Some
popular Operating Systems include Linux, Windows, OS X, VMS, OS/400, AIX, z/OS,
etc.
Definition
An
operating system is a program that acts as an interface between the user and
the computer hardware and controls the execution of all kinds of programs.
Following
are some of important functions of an operating System.
- Memory
Management
- Processor
Management
- Device
Management
- File Management
- Security
- Control over
system performance
- Job accounting
- Error detecting
aids
- Coordination
between other software and users
Memory Management
Memory
management refers to management of Primary Memory or Main Memory. Main memory
is a large array of words or bytes where each word or byte has its own address.
Main
memory provides a fast storage that can be accessed directly by the CPU. For a
program to be executed, it must in the main memory. An Operating System does
the following activities for memory management −
·
Keeps tracks of primary memory, i.e., what part of it are in use
by whom, what part are not in use.
·
In multiprogramming, the OS decides which process will get memory
when and how much.
·
Allocates the memory when a process requests it to do so.
·
De-allocates the memory when a process no longer needs it or has
been terminated.
Processor Management
In
multiprogramming environment, the OS decides which process gets the processor
when and for how much time. This function is called process scheduling.
An Operating System does the following activities for processor management −
·
Keeps tracks of processor and status of process. The program
responsible for this task is known as traffic controller.
·
Allocates the processor (CPU) to a process.
·
De-allocates processor when a process is no longer required.
Device Management
An
Operating System manages device communication via their respective drivers. It
does the following activities for device management −
·
Keeps tracks of all devices. Program responsible for this task is
known as the I/O controller.
·
Decides which process gets the device when and for how much time.
·
Allocates the device in the efficient way.
·
De-allocates devices.
File Management
A file
system is normally organized into directories for easy navigation and usage.
These directories may contain files and other directions.
An
Operating System does the following activities for file management −
·
Keeps track of information, location, uses, status etc. The
collective facilities are often known as file system.
·
Decides who gets the resources.
·
Allocates the resources.
·
De-allocates the resources.
Other Important Activities
Following
are some of the important activities that an Operating System performs −
·
Security − By means of
password and similar other techniques, it prevents unauthorized access to
programs and data.
·
Control over system performance −
Recording delays between request for a service and response from the system.
·
Job accounting − Keeping track of
time and resources used by various jobs and users.
·
Error detecting aids −
Production of dumps, traces, error messages, and other debugging and error
detecting aids.
·
Coordination between other softwares and users −
Coordination and assignment of compilers, interpreters, assemblers and other
software to the various users of the computer systems.
Operating systems are there from the very first computer
generation and they keep evolving with time. In this chapter, we will discuss
some of the important types of operating systems which are most commonly used.
Batch operating system
The users of a batch operating system do not interact with the
computer directly. Each user prepares his job on an off-line device like punch
cards and submits it to the computer operator. To speed up processing, jobs
with similar needs are batched together and run as a group. The programmers
leave their programs with the operator and the operator then sorts the programs
with similar requirements into batches.
The problems with Batch Systems are as follows −
- Lack of
interaction between the user and the job.
- CPU is often
idle, because the speed of the mechanical I/O devices is slower than the
CPU.
- Difficult to
provide the desired priority.
Time-sharing operating
systems
Time-sharing is a technique which enables many people, located at
various terminals, to use a particular computer system at the same time.
Time-sharing or multitasking is a logical extension of multiprogramming.
Processor's time which is shared among multiple users simultaneously is termed
as time-sharing.
The main difference between Multiprogrammed Batch Systems and
Time-Sharing Systems is that in case of Multiprogrammed batch systems, the
objective is to maximize processor use, whereas in Time-Sharing Systems, the
objective is to minimize response time.
Multiple jobs are executed by the CPU by switching between them,
but the switches occur so frequently. Thus, the user can receive an immediate
response. For example, in a transaction processing, the processor executes each
user program in a short burst or quantum of computation. That is, if n users are present, then
each user can get a time quantum. When the user submits the command, the
response time is in few seconds at most.
The operating system uses CPU scheduling and multiprogramming to
provide each user with a small portion of a time. Computer systems that were
designed primarily as batch systems have been modified to time-sharing systems.
Advantages of Timesharing operating systems are as follows −
- Provides the
advantage of quick response.
- Avoids
duplication of software.
- Reduces CPU idle
time.
Disadvantages of Time-sharing operating systems are as follows −
- Problem of
reliability.
- Question of
security and integrity of user programs and data.
- Problem of data
communication.
Distributed operating
System
Distributed systems use multiple central processors to serve
multiple real-time applications and multiple users. Data processing jobs are
distributed among the processors accordingly.
The processors communicate with one another through various
communication lines (such as high-speed buses or telephone lines). These are
referred as loosely coupled systems or distributed systems. Processors in a distributed system may
vary in size and function. These processors are referred as sites, nodes,
computers, and so on.
The advantages of distributed systems are as follows −
- With resource
sharing facility, a user at one site may be able to use the resources
available at another.
- Speedup the
exchange of data with one another via electronic mail.
- If one site
fails in a distributed system, the remaining sites can potentially
continue operating.
- Better service
to the customers.
- Reduction of the
load on the host computer.
- Reduction of
delays in data processing.
Network operating
System
A Network Operating System runs on a server and provides the
server the capability to manage data, users, groups, security, applications,
and other networking functions. The primary purpose of the network operating
system is to allow shared file and printer access among multiple computers in a
network, typically a local area network (LAN), a private network or to other
networks.
Examples of network operating systems include Microsoft Windows
Server 2003, Microsoft Windows Server 2008, UNIX, Linux, Mac OS X, Novell
NetWare, and BSD.
The advantages of network operating systems are as follows −
- Centralized
servers are highly stable.
- Security is
server managed.
- Upgrades to new
technologies and hardware can be easily integrated into the system.
- Remote access to
servers is possible from different locations and types of systems.
The disadvantages of network operating systems are as follows −
- High cost of
buying and running a server.
- Dependency on a
central location for most operations.
- Regular
maintenance and updates are required.
Real Time operating
System
A real-time system is defined as a data processing system in which
the time interval required to process and respond to inputs is so small that it
controls the environment. The time taken by the system to respond to an input
and display of required updated information is termed as the response time. So in this method, the response time is very less as compared to
online processing.
Real-time systems are used when there are rigid time requirements
on the operation of a processor or the flow of data and real-time systems can
be used as a control device in a dedicated application. A real-time operating
system must have well-defined, fixed time constraints, otherwise the system
will fail. For example, Scientific experiments, medical imaging systems,
industrial control systems, weapon systems, robots, air traffic control systems,
etc.
There are two types of real-time operating systems.
Hard real-time systems
Hard real-time systems guarantee that critical tasks complete on
time. In hard real-time systems, secondary storage is limited or missing and
the data is stored in ROM. In these systems, virtual memory is almost never
found.
Soft real-time systems
Soft real-time systems are less restrictive. A critical real-time
task gets priority over other tasks and retains the priority until it
completes. Soft real-time systems have limited utility than hard real-time
systems. For example, multimedia, virtual reality, Advanced Scientific Projects
like undersea exploration and planetary rovers, etc.
An
Operating System provides services to both the users and to the programs.
- It provides programs
an environment to execute.
- It provides
users the services to execute the programs in a convenient manner.
Following
are a few common services provided by an operating system −
- Program
execution
- I/O operations
- File System
manipulation
- Communication
- Error Detection
- Resource
Allocation
- Protection
Program execution
Operating
systems handle many kinds of activities from user programs to system programs
like printer spooler, name servers, file server, etc. Each of these activities
is encapsulated as a process.
A process
includes the complete execution context (code to execute, data to manipulate,
registers, OS resources in use). Following are the major activities of an
operating system with respect to program management −
- Loads a program
into memory.
- Executes the
program.
- Handles
program's execution.
- Provides a
mechanism for process synchronization.
- Provides a
mechanism for process communication.
- Provides a
mechanism for deadlock handling.
I/O Operation
An I/O
subsystem comprises of I/O devices and their corresponding driver software.
Drivers hide the peculiarities of specific hardware devices from the users.
An
Operating System manages the communication between user and device drivers.
- I/O operation
means read or write operation with any file or any specific I/O device.
- Operating system
provides the access to the required I/O device when required.
File system manipulation
A file
represents a collection of related information. Computers can store files on
the disk (secondary storage), for long-term storage purpose. Examples of
storage media include magnetic tape, magnetic disk and optical disk drives like
CD, DVD. Each of these media has its own properties like speed, capacity, data
transfer rate and data access methods.
A file
system is normally organized into directories for easy navigation and usage.
These directories may contain files and other directions. Following are the
major activities of an operating system with respect to file management −
- Program needs to
read a file or write a file.
- The operating
system gives the permission to the program for operation on file.
- Permission
varies from read-only, read-write, denied and so on.
- Operating System
provides an interface to the user to create/delete files.
- Operating System
provides an interface to the user to create/delete directories.
- Operating System
provides an interface to create the backup of file system.
Communication
In case
of distributed systems which are a collection of processors that do not share
memory, peripheral devices, or a clock, the operating system manages
communications between all the processes. Multiple processes communicate with
one another through communication lines in the network.
The OS
handles routing and connection strategies, and the problems of contention and
security. Following are the major activities of an operating system with
respect to communication −
- Two processes
often require data to be transferred between them
- Both the
processes can be on one computer or on different computers, but are
connected through a computer network.
- Communication
may be implemented by two methods, either by Shared Memory or by Message
Passing.
Error handling
Errors
can occur anytime and anywhere. An error may occur in CPU, in I/O devices or in
the memory hardware. Following are the major activities of an operating system
with respect to error handling −
- The OS
constantly checks for possible errors.
- The OS takes an
appropriate action to ensure correct and consistent computing.
Resource Management
In case
of multi-user or multi-tasking environment, resources such as main memory, CPU
cycles and files storage are to be allocated to each user or job. Following are
the major activities of an operating system with respect to resource management
−
- The OS manages
all kinds of resources using schedulers.
- CPU scheduling
algorithms are used for better utilization of CPU.
Protection
Considering
a computer system having multiple users and concurrent execution of multiple
processes, the various processes must be protected from each other's
activities.
Protection
refers to a mechanism or a way to control the access of programs, processes, or
users to the resources defined by a computer system. Following are the major
activities of an operating system with respect to protection −
- The OS ensures
that all access to system resources is controlled.
- The OS ensures
that external I/O devices are protected from invalid access attempts.
- The OS provides
authentication features for each user by means of passwords.
Batch processing
Batch processing is a technique in which an Operating System
collects the programs and data together in a batch before processing starts. An
operating system does the following activities related to batch processing −
·
The OS defines a job which has predefined sequence of commands,
programs and data as a single unit.
·
The OS keeps a number a jobs in memory and executes them without
any manual information.
·
Jobs are processed in the order of submission, i.e., first come
first served fashion.
·
When a job completes its execution, its memory is released and the
output for the job gets copied into an output spool for later printing or
processing.
Advantages
·
Batch processing takes much of the work of the operator to the
computer.
·
Increased performance as a new job get started as soon as the
previous job is finished, without any manual intervention.
Disadvantages
- Difficult to
debug program.
- A job could
enter an infinite loop.
- Due to lack of
protection scheme, one batch job can affect pending jobs.
Multitasking
Multitasking is when multiple jobs are executed by the CPU
simultaneously by switching between them. Switches occur so frequently that the
users may interact with each program while it is running. An OS does the
following activities related to multitasking −
·
The user gives instructions to the operating system or to a
program directly, and receives an immediate response.
·
The OS handles multitasking in the way that it can handle multiple
operations/executes multiple programs at a time.
·
Multitasking Operating Systems are also known as Time-sharing systems.
·
These Operating Systems were developed to provide interactive use
of a computer system at a reasonable cost.
·
A time-shared operating system uses the concept of CPU scheduling
and multiprogramming to provide each user with a small portion of a time-shared
CPU.
·
Each user has at least one separate program in memory.
·
A program that is loaded into memory and is executing is commonly
referred to as a process.
·
When a process executes, it typically executes for only a very
short time before it either finishes or needs to perform I/O.
·
Since interactive I/O typically runs at slower speeds, it may take
a long time to complete. During this time, a CPU can be utilized by another
process.
·
The operating system allows the users to share the computer
simultaneously. Since each action or command in a time-shared system tends to
be short, only a little CPU time is needed for each user.
·
As the system switches CPU rapidly from one user/program to the
next, each user is given the impression that he/she has his/her own CPU,
whereas actually one CPU is being shared among many users.
Multiprogramming
Sharing the processor, when two or more programs reside in memory
at the same time, is referred as multiprogramming.
Multiprogramming assumes a single shared processor. Multiprogramming increases
CPU utilization by organizing jobs so that the CPU always has one to execute.
The following figure shows the memory layout for a
multiprogramming system.
An OS does the following activities related to multiprogramming.
·
The operating system keeps several jobs in memory at a time.
·
This set of jobs is a subset of the jobs kept in the job pool.
·
The operating system picks and begins to execute one of the jobs
in the memory.
·
Multiprogramming operating systems monitor the state of all active
programs and system resources using memory management programs to ensures that
the CPU is never idle, unless there are no jobs to process.
Advantages
- High and
efficient CPU utilization.
- User feels that
many programs are allotted CPU almost simultaneously.
Disadvantages
- CPU scheduling
is required.
- To accommodate
many jobs in memory, memory management is required.
Interactivity
Interactivity refers to the ability of users to interact with a
computer system. An Operating system does the following activities related to
interactivity −
- Provides the
user an interface to interact with the system.
- Manages input
devices to take inputs from the user. For example, keyboard.
- Manages output
devices to show outputs to the user. For example, Monitor.
The response time of the OS needs to be short, since the user
submits and waits for the result.
Real Time System
Real-time systems are usually dedicated, embedded systems. An
operating system does the following activities related to real-time system
activity.
- In such systems,
Operating Systems typically read from and react to sensor data.
- The Operating
system must guarantee response to events within fixed periods of time to
ensure correct performance.
Distributed Environment
A distributed environment refers to multiple independent CPUs or
processors in a computer system. An operating system does the following
activities related to distributed environment −
·
The OS distributes computation logics among several physical
processors.
·
The processors do not share memory or a clock. Instead, each
processor has its own local memory.
·
The OS manages the communications between the processors. They
communicate with each other through various communication lines.
Spooling
Spooling is an acronym for simultaneous peripheral operations on
line. Spooling refers to putting data of various I/O jobs in a buffer. This
buffer is a special area in memory or hard disk which is accessible to I/O
devices.
An operating system does the following activities related to
distributed environment −
·
Handles I/O device data spooling as devices have different data
access rates.
·
Maintains the spooling buffer which provides a waiting station
where data can rest while the slower device catches up.
·
Maintains parallel computation because of spooling process as a
computer can perform I/O in parallel fashion. It becomes possible to have the
computer read data from a tape, write data to disk and to write out to a tape
printer while it is doing its computing task.
Advantages
- The spooling
operation uses a disk as a very large buffer.
- Spooling is
capable of overlapping I/O operation for one job with processor operations
for another job.
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