii. Structured application.
iii. Operating system structure.
In a single processor multiprogramming system, processes are int leaved in time but
still they appear to be simultaneously executing. In a multiprocessor system the processes
are overlapped that is two or more processes simultaneously execute. In both the situation
the following difficulties arise.
a) Sharing of global resources.
b) Allocation of resources.
c) Locating a programming error.
Consider a simple example in a uni-processor environment.
void echo()
{
chin = getchar();
chout = chin;
putchar(chout);
}
Operating System Concerns: The following are the issues with respect to existence
of the concurrency.
1. The operating system must be able to keep track of various processes.

2. The operating system must allocate and de-allocate various resources for each
process. The resources are processor time, memory, input output device, files.
3. The operating system must protect the data and critical resources of each
process.
4. The functioning of the process and its result must be independents of its speed at
which its execution is carried out with respect to other processes.
Process Interaction: The following are the various process interaction.
 Processes unaware of each other.
 Processes directly aware of each other.
Process interaction:
Degree of
awareness
Relationship Influence that one
process has on the
other
Potential control
problem
Processes
unaware of each
other
competition  Results of
process
independent
of the action
of others
 Timing of
process may
be affected.
 Mutual
exclusion.
 Deadlock
(renewable
resource)
 Starvation.
Process indirectly
aware of each
other (eg: shared
object)
Cooperation by
sharing
 Results of one
process may
depend on
information’s
obtained
from others.
 Timing of
process may
be affected.
 Mutual
exclusion.
 Deadlock
(renewable
resource).
 Starvation.
 Data
coherence.
Process directly
aware of each
other (have
communication
primitives
available to
them)
Cooperation by
communication.
 Results of one
process may
depend on
information
obtained
from others.
 Timing of
process may
be affected.
 Deadlock
(consumable
resource).
 Starvation.

Requirements For Mutual Exclusion: Any solution for enforcing mutual
exclusion must satisfied the following requirements.
1. Mutual exclusion must be enforced that is when one process is in its critical
section no other process should be in its critical section.
2. A process that halts in its non critical section must not interfere with other
processes.
3. A process should not wait indefinitely for entry into the critical section.
4. When no process is in its critical section, any process which request entry to
the critical section must be granted without delay.
5. A process must remain in its critical section for a finite time.
Mutual Exclusion: Algorithm Approach
Algorithm 1
/* process 0 */ /* process 1*/
While (turn!=0) While (turn!=1)
/* do nothing */; /*do nothing */;
/* critical section */; /* critical section */;
Turn 1; Turn 0;
In this case the two process share a variable turn. A process which wants to enter the critical
section checks the turn variable. If the value of the turn is equal to the number of process,
then the process may go into the critical section. The drawback of these algorithm is that are
get busy waiting. If one process fails the other is permanently blocked.
Algorithm 2
/* process 0 */
While (flag [1])
/* do nothing */
Flag [0]= true;
/* critical section */
Flag [0]= false;
/* process 1 */
While (flag [0])
/* do nothing */
Flag [1]= true;

Flag [1]= false;
In this algorithm we used a Boolean variable flag. When a process wants to enter its critical
section, it checks other process flag, if it is false, it indicates that other process is not in its
critical section. The checking process immediately sets its own flag to true and goes into the
critical section. After leaving its critical section it resets its flag to false.
This algorithm does not ensure mutual exclusion. It can happen that both the process
check each other’s flag and find it false, hence set their own flag to true and both the critical
section simultaneously.
Algorithm 3
/* process 0 */
Flag [0]= true
While (flag[1])
/* do nothing */
/* critical section*/
Flag[0]= false;
/* process 1 */
Flag [1]= true
While (flag[0])
/* do nothing */
/* critical section*/
Flag[1]= false;
In this algorithm a process which wants to enter the critical section sets its flag to true and
check other process flag if it is not set it enter the critical section if it is set the process wait.
The drawback of this algorithm is that it can happen that both process set their flag to
true and check each other’s flag causing deadlock. Also if a process fails inside the critical
section, the other process is blocked.
Algorithm 4
/* process 0 */
Flag [0]= true
While (flag[1])
{
Flag[0]= false;

/* delay */
Flag[0]=true;
}
Flag [0]= false;
/* process 1 */
Flag [1]= true
While (flag[0])
{
Flag[1]= false;
/* delay */
Flag[1]=true;
}
Flag [1]= false;
In this algorithm it can be shown that a live lock situation occurs in which the two processes
continuously set and reset their flag without doing any useful works. If one of the process
slows down than live lock situation would be broke down and one of the process enter the
critical section.
Peterson’s Algorithm:
Boolean flag[2];
Void p0()
{
While (true)
{
Flag[0]=true;
Turn=1;
While(flag[1] && turn==1)
/*do nothing */
Flag[0]=false;
/* remainder section */
}
}
Void p1()
{
While (true)
{
Flag[1]=true;
Turn=0;

While(flag[0] && turn==0)
/*do nothing */
Flag[1]=false;
/* remainder section */
}
}
Void main()
{
Flag[0]=false;
Flag[1]=false;
Par begin(p0,p1)
}
In Peterson algorithm gives the simple solution to the problem of mutual exclusion. A global
variable turn decides which process must go into the critical section. We see that mutual
exclusion is easily preserved.
Suppose p0 wants to enter its critical section, it sets its flag to true and the turn value
to 1 because of which p1 can not enter its critical section. If p1 is already in its critical section
than p0 is blocked from entering its critical section. It can be shown that Peterson’s
algorithm provide a better solution which is free from deadlock and live lock. This algorithm
can be easily generalized for n number of processes.
Semaphores: Two or more processes can co-operate by means of simple signals such that a
process can be forced to stop at a specific place until it has receive a specific signal. Any co
ordination requirement can be satisfied by proper signals. For signaling special variable
called semaphores are used to transmit a signal through semaphores S a process execute the
primitive semSignal (S). To receive a signal through semaphores S a process executes the
primitive semWait(S).
To achieve the desired effect we can view the semaphore as a variable that has an integer
value on which three operations are defined.
1) A semaphore may be initialize to a non negative value.
2) The semWait Operation: The semWait decrements the semaphore value. If the value
become negative then the process executing the semWait is blocked, otherwise the
process continue.

3) The semSignal Operation: The semSignal operation increments the semaphore value.
If the value is less than or equal to 0, a process blocked by semWait operation is
unblocked.
Semaphore primitives are defined as follows
Struct semaphore
{
Int count;
Queue type queue;
}
Void semaWait(semaphore.S)
{
S.count--;
If (S.count<0)
{
Place this process in S.queue;
Block this process
}
}
Void semaSignal(semaphore.S)
{
S.count++;
If (S.count<=0)
{
Remove a process p from S.queue;
Place process p on ready list;
}
}
Binary Semaphores: Binary semaphore may only take the value 0 and 1 and can be defined
by the following three operation.
1. Initialization: A binary semaphore may be initialized to 0 or 1.
2. SemWaitB operation: The semWait B operation checks the semaphore value. If the
value is 0, than the process executing the semWait B is blocked. If it is 1 it is change
to 0 and the process continues.
3. SemSignal B operation: The semSignal B operation checks to see if any processes are
blocked on this semaphore. If so, than a process blocked by a semwait B operation is
unblocked.
If no processes are blocked, than the value of semaphore is set to 1.

Binary semaphore primitives are defined as follows.
Struct binary—semaphore
{
Enum {zero, one} value
Queue type queue;
}
Void semWait B (binary-semaphore.S)
{
If(S.value==1)
S.value=0;
{
Place this process in S.queue;
Block this process
}
}
Void semSignal(binary-semaphore.S)
{
If (S.queue is empty())
S.value=1;
{
Remove a process p from s. queue;
Place process p on ready list;
}
}

Unit - IV
MEMORY MANAGEMENT
Memory consists of a large array of words or bytes each having its own address.The CPU fetches
instructions from the memory according to the value of the program counter.To improvethe utilization
of CPU the computer must keep several processes in the memory.To utilize the memory many memory
management schemes are proposed.Selection of a memory management schemes depends on many
factors such as hardware of the system.
ADDRESS BINDING
A user program goes through several steps such as compiling ,loading ,linking.Addresses may be
represented in different ways during these steps.Addresses in the source program are generally
symbolic.A compiler will bind these addresses to relocatable addresses.The loader will inturn bind these
addresses to absolute addresses.Each binding is a mapping from one address space to another.
The binding of instructions and data can be done in the following ways.
1..Compile time-If it is known at compile time where the process will reside in memory ,then absolute
code can be generated.If address changes at compile time the program is recompiled.
2..Load time-If it is not known at compile time where the process will reside in the memory,than the
compiler must generate relocatable code.
3..Execution time-Binding delayed until run time if the process can be moved during its execution from
one memory segment to another.Need hardware support for address maps.(eg.base and limit registers).
LOGICAL V/S PHYSICAL ADDRESS SPACE
Logical address-generated by the CPU,also referred to as virtual address.
Physical address-address seen by the memory unit.
Logical and physical addresses are the same in compile time and load time address binding
schemes,logical(virtual)and physical addresses differ in execution time address binding scheme.

DYNAMIC LOADING
 Routine is no loaded until it is called .
 Better memory space utilization,unused routine is never loaded.
 Useful when large amounts of code are needed to handle in frequently occurring cases.
 No special support from the OS is required implemented through program design.
OVERLAYS :
OVERLAYS FOR A TWO PASS ASSEMBLER
 Keep in memory only those instructions and data that are needed at any given time.
 Needed when process is larger than amount of memory allocated to it.
 Implemented by user, no special support needed from OS, programming design of overlay
structure is complex.
SWAPPING

A process needs to be in memory to be executed. A process can be swapped out temporarily
from the main memory to a backing store and then brought back into main memory for
continued execution. When a process completes its time slice it will be swapped with another
process (in case of RR scheduling algorithm)
Another type of swapping policy which is used for priority based algorithm is that is that a higher
priority process arrives and wants service,the memory manager can swap out the lower priority
process and swap in the higher priority process.This type of swapping is called as roll out –roll in.
Swapping requires a backing store.The backing store is commonly a fast disk and is large enough
to accommodate copies of all memory images for all users,and it must provide direct access to
these memory images.
Major part of swap time is transfer time,total transfer time is directly proportional to the
amount of memory swapped.
SCHEMATIC VIEW OF SWAPPING

CONTIGUOUS ALLOCATION:
The memory is usually divided into two partitions one for the OS and other for the user processes.We
want several user process to reside in memory at the same time.Hence we need to consider how to
allocate the available memory to the processes.In contiguous memory allocation each process is
contained in a single contiguous part of the memory.
When the CPU scheduler selects a process for execution,the dispatcher loads the relocation and limit
register and every address generated by the CPU is checked with these registers.So that the OS and user
programs are not modified by the running process.
Limit
register
Relocation
register
+ Memory
Logical add yes physical add
No
<
Trap;addressing error
HARDWARE SUPPORT FOR RELOCATION AND LIMIT REGISTERS
CPU
MEMORY ALLOCATION
One of the simplest method for memory allocation is to divide memory into several fixed sized
partitions.Each partition may contain exactly one process.When a partition is free a process is selected
from the input queue and is loaded into the free partition.When the process terminates the partition
become available for another process.
The OS keeps a table indicating which parts of memory are available and which are occupied.Initially all
memory is available for user processes and is considered as one large block of user memory called
hole.When a process arrives and needs memory we search for a large enough for this process.The set of
holes is searched to determine which hole is best to allocate.The following strategies are considered to
select a free hole.

1..First-fit:-Allocate the first hole that is big enough.We can search the set of holes either at the
beginning or at the end of the previous first fit search.We can stop searching as soon as we find a free
hole that is large enough.
2..Best fit:-Allocate the smallest hole that is big enough.For this purpose we must search entire list.It
produces the smallest left over hole.
3..Worst fit:-Allocate the largest hole.For this purpose we must search the entire list.It produces the
largest left over hole.First-fit and best fit are better than worst fit in terms of speed and storage
utilization.
All the above algorithms suffer from external fragmentation.Memory fragmentation can be internal or
external.
 External fragmentation –total memory space exists to satisfy a request,but it is not contiguous.
 Internal fragmentation-allocated memory may be slightly larger than requested memory,the
size difference is memory internal to a partition,but not being used.
PAGING:
Paging is a memory management scheme which permits the physical address space of a process to
be non contiguous.Paging avoids the problem of fitting the different process of different sizes into
the memory.In this scheme physical memory is broken into fixed sized blocks called frames while
the logical memory is broken into blocks of same size calles pages.When a process is to be
executed,its pages are loaded into any available memory frames from the backing store.
The following diagram gives the required paging hardware.

Every address generated by the CPU is divided into two parts –a page number p and page offset d.The
page number is used as index into a page table. The page tables contains the base address of each page
in physical memory .This base address is combined with the page offset to define the physical memory
address which is sent to the memory unit.
The paging model of a memory is as shown below

The size of a page is a power of 2 and lies between 512 bytes and 16mb per page depending on the
computer architecture. The selection of power of 2 as the page size makes the translation of logical
address into a page number and page offset easy.
When we use the paging scheme we have no external fragmentation. However there may be some
internal fragmentations. When a process arrives in a system to be executed its size in terms of pages
is examined. Each page of the process needs one frame. The first page of the process is loaded into
one of the allocated frame and the frame number is put in the page table for this process. This is
done for all the pages. The OS maintains a list of free frames in a data structure called the frame
table as given in the diagram below.
SEGMENTATION :
A program may consists of a main program, subroutines, procedures, functions, etc. Each of which
can be considered as a segment of variable length. Elements within a segment are identified by their
offset from the beginning of the segment. Segmentation is a memory management scheme which
supports this view of the memory. A logical address space is a collection of segments. Each segment
has a name and a length. The addresses specify the segment name and the offset within the
segment. A logical address consists of segment number and offset.

SEGMENTATION HARDWARE:
A logical address consists of two parts a segment number s and an offset d. The segment number is used
as an index into the segment table. The offset d of the logical address must be between O and the
segment limit. If it is not we trap to the OS. If this offset is valid it is added to the segment base to
produce the address in the physical memory.
A particular advantage of segmentation is the association of protection with the segments.
Another advantage of segmentation involves the sharing of code or data.
Segmentation may cause external fragmentation when all blocks of free memory are too small to
accommodate a segment.
SEGMENTATION WITH PAGING:
By combining segmentation and paging we can get best of both. In this model the logical address
space of a process is divided into two partitions. The first partition consist of upto 8kb segments that
are private to that process. The second partition consist of upto 8 kb segments which are shared
among all the processes. Information about the first partition is kept in the local description
table(LDT) while the information about the second partition is kept in the global descriptor table
(GDT). Each entry in the LDT and GTD cosist of 8 bytes with detailed information about a particular

segment including the base location and length of that segment. The logical address consist of
selector and offset, the selector is 16 bit number given as
S G P
13 1 2
Where s = segment number.
G= indicates whether segment is LDT of GDT
P = deals with protection / for protection
The offset is a 32 bit number specifying the location of the byte within the segment. It is given as
Page number page offset
P1 P2 D
10 10 12
VIRTUAL MEMORY
Virtual memory is a technique that allows the execution of processes which may not be completely in
memory. One major advantage of this scheme is that we can have programs which are larger than the
physical memory. It is seen that many programs have code to handle unusual error conditions and these
errors never occur hence these codes are never executed. Also programs may contains arrays, list and
tables which may be allocated more memory than required. Apart from this certain options and features
of a program are rarely used. Virtual memory makes the task of programming much easier because the
programmer no longer needs to worry about the amount of physical memory. Virtual memory is
implemented by demand paging as well as demand segmentation
DEMAND PAGING
A Demand paging system is similar to paging system with swapping in which we have a swapper which
swaps only those pages which are needed and does not swaps the entire process. When a process is to
be swapped in the swapper(pager) guesses which pages will be used before the process is swapped out
again, it brings only those pages. Hence the pager decreased the swap time and the amount of memory
needed.
To implement demand paging we need some hardware to differentiate between those pages which are
in the memory and those pages that are on the disk.

When the valid bit is set it means that the page is legal and is in the memory. If the bit is set to invalid it
means that either the page is not valid or is valid but is currently on the disk.

STEPS FOR HANDLING PAGE FAULT:
The procedure for handling page fault is as follows.
1. We check an internal table to determine whether the reference was valid or invalid.
2. If it was valid but is not in the physical memory we now bring it.
3. We find a free frame.
4. Bring the desired page from disk into the frame.
5. Modify the table.
6. Restart the instruction.
PAGE REPLACEMENT
If no frame is free, we find a frame that is not currently being used and free it, we can free a frame by
writing the contents to swap space and change the page table entries. It is done by the following steps.
1. Find the location of the desired page on disk.
2. Find a free frame:

 If there is a free frame, use it,
 If there is no free frame, use a page replacement algorithm to select a victim frame.
3. Read the desired page into the (newly) free frame. Update the page and frame table.
4. Restart the process.
If no frames are free, two pages transfers are required (one out, one in). We can reduce this overhead
by using a modify bit (dirty bit). The dirty bit is set for a page if the page is modified. Hence in case of
page replacement we need to replace that page. If the dirty bit is reset we don’t need to replace that
page, it can be overwritten by other page.
PAGE REPLACEMENT ALGORITHMS.
First –In – first –Out (FIFO) Algorithm.
The simplest page replacement algorithm is the FIFO algorithm. A FIFO replacement algorithm
associates each page the time when that page was brought into the memory. When a page must be
replaced the oldest page is choosen. FIFO algorithm is easy to understand and program. However its
performance is not always good.

Prob: Consider the following reference string
7,0,1,2,0,3,0,4,2,3,0,3,2,1,2,0,1,7,0,1. Find the number of page faults using FIFO page replacement
algorithm with three frames.
Belady,s Anamoly :
In some page replacement algorithms, the number of page faults may increase as the number of
allocated frames increased. Consider the following curve showing Belady’s Anamoly.

From the above graph we see that if the number of frames are three we get a page faults and when the
number of frames are 4 we get 10 page faults.
OPTIMAL PAGE REPLACEMENT ALGORITHM:
In the algorithm we replace the page which will not be used for the longest period of times. This
algorithm has the lowest page fault rate as compared to other algorithms and does not suffers from
Belady’s Anamoly. This algorithm is difficult to implement because it requires the future knowledge of
the reference string. This algorithm is mainly used for comparison studies.
Eg: For the following reference string find the number of page faults using optimal page replacement
algorithm with 3 frames.
RS : 7,0,1,2,0,3,0,4,2,3,0,3,2,1,2,0,1,7,0,1.
9 pagefaults
LEAST RECENTLY USED ALGORITHM(LRU):
In this algorithm when we need to replace a page we replace it with the page that has not been used for
the longest period of time LRU Replacement associates with each page the time of that pages last use.
When a page must be replaced we choose a page which has not been used for the longest period of
time. Hence we look into the part.
The major problem with this algorithm is to implement it for which additional hardware is require. The
two types of implementations are
1. Counter implementation
 Every page entry has a counter, every time page is referenced through this entry, copy the
clock into the counter.

 When a page needs to be changed look at the counters to determine which are to change.
2. Stack implementation.
Another approach to implement is to keep a stack of page number. Whenever a page is referenced,
it is removed from the stack and put on the top. Hence the top of the stack is the most recently used
page and the bottom of the stack is the least recently used.
Eg : For the following reference string find the number of page faults using LRU algorithm with 3
frames.
RS-7,0,1,2,0,3,0,4,2,3,0,3,2,1,2,0,1,7,0,1
THRASHING:
For execution, a process needs free frames, if the process does not have the required number of
free frames, we get a page fault. Hence we must replace some page, since all the pages are in
active use, we replace a page which is needed. Hence we get a page fault again, and again, and
again. The process continued to fault, this high paging activity is called thrashing.
CAUSE OF THRASHING:
The OS monitors the CPU utilization, if it is low we increase the degree of multiprogramming by
introducing new process. A global page replacement algorithm is used which replaces process
they belong. Suppose a process needs more frames, hence it starts faulting and takes frames
away from the other processes. These processes need those pages, hence they also fault taking
away frames from other processes. Hence we get a high paging activity due to which the
utilization decreases.

As the CPU utilization decreases, the CPU scheduler brings in more new processes causing more
page faults as a result CPU utilization drops even further. The CPU schedule again brings in more
processes. At this stage thrashing has occurred.

Unit - V
FILE SYSTEM.
A file is a collection of related information which is recorded on the secondary storage.From users point
of view data cannot be written to secondary storage unless they are within a file.Files represent
programs and data.A file has a certain designed structure according to its type.A text file is a sequence
of characters.A source file is a sequence of subroutines and functions.An object file is a sequence of
bytes organize into blocks.An executable file is a series of codes.
FILE ATTRIBUTES:
A file has certain attributes which may change from OS to another and consists of the following
1..Name – The symbolic file name is the only information through which the user can identify the file.
2..Identifier – It consists of a number which identifies the file within the file system.
3..Type – It is required for those systems which support different file types.
4..Location – It provides information regarding the location of the file and to the device on which the file
resides.
5..Size – It gives the information regarding the size of the file.
6..Protection – It gives the access control information so as to decide who can do reading writing and
executing.
7..Time,date and user identification – This information may be kept for creation,last modification and
last use.The data can be useful for protection,security as well as monitoring the usage.
FILE OPERATIONS:
The various file operations are:
1..Creating a file – To create a file two steps are necessary.

 Space in the file system must be found.
 An entry for the new file must be made in the directory.
2..Writing a file – To write a file we make a system call specifying the name of the file and the
information to be written to the file.The system must keep a write pointer to the location in the file
where the next write is to take place.
3..Reading a file – To read from a file we make a system call specifying the name of the file and the
blocks of information which is to be read.The system searches the directory to find the file and
maintains a read pointer from where the next read is to take place.
4..Repositioning within a file – The directory is searched for a particular entry and the current file
position is set to the given value.
5..Deleting a file – To delete a file we search the directory for the given file and release all file space so
that it can be reused by other files and erase the directory entry.
6..Truncating a file – This operation allows a user to retain the file attribute and erase the contents of
the file.
The other common attribute include file appending and file renaming.
The following information are associated with an open file.
1..File pointer – It keeps the track of last read-write operation.
2..File open count – It keeps a tarck of the counter giving the number of open and closes of a file and
becomes zero on the last close.
3..Disk location of the file – It is required for modifying the data within the file.
4..Access rights – This information can be used to allow or deny any request.
FILE TYPES:
A common technique for implementing file types is to include the type as part of the file name .The
name is split into 2 parts that is name and extension.The common file types are
File type usual extension function
1..executable exe,com,bin,or none read to run machine language program
2..object obj,o compiled,machine language,not linked

3..text txt,doc textual data,documents
4..batch bat,sh commands to the command interpreter
5..word processor wp,tex,rrf,doc various word processor formats.
FILE STRUCTURE :
Since there are various files.It is required that the OS must support multiple file structures,due to this
the resulting size of the OS would become very large.If the OS defines five different file structures, it
needs to contain the code to support these file structures. Severe problems may arise if new
applications require different file structure not supported by the OS.
UNIX consider each file to be a sequence of 8 bits (byte), no interpretation of these bits is made by the
OS. Each application program must include its own code to interpret an input file into proper structure.
However all OS must support at least executable file structure.
ACCESS METHODS
The various access methods are
1..Sequential access
2..Direct access
3..Indirect access
1..Sequential access-The simplest access method is sequential access which is used by editors
compilers,etc.Information in the file is processed in order one record after the other.The operations on
the files are reads and writes.A read operation reads the next portion of the file and advances the
pointer.The write operations appends to the end of the file and advances the pointer.Sequential access
is based on a tape model of a fil.
2..Direct access-We consider file to be made up of fixed length logical records which allows program to
read and write records without following any order.The direct access method is based on disc model of
a file,since disc allows random access to any file block.For direct access the file is viewed as a numbered
sequence of blocks or records.Since there is no restriction to follow any order we may read block 14
then block 54 and write block 6.

For the direct access method ,the file operations are Read n and Write n.
3..Indexed access-
With large files we see that the indexed file become too large to be kept in the memory.Hence we
create an indexed for the index file.The primary index file would contain pointers to secondary index
files which would point to the actual data items.
DIRECTORY STRUCTURES
The directory can be viewed as a symbol table which translates file names into their directory entries.A
directory can be organize in many ways .The following are the various operations perform on a
directory.
1..Search for a file
2..Create a file
3..Delete a file
4..List a directory
5..Rename a file

A directory has the following logical structures
1..Single level directory
The simplest directory structure is the single level directory.Since all files are stored in the same
directory.The advantage of this structure is that it is easy to support and understand.
The drawback of this implementation is that if the number of files increases the user may find it difficult
to remember the names of all the files.If thesystem has more than one user the file naming issue would
also arise because each file must have a unique name.
2..Two level directory
In two level directory structure each user has his own user file directory (UFD).Each UFD has a similar
structure and contains the files of a single user.When a user logs in the systems master file
directory(MFD) is searched.When a user refers to a particular file,only his own UFD is searched.Hence
different users may have files with the same name as long as all the file names within each UFD are
unique.

To create a file for a user ,the OS searches only that users UFD to confirm whether another file of that
name exits.To delete a file,the OS confines its search to the local UFD,hence it cannot accidentally delete
another users file which has the same name.
UFD are created by a special system program through proper user name and account information.The
program creates a new UFD and adds entry in the MFD.
The disadvantage of two level directory structure is that it isolates one user from another.In some
systems if the path name is given and access is possible to the file residing in other UFD.A two level
directory can be thought as a tree of height 2.The root of the tree is the MFD,UFD’s are the branches
and the files are the leaves.
3..TREE DIRECTORY DIRECTORY
The tree structured directory is the most common directory structure.It contains a root directory.Every
file in the system has a unique path name.A directory (or subdirectory) contains a set of files or
subdirectories.A directory is simply another file treated in a special way.All directories have the same
internal format.One bit in each directory entry defines the entry as a file(0) or as a
subdirectory(1).Special system calls are used to create and delete directories.
Each user has a current directory,when a reference is made to a file the current directory is searched.If
the file is not in the current directory than the user must specify a path name or change the current

directory to the directory containing that file.The user can change his current directory whenever
required.
In this structure path names can be of two types.Absolute path names or Relative path names.
There are two approaches to handle the deletion of a directory.
1..Some systems will not delete a directory unless it is empty.
2..Some systems delete a directory even if it contains subdirectories or files.
4..ACYCLIC – GRAPH DIRECTORIES
An Acyclic graph directory structure allows directories to have shared subdirectories and files such that
same file or subdirectories may be in two different directories.A shared fileor directory is not the same
as two copies,it is such that any changes made by one user are immediately visible to the othe.A new
file created by one person will automatically apper in all the shared sub directories.
This structure is more complex since a file may have multiple absolute path names.Another problem
involves deletion of aq shared directory or a file.

ALLOCATION METHODS:
1..Contiguous Allocation- The contiguous allocation method requires each file to occupy a set of
contiguous blocks on the disk.Contiguous allocation of the first block and the length of the
file.Thedirectory entry for each file indicate the address of the starting block and the length of the area
allocated for this file.Accessibg a file in this method is very easy,it supports both sequential and direct
access method.
The main drawback of this method is that it suffers from external fragmentation,another problem is
finding space for new file.Also with contiguous allocation there is a problem of determining how much
space is needed for a file.
2..Linked Allocation – In Linked allocation each file is a linked list of disk blocks,the disk blocks ,the disk
blocks may be scattered anywhere on the disk.The directory contains a pointer to the first and last block
of the file.Each block contains a pointer to the next block.These pointers are not made available to the
user.In the allocation there is no external fragmentation and any free block can be used for a file.A file
can continue to grow as long as free blocks are available.The disadvantage with linked allocation is that
it does not supports direct access methods.Another disadvantage is the space required for the
pointers.A problem may arise if a pointer is damage or lost hence this type of allocation would be
unreliable.
3..Indexed Allocation – In this allocation all the pointers are occupied in a block called as indexed
block.The directory contains the address of the indexed block.When a file is created,all pointers in the
indexed block are set to nil.When a block is first written its address is put in the indexed block.
Indexed allocation supports direct access and does not suffers from external fragmentation.The main
drawback of this allocation is that it suffers from wasted space used to store blocks information.
FREE SPACE MANAGEMENT
1..Bit vector-In this approach each block is represented by 1 bit.If the block is free,the bit is 1,if the block
is allocated the bit is 1
The main advantage of this approach is its relative simplicity and efficiency in finding the free
blocks.However the disadvantage is that for fast access these bit vectors are kept in main memor
y.Hence they occupy large spaces.
2..Linked list-In this approach we keep a pointer to the first free block.This free block contains a pointer
to the next free block and so on.This scheme is not efficient because to go through the list of free blocks
we must read each block which takes much time.

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