- Operating SystemsOverview - system - -Overview and
Functions of operating systems
- What does an OS do?
- Operating system Operations
- Operating system structure
- Protection and security
- Computing Environments - Traditional, mobile ,
distributed, Client/server, peer to peer computing
- Open source operating System
- Booting
- Operating System services,
- System calls Types of System calls and their working.
Introduction to Operating
Systems
3.
Introduction
Operating systemis a chief program that manages all
of the hardware and software.
It has control of every process, file section of memory
and device.
It is very important as it works as a middle man
making sure both hardware and software interact
correctly.
Windows, Mac OS, LINUX and UNIX are the big
operating system contenders and all have their own
advantages and or disadvantages but they are all
based on the same concepts.
4.
Continued…
There arefour essential managers –
Process manager,
Memory manager,
File manager,
Device Manager
of every operating system and they all work together
like a team to get jobs completed.
The operating system works as a boss and to make
sure they are all working in harmony.
5.
Operating Systems Overview
An Operating System is one of the most important
software that runs on computer.
It is an integrated set of programs that are used to
manage Computer memory and processes, as well as
all of its software and hardware.
It helps users to communicate with the computer
without knowing the Computer’s Language i.e. it is a
program that are used to manage various and overall
operations of computer system.
6.
Continued..
If wepurchase a Computer Machine and try to work
on it then it will be like a dump machine like body
without a soul. If you want a computer machine to
work its soul i.e. Operating system must be installed
in it. So we can say that without an operating system
a Computer is useless.
An operating system acts as an intermediary between
the user of the computer and computer hardware.
The most important purpose of O.S is to provide an
environment in which a user can execute programs
conveniently and efficiently
7.
Objectives or Goalsof OS
1) Convenience: An OS is designed or constructed in
such as way that it makes the Computer System
more convenient to use.
2) Efficiency: An OS allows the computer system
resources to be used in an efficient manner.
3) Ability to evolve: An OS should be designed or
constructed in a such a way that the testing ,
development as well as addition of new system
components could be performed effectively without
interfering with existing services.
8.
Categories of OS
1)Single user, Single tasking OS
2) Single User, Multiple tasking OS
3) Multi-user, Multi –tasking OS
4) Real Time OS : Airline traffic control systems, Network Multimedia Systems,
Robot
5) Distributed OS
6) Time Sharing OS
7) Multiprogramming OS
8) Batch Processing OS
9) Multithreading OS
10) Multiprocessing OS
11) Network OS
12) Embedded OS
13) Mobile OS
9.
Definition:
An operatingsystem can be defined as “A system software
which acts as an interface or bridge between an application
program and computer hardware by controlling the
execution of user/application program.”
An operating system is defined as,” an organized collection of
programs that controls the execution of the application
programs and acts as interface between computer system
and computer hardware.”
An OS is defined as, “a program that acts as an intermediary
between the user and the computer hardware and controls
the execution of all kinds of programs”.
OS is a “set of programs which controls/manages all the
computers resources (hardware and software) and provide
an OS environment in which users can execute programs.
10.
Computer System Overview
An Computer is an electronic device which is used to
store, retrieve and process the data
A Computer system is a collection of hardware and
software components designed to provide an
effective tool for computation.
Computer System Overview
Every Computer system is divided into hardware,
system and application programs, the operating
system and users.
The hardware consists of CPU, Memory and
Input/Output devices which are altogether used to
provide the basic computing resources for the
computer system.
An OS is core software that controls the execution of
an application program and acts as an interface
between the user of a computer and computer
hardware.
13.
Continued..
The OSmainly coordinates the use of the hardware
among the various system programs and application
programs for various users.
Application programs defines the ways in which the
system resources are used to solve the computing
problems of the users (word processor, Video games,
spreadsheet etc.)
System Program are the set of utility programs
supplied with an OS to provide basic services for the
computer users (Compilers, Text editors, assemblers et).
Users are peoples like system analyst, programmers and
operational users etc who interact with computer system.
14.
Functions of operatingsystems
Process
Management
Memory
Management
Input/Output
Management
File
Management
Device
Management
Security
Command Interpretation
Job Priority
15.
Functions of operatingsystems
1) Process Management:
The process management module of an operating
system take care of the creation and deletion of
processes, scheduling of various system resources to
the different processes requesting them and
providing mechanism for synchronization and
communication among processes.
16.
Functions of operatingsystems
2) Memory Management:
To execute a program, it must be loaded, together
with the data it accesses in the main memory.
To improve CPU utilization and to provide better
response time to its user, a computer system
normally keeps several programs in main memory.
The memory management module of an operating
system takes care of the allocation and deallocation
of memory space to the various programs in need of
this resource.
17.
Functions of operatingsystems
3) Input/output Management:
One of the purposes of an operating system is to hide the
peculiarities of specific hardware devices from the user.
For example, in UNIX, the peculiarities(uniqueness) of I/O
devices are hidden from the bulk of the operating system itself
by the I/O subsystem.
The I/O subsystem consists of several components:
Memory-management component that includes buffering,
caching and spooling.
A general device-driver interface.
Drivers for specific hardware devices.
Only the device driver knows the peculiarities of the specific
device to which it is assigned.
18.
Functions of operatingsystems
4) File Management:
All computer systems are used for storage, retrieval and
sharing of information. A computer normally stores such
information in units called files.
Processes can read information from files and can create new
files for storing newly generated information. Information
stored in files is made persistent by storing them on a
secondary storage media such as a magnetic disk.
Files provide a natural and easy means of information
sharing. That is, a file can be created by one application and
then shared with different applications at a later time.
The file management module of an operating system takes
care of file-related activities such as organization, storing,
retrieval, naming, sharing and protection of files. It
allows programs to use a set of operations that allocation and
layout of the secondary storage devices.
19.
Functions of operatingsystems
5) Device Management:
A computer system normally consists of several l/O
devices such as terminal, printer, disk, and tape. The
device management module of an operating system takes
care of controlling all the computers l/O devices.
It keeps track of l/O request from process, issues
commands to the l/O devices and ensures correct data
transmission to/from and I/O devices.
It also provides an interface between the device and the
rest of the system that is simple and easy to use. Often,
this interface is device independent, that is, the interface
is same for all types of I/O devices.
20.
Functions of operatingsystems
6) Security:
Computer systems often store large amount of
information, some of which is highly sensitive and
valuable to their users.
Users can trust the system and rely on it only if the
various resources and information of a computer system
are protected against destruction and unauthorized
access.
The security module of an operating system ensures this.
This module also ensures that when several disjoint
processes are being executed simultaneously one process
does not interfere with the others, or with the operating
system itself.
21.
Functions of operatingsystems
7) Command Interpretation:
A user communicates with the operating system, for
using the various operations, via a set of command
provided by the operating system.
The operating system also provides a simple language,
known as command language (CL) using which a user
can put several commands together from the command
set to describe the resource requirements of the job.
The command interpretation module of an operating
system take care of interpreting user command, supplied
individually or in the form of command language and
directing the system resources to handle the request.
22.
Functions of operatingsystems
8) Job Priority:
Establishment and enforcement of a job priority
system, i.e., it determines the order in which jobs are
to be executed in the computer system.
23.
What does anOS do?
1) The OS schedules all the operations and manages all the
computing resources.
2) The OS provides the proper use of the resources in the operations
of the computer system.
3) It provides an environment for running user program.
4) It supports operations and services that required communications
over network
5) It provides security – ensures that only authorized persons can
access data and information or perform certain tasks on the
computer system.
6) OS executes user programs and to make solving user problems
easier.
7) It provides file management
8) It provide memory management function.
9) It manages the processes.
10) It manages all I/O device communication.
Operating system Operations
Inorder to ensure the proper execution of the operating system,
we must be able to distinguish between the execution of operating-
system code and user defined code.
The approach taken by most computer systems is to provide
hardware support that allows us to differentiate among various
modes of execution.
At the very least, we need two separate modes of operation: user
mode and kernel mode (also called supervisor mode, system
mode or privileged mode).
A bit, called the mode bit, is added to the hardware of the
computer to indicate the current mode: kernel (0) or user (1).
With the mode bit, we are able to distinguish between a task that
is executed on behalf of the operating system and one that is
executed on behalf of the user.
26.
Operating system Operations
When the computer system is executing on behalf of a user
application, the system is in user mode.
However, when a user application requests a service from
the operating system (via a system call), it must transition
from user to kernel mode to fulfill the request.
As we shall see, this architectural enhancement is useful for
many other aspects of system operation as well.
At system boot time, the hardware starts in kernel mode.
The operating system is then loaded and starts user
applications in user mode.
27.
Continued…
Whenever a trapor interrupt occurs, the hardware
switches from user mode to kernel mode (that
is, changes the state of the mode bit to 0).
Thus, whenever the operating system gains control
of the computer, it is in kernel mode.
The system always switches to user mode (by
setting the mode bit to 1) before passing control to
a user program.
28.
Operating system Operations
The dual mode of operation provides us with the means
for protecting the operating system from errant users
We accomplish this protection by designating some of
the machine instructions that may cause harm as
privileged instructions.
The hardware allows privileged instructions to be
executed only in kernel mode.
If an attempt is made to execute a privileged instruction
in user mode, the hardware does not execute the
instruction but rather treats it as illegal and traps it to the
operating system.
The instruction to switch to user mode is an
example of a privileged instruction.
29.
Timer
We mustensure that the operating
system maintains control over the CPU.
We must prevent a user program from getting
stuck in an infinite loop or not calling system
services and never returning control to the operating
system.
To accomplish this goal, we can use a timer. A timer
can be set to interrupt the computer after a specified
period.
The period may be fixed (for example, 1/60 second)
or variable (for example, from 1 millisecond to 1
second). A variable timer is generally implemented
by a fixed-rate clock and a counter.
30.
Timer
We mustensure that the operating system maintains
control over the CPU. We must prevent a user
program from getting stuck in an infinite loop or
not calling system services and never returning control to
the operating system.
To accomplish this goal, we can use a timer.
A timer can be set to interrupt the computer after a
specified period. The period may be fixed (for example,
1/60 second) or variable (for example, from 1 millisecond
to 1 second).
A variable timer is generally implemented by a fixed-rate
clock and a counter.
31.
The operatingsystem sets the counter. Every time
the clock ticks, the counter is decremented. When
the counter reaches 0, an interrupt occurs.
If the timer interrupts, control transfers
automatically to the operating system, which may
treat the interrupt as a fatal error or may give the
program more time.
Clearly, instructions that modify the content of the
timer are privileged.
32.
Timer
Thus, wecan use the timer to prevent a user program
from running too long. A simple technique is to
initialize a counter with the amount of time that a
program is allowed to run. A program with a 7-
minute time limit, for example, would have its
counter initialized to 420.
33.
Operating system structure
OperatingSystem Structure
When we develop a large software or program then , we split the
program into small modules or units.
Similarly when we going to design the operating system, we decompose
it into small parts or modules, so that we can easily develop or modify
in the modules.
For efficient performance and implementation an OS should be
partitioned into separate subsystems, each with carefully defined tasks,
inputs, outputs, and performance characteristics.
These subsystems can then be arranged in various architectural
configurations:
Following approach is used to design the operating system
1) Simple Structure
2) Layered Approach
3) Microkernel's
4) Modules
34.
Simple Structure
Manycommercial operating systems do not have well-defined structures.
Frequently, such systems started as small, simple, and limited systems and
then grew beyond their original scope.
When DOS was originally written its developers had no idea how big and
important it would eventually become.
It was written by a few programmers in a relatively short amount of time,
without the benefit of modern software engineering techniques, and then
gradually grew over time to exceed its original expectations.
It does not break the system into subsystems, and has no distinction between
user and kernel modes, allowing all programs direct access to the hardware.
MS-DOS – written to provide the most functionality in the least space
Not divided into modules
Although MS-DOS has some structure, its interfaces and levels of
functionality are not well separated
35.
MS-DOS Layer Structure
written for intel
8088
Provides no dual
mode and no
hardware protection
36.
MS-DOS Layer Structure
Advantageof Simple Structure:
MS-DOS – written to provide the most functionality in the least space
Disadvantages of Simple Structure:
Not divided into modules, Although MS-DOS has some structure, its
interfaces and levels of functionality are not well separate
37.
Layered structure UNIXO.S
The original UNIX OS used a simple layered approach, but almost all the
OS was in one big layer, not really breaking the OS down into layered
subsystems:
UNIX – limited by hardware functionality, the original UNIX operating
system had limited structuring.
The UNIX OS consists of two separable parts
Systems programs (system utilities or system applications.)
The kernel
Kernel is further divided into a series of interfaces and device drivers.
Kernel provides the functions such as
- CPU scheduling
- memory mgmt
- File mgmt
- Paging etc
Layered Approach
Theoperating system is divided into a number of layers (levels), each
built on top of lower layers.
The bottom layer (layer 0), is the hardware
The highest (layer N) is the user interface.
OS is divided into a number of smaller layers, each of which rests
on the layer below it, and relies solely on the services
provided by the next lower layer.
This approach allows each layer to be developed and debugged
independently, with the assumption that all lower layers have
already been debugged and are trusted to deliver proper services.
The problem is deciding what order in which to place the layers, as
no layer can call upon the services of any higher layer.
Advantages of Layeredapproach
Simplicity in design and implementation
Debugging is easy. Debugging is done from layer 0 to layer N
Once the first layer debugging is done, then second layer is debugged and
so on.
When the error is found, the error is only of that layer because bottom
layers are already debugged.
Each layer is implemented with only those operations provided by lower
level layers.
A layer does not need to know how these operations are implemented it
needs to know only what these operations do.
Each layer hides the existence, hardware from higher level layer.
43.
Disadvantages of Layeredapproach
A layer can use only lower level layers, so careful planning
is necessary.
e.g: Device driver for the backing store must be also at a
lower level than the memory mgmt routines, because
memory mgmt requires the ability to use the backing
stores.
Efficiency is less than other type.
44.
Microkernel System Structure
The basic idea behind micro kernels is to remove all non-essential
services from the kernel, and implement them as system
applications instead, there by making the kernel as small and efficient as
possible.
The result is smaller kernel
It provides minimal process and memory mgmt in addition to the
communication facility
The main function is to provide a communication facility between the client
program and the various services that are also running in user space.
Communication is provided by message passing
If the client program wishes to access a file it must interact with the file
system
The client program and service never interact with the file server.
Rather they communicate indirectly by exchanging messages with the
microkernel
45.
Microkernel System Structure
Benefits:
Easier to extend a microkernel
All new services are added to user space without modifying the kernel
Easier to port the operating system to new architectures
More reliable (less code is running in kernel mode)
More secure
Disadvantages
The performance of microkernel decreases due to increased function
overheads
Modules
Most modernoperating systems implement kernel modules
Uses object-oriented approach
Each core component is separate
Each talks to the others over known interfaces
Each is loadable as needed within the kernel
Such type of design is combination of layered structure and
microkernel approach
But it is flexible than layered system as any module can call any other modules.
It is also efficient than microkernel approach because modules do not
need to invoke message passing in order to communicate.
E.g
Solaris, linux and Mac OS X
Protection and Security
The owners of information stored in a multi-user or networked
computer system many want to control use of that information.
i.e. access to data and other related privileged like reading , writing,
executing etc. Even the concurrent processes should not interfere
with each other which cause damage to the information.
Protection:
When several jobs belonging to different users are executed, at
that time program or data of one job is protected from other jobs by
operating system. It also ensures that all access to system resources
is controlled.
Security:
Security of the system from unwanted users requires user
authentication usually by means of password. External I/O must
also be defended from invalid access.
Keeping the system protected and secure is one of the
important aspects taken into consideration.
51.
Computing Environments :Traditional
Computing:
Consider the ‘typical office environment.’ Just a few years ago, it
considered PCs connected to a network, with servers providing file and
print services.
Remote access was awarded and portability was achieved by the use of
laptop computers
Terminals attached to the mainframe were prevalent at many
companies as well, with even fewer remote access and portability
options.
The current trend is toward providing more ways to access
these computing environments. Web technologies and
increasing one WAN bandwidth are stretching the
boundaries of traditional computing.
Companies established portals, which provide web access to their
internal servers. Network computers are essentially terminals that
understand web-based computing which used in place of the traditional
workstation where more security or easier maintenance is desired.
52.
Mobile Computing:
Mobilecomputing refers to computing on handheld
smart phones and tablet computers. These device
distinguishing physical features of being portable and
lightweight.
Historically, compared with desktop and laptop
computers, mobile systems gave up screen size, memory
capacity, and overall functionality in return for handheld
mobile access to services such as email and web
browsing.
Over the past few years, however, features of mobile
devices have been so rich that the distinction in
functionality between, say, a consumer laptop and a
tablet computer may be difficult to
distinguish/differenciate.
53.
Mobile Computing:
Infact, we might argue that the features of a contemporary
mobile device allow it to provide functionality that is easily
unavailable or impractical on a desktop or a laptop
computer.
Today, mobile systems are used not only for e-mail
and web browsing but also for playing music and
video, reading digital books, taking photos, and
recording high-definition video.
Two operating systems currently dominate mobile
computing: Apple iOS and Google Android.
iOS was designed to run on Apple iPhone and iPad mobile
devices.
Android powers smartphones and tablets computers
available from many manufactures.
54.
Distributed Systems:
Thissystem was developed during 1980’s. it depends on
networks.
It consists of many processors connected through networking.
The networking are of different types and depends on the
distance between nodes such as: LAN (Local Area Network),
exists in limited area, WAN (Wide Area Network), exists for
long distance and MAN (Metropolitan Area Network), could
link building within the city.
In distributed operating system, user are not be aware of
where the programs are located and processed, that should all
be handled automatically by the operating system.
55.
Distributed Systems:
Adistributed computing environment contains
multiple nodes that are physically separate but
linked together using the network.
A Distributed system is managed by a distributed
OS.
A distributed system is a collection of
physically separate, possibly heterogeneous,
computer system that are networked to
provide users with access to the various
resources that the system maintain.
Distributed Systems:
Schemesfor building distributed system:
There are two schemes for building distributed system:
1) Tightly Coupled:
In this the processors share memory and a clock. A
communication between the processors usually through
the shared memory.
2) Loosely Coupled:
In which the processor do not share memory or a
clock.
However, each processor has its own local memory and
the processor communicate with one another through
various communication lines such as telephone lines
or high speed buses.
58.
Distributed Systems:
Some ofthe reasons for using distributed operating system:
1) Reliability:
It is more reliable. If one node (site) fails the remaining nodes are
continue operating.
2) Communication:
The several sites are connected to one another in the network; the
information can be exchanged among different sites or users.
3) Speed:
It allows to share the computation among various nodes to run it
concurrently so execution speed increases.
4) Resource Sharing:
It allows sharing resources computational task and providing a rich
set of features to users.
59.
Distributed Systems:
Some ofthe reasons for using distributed operating system:
1) Reliability:
It is more reliable. If one node (site) fails the remaining nodes are
continue operating.
2) Communication:
The several sites are connected to one another in the network; the
information can be exchanged among different sites or users.
3) Speed:
It allows to share the computation among various nodes to run it
concurrently so execution speed increases.
4) Resource Sharing:
It allows sharing resources computational task and providing a rich
set of features to users.
60.
Distributed Systems:
Advantagesof Distributed system:
With resource sharing facility, user at one site may be able to use the
resources available at another.
Speedup the exchange of data with one another via electronic mail.
If one site fails in a distributed system, the remaining sites can
potentially continue operating.
Better service to the customers.
Reduction of the load on the host computer.
Reduction of delays in data processing
Disadvantages of Distributed system:
o Failure of main network will stop the entire communication.
o To establish distributed systems the language which are used are not
well defined.
o These types of systems are not readily available as they are very
expensive
61.
Client-Server
Now aday Client- server system is popularly used to satisfy the user needs. In this, we access
and use different database or services via. a network.
Example: Internet
Components used in Client-server system:
1) Client:
It is a single user workstation which is used to access the data. It performs task like:-validation
of data, send user requests and receive information from the server, gives interface.
2) Server:
It is a multi-user processor that manages the database and provides information to the user. It
perform tasks like:- manage database of several users, security to database is maintained,
processes user requests.
Server system has two types:-
a) Computer-server:
The request from the client is served and result is send back to the client.
b) File-server:
Client can create, delete, update, read the files using file-system interface.
3) Database:
It is used to store data.
4) Network:
It is used to establish the connection between client and server.
Client-Server
Advantages of Client-serversystem:
Reduce total execution time.
Increases client CPU usage
Use of client memory is reduce
Network traffic is reduced.
Disadvantages of Client-server system:
Traffic issue
Very expensive
Server dependency.
64.
peer to peercomputing
InP2P Computing, client and servers are not
distinguishing from one another. All nodes in the
network are considered peers and they can act as server
or client depending upon the service provided
It Contains nodes that are equally participating in data
sharing
In this all the tasks are equally divided between all the
nodes. The nodes interact with each other as required as
share resource.
The nodes in peer to peer computing both use resource
and provide resources. Most modern OS such as
Windows and Mac Os contain software to implement
peer to peer networks.
65.
peer to peercomputing
To participate in peer to peer computing the node
must first join the network of peers. Once a node has
joined, it services to or from other nodes.
If Any node want specific service, it first contacts
with centralized lookup service to determine which
node provides a service
The node can broadcast a request for a service on
network and finds the response from the peer who is
going to provide a service.
peer to peercomputing
Advantages
1) Each computer in the peer to peer computing Manages
itself. So, the network is quite easy to set up and maintain.
2) It is easy to scale the peer to peer network and add more
nodes
3) In this the cost of the server is saved
Disadvantages:
1) It is difficult to backup the data as it is stored in different
computer system and there is no central server.
2) It is difficult to provide overall security in the peer to peer
computing as each system is independent and contains its
own data.
68.
Open source operatingSystem
Its a OS whose code has been made publicly and freely available to anyone who want
to see it and modify it
Its available in Source code format instead of Compiled binary code.
Open source is a term that originally referred to Open Source Software(OSS).
It is released under licence where the copyright holder allows other to study, change
as well as distribute the software to other people without any cost or charges.
E.g GNU/ Linux released under GNU General Public Licence(GPL).
Advantages :-
1) Its Available Freely on Internet without any charges
2) User can modify and edit changes as per their choice and requirement
Disadvantages:-
1) Constant updates are headaches sometimes as users have to keep updating their
system and code
2) Frequent modifications are challenges for users to decide which Open source
system is better.
3) Sometimes Hardware not Compatible with this System.
70.
Booting
The procedureof starting a computer by loading the kernel is know
as booting the system
On most computer system a small piece of code known as bootstrap
program or bootstrap loader locates the kernel loads it into
main memory and starts its execution (booting)
Some times this booting process is two-step process
1) A simple bootstrap loader fetches and loads more complex boot
program.
2) which in turn loads the kernel.
When the CPU is powered on or rebooted, the instruction register
starts execution at predefined memory location where the initial
bootstrap program stored. This program is in the form of read only
memory(ROM)
71.
The bootstrapprogram can perform a variety of tasks.
1. One task is to run diagnostics to determine the state of the
M/C
2. If the diagnostics pass, the program can continue with the
booting steps
3. It also initializes all aspects of the system from CPU
registers to device controllers and the contents of main
Memory.
4. Once the full bootstraps program is loaded it traverse the
file system to search the O.S kernel, loads it into memory
and starts its execution.
Types of Booting:-
Cold Booting and Warm Booting
Booting
Operating System services
Operatingsystem provides number of services to the user. It varies system to
system. But there are some common services which are provided by all
operating system.
Common services provided by operating system:
1) User Interface:
It is a medium through which user actually interacts with the computer
system via operating system.
User interface are categorized into three types : Command Line
Interface (CLI), Batch Interface, Graphical User Interface (GUI).
Command Line Interface (CLI):
It provides an environment where user can write their command in text
format on command prompt.
Batch Interface:
In this user creates a batch file which contains multiple executable
commands in a sequence. This batch file is executed to execute the set of
commands included in it one by one or sequentially.
Graphical User Interface (GUI):
Most of the operating system provides graphical interface which have
tools like pointing devices which directs the input/ouput operations with the
help of graphical control like menus etc.
74.
Operating System services
2)Program execution:
To get any work done from computer, user
writes a program. Operating system must let the
user to load his program into memory and execute
it. Execution of program must end normally or
abnormally.
3) Input/Output operations:
Operating system must provide means to
perform I/O for any of user program. I/O
operation can be performed from file or
input/output device.
75.
Operating System services
4)File system Manipulation:
Files are used by users for different purpose. There
should be some means to perform operations like
read/write, create/delete etc on file.
5) Communication:
It refers to communication that takes place among
the different processes which may be present on the
same machine/computer or another machine/computer
connected through computer network.
Communication might happen that processes need
to communicate with each other for some purpose like
data transfer between two processes.
Communication may be implemented by two methods:
shared memory, message passing.
76.
Operating System services
6)Error Detection:
An error is one of the parts of the system that
may cause malfunctioning of the complete system.
Error may occur in CPU and memory hardware, user
program (due to wrong/illegal memory
access/operations), input/output (e.g. keyboard not
connected properly, printer out of paper etc.).
Operating system must be aware of such error and
must be able to take appropriate actions to ensure
correct and consistent computing.
77.
Operating System services
Fewother services in addition to above mentioned services are:
7) Resource Allocation:
When multiple user or multiple jobs running concurrently, resources
must be allocated to each of them. Many types of resources such as CPU cycle,
main memory and file storage etc. which has special allocation code, other such
as I/O devices has general request and release code. These resources are
managed by operating system e.g. status of resource – whether available/ not
available, whether a process request for a resource is valid or not, status of the
resource is updated in case it is released, checking if resource can be shared or
not.
8) Accounting:
Information about available resources and user (which users use how
much and what kinds of resources) is maintained by operating system.
It also helps in finding out the usage statistics. It is even useful for the
researchers to find out the usage and the requirements to reconfigure the
system.
78.
Operating System services
9)Protection and Security:
The owners of information stored in a multi-user or networked computer
system many want to control use of that information. i.e. access to data and
other related privileged like reading , writing, executing etc. Even the
concurrent processes should not interfere with each other which cause damage
to the information.
Protection:
When several jobs belonging to different users are executed, at that time
program or data of one job is protected from other jobs by operating system. It
also ensures that all access to system resources is controlled.
Security:
Security of the system from unwanted users requires user authentication
usually by means of password. External I/O must also be defended from invalid
access.
Keeping the system protected and secure is one of the important aspects
taken into consideration.
79.
System calls
Asystem call is a request by the user to the operating
system to do something on user’s behalf. It provides an
interface between a running program and an
operating system.
System calls allow user-level processes to request some
services from the operating system which the process
itself is not allowed to do.
It is a programmatic way in which a computer
program requests a service from the kernel of the
operating system.
Some system may allow system calls to be written in
higher-level language program, in which the calls
normally like predefined function or subroutine
calls.
80.
Working Of SystemCall
Consider the above example i.e. to copy one file into another file
Initially we need to input two filenames- input file and output file.
In first approach we can ask user to enter filenames by prompting a
message. It requires sequence of system calls, first to prompt a
message on the screen and then read filenames entered through
keyboard.
In second approach user can select appropriate filenames through
menus with the help of mouse. This also requires many I/O system
calls.
Once filenames are obtained, we need system calls to check the
existence of input file, to print a message if file does not
exist, to open an input file if file exist, to create an output
file.
81.
Working Of SystemCall
After opening files, we use a sequence of system calls
in the loop that reads from input file and writes into
output file.
Finally, after copy operation is over, both files must
be closed for this system call is used, write message
on screen which indicates the completion of copy
process also requires the system call.
In this way, almost every program or operation
makes use of system calls of operating system.
Passing Parameter inSystem Call
There are three methods available for passing parameter
to operating system through system call.
1) Pass the parameter in registers. Parameters are
accessed much faster in registers. But there may be
more number of parameter than registers.
2) Parameters are stored in memory block or table.
Then address of that memory block or table is passed as
a parameter through register. This approach is used by
LINUX OS.
3) Parameters are pushed into stack by the program and
operating system popped off the stack to get parameter.
Types of Systemcalls
The system calls can categorized into five major
categories:
1) Process and job control
2) Device management
3) File management
4) Information maintenance
5) Communication
87.
Process and JobControl:
Process and Job Control:
System call under this category:
1. End, abort
2. Load, execute
3. Create Process, terminate process
4. Get process attribute, set process attribute
5. Wait for time, Wait event
6. Allocate memory, free memory
Process need to be controlled as in a running. Process must able to halt
its execution either normally (end) or abnormally (abort).
If error occurs in input data of the program, then error messages is
generated.
In both normal and abnormal halt, the operating system transfer
control to the command interpreter. The command interpreter then
reads the next command.
88.
Process and JobControl:
In multiuser system, command interpreter continues
with next command, while in batch system, it
terminates the entire job and continue with next job.
We should able to control the execution to the newly
created job or processes by assigning the job priority,
or reset the attribute of the processes or sometimes
terminate a created process if we find that it is wrong.
When the job is created, we should wait for its
execution. i.e. wait for time
When a process is going to be executed it may need
memory or may need access to the memory so we have
to allocate memory (allocate memory) and when the
process finishes its execution the allocated memory
has to be freed.
89.
Device management
System callunder this category:
1. Request device, release device
2. Read and write operation
3. Get device attribute, set device attribute
4. Logically attach or detach devices.
If users are using the system, then user must first request the device. If the
requested resources are available in the system then they can be granted to the
user process. Otherwise process will have to wait till requested resources are
available. When the work is over with the device, we must release it (release
device). These system calls are similar to open and close system calls
After device is granted to process, we must read, write and reposition the device
just as we can with files.
If any process wants to attach to any device then it will use attach device system
call and if any point the process want to disconnect any device then it will
make use of this detach system call.
Many operating systems like UNIX or LINUX treats every device as a file. All
operations to perform device management are almost similar to file
management. The user interface can also make files and devices appear to be
similar.
90.
File management
Systemcall under this category:
1. Create file, delete file
2. Open, close
3. Read, write, reposition
4. Get file attribute, set file attribute
Computers can store information on various storage media, such as magnetic disks, magnetic tapes,
and optical disks.
Files are mapped by the operating system onto physical devices. These storage devices are usually
nonvolatile, so the contents are persistent through power failures and system reboots.
There are some common system calls related to file management available in operating systems. Every
file is created using create system call and deletion is possible using delete system call.
Once the file is created or already available, it must be opened using open system call before any
operations are performed. We must have to pass the filename and operation mode as parameter to the
open system call.
We read the content of the file using read system call and write operation can be performed using write
system call.
After read or write operation file pointer is advanced automatically. File pointer can also be
repositioned manually using reposition system call.
Finally file should be closed for which close system call is used to indicate that we are no longer using
it.
Every file has a set of attribute such as file type, owner identifier, access permission, accounting
information and so on. Get file attribute and set file attribute are the system call used to retrieve
attributes of file.
91.
Information Maintenance
Ithandles information and its transfer between the OS
and the user program.
System call under this category:
Get or set time and date
Get process and device attributes
Get system data, set system data.
The system have a system call to return the current time
and date. Some system calls provide the information
about number of users, version of operating system, free
disk space, free memory and so on. Some system calls
gives the job or process information.
92.
Communication:
These typesof system calls are specially used for
inter process communications.
System call under this category:
Create, delete communications connections
Send, receive message
Help OS to transfer status information
Attach or detach remote devices
Shared Memory
Inthis model a region of memory that is shared by
cooperating processes is established .
Process can exchange information by reading and writing
data to the shared region.
It is faster than message passing because in shared
memory systems, system calls are required only to
establish shared memory region.
Normally O.S tries to prevent one process from accessing
other process memory
The process are responsible for ensuring that they are not
writing to the same location simultaneously
95.
Message Passing system
In this communication takes place by means of message
exchanged between the cooperating processes.
They are useful for exchanging smaller amounts of data.
Easier to implement than shared memory for inter
computer communication
Slower than shared memory because it is implemented
using system calls and thus require the more time
consuming task of kernel intervention
E.g a chat program used on www could be designed
so that chat participants communicate with one
another by exchanging messages.
This model provides a mechanism to allow processes to
communicate and to synchronize their actions.
96.
Message Passing system
It provides at least two operations: send (message)
and receive (message)
If process P and Q want to communicate they must
send messages to and receive messages from each
other
A communication link must exist between them
This communication link can be implemented in a
variety of ways.
1. Direct or indirect communication
2.Synchronous or Asynchronous communication
3.Automatic or explicit buffering
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