In general, package can be divided into two chief classs, System package and Application package. Application package includes plans that perform user undertakings. In other words, application package are designed to be used by terminal users and do non interact straight with the hardware of a system. Examples of application package are databases, MS-Office, media participant, games and so on. In contrast to application package, system package straight interacts with the hardware of a system in order to put to death applications. It includes assorted plans that are required for a computing machine or system to map. These include Operating System ( OS ) , drivers for hardware devices, compilers, debuggers, linkers and so on. System package is designed to be used by a system itself and non for terminal users.

An OS is a plan that acts as an mediator between the user of a computing machine and the computing machine hardware. An OS shields low-level information of the machine operation and provides often needed services to the terminal user. It directs the processor in the usage of system resources and uses the computing machine hardware in an efficient mode

An OS has the undermentioned few chief duties:

It executes or performs basic undertakings such as input and end product operations, maintaining path of file and directories on the disc and commanding peripheral devices, such as pressmans, scanners and disc thrusts.

It ensures that no intervention occurs between different plans running at the same clip.

It provides platform for other 3rd party application packages.

The listed duties address the demand for pull offing the hardware and application plans. It besides focuses on supplying an interface between the hardware and the application package for efficient executing of the package.

Definition and Function of OS

An OS integrates the terminal user and the hardware of the system and this enables the user to execute individual or multiple undertakings together. It acts as an mediator between the hardware and the terminal user. An OS resides on top of the hardware bed as illustrated in Figure 1.1.

Figure 1.1: Overview of an OS

An OS has the undermentioned cardinal elements:

A proficient bed of package for driving the hardware of the computing machine, such as disc thrusts, keyboard and the screen.

A file system that helps organize files in a logical manner.

A simple bid linguistic communication that enables users to run their ain plans and pull strings their files in a simple manner. Some OSs besides provide text editors, compilers, debuggers and a assortment of other tools.

Since the OS is in charge of a computing machine, all petitions to utilize its resources and devices need to travel through the OS. Therefore, an OS provides legal entry points into its codification for executing basic operations, such as composing to devices.

Os may be classified in two ways, based on the figure of undertakings they can execute at the same time and based on the figure of users utilizing the system at the same time. Therefore, an OS can be single-user or multi-user and single-task or multi-tasking. An OS performs assorted maps to move as an mediator between a user and the hardware of a system. Most of the OSs available today execute the undermentioned cardinal maps to supply the needed services to the users:

Management of the Processor: An OS is responsible for apportioning assorted undertakings of different plans to the processor.

Management of Memory: An OS besides allocates the chief memory and secondary storage countries to informations, user plans and system plans.

Management of Input/Output ( I/O ) : An OS allows fusion and control of entree of plans to different input and end product devices while plans are being executed.

Execution of Applications: An OS is responsible for smooth executing of applications by apportioning the resources required for them to run.

Management of Security: An OS manages the security of a system by leting authorized plans to put to death and authorised users to execute.

File Management: An OS maintains a-file system so that users can execute file creative activity, alteration and omission undertakings.

Information Management: An OS provides a certain figure of indexs ( such as debugger ) that can be used to name the right operation of the machine.

Precedence System: An OS besides specifies and maintains the sequence in which undertakings need to be executed in a system.

Types of Operating Systems

To understand the cardinal characteristics of an OS and its architecture, it is of import to see assorted types of OSs that were used earlier and are being used today. OSs can be categorised as follows:

Simple Batch Processing

Multiprogrammed Batched

Time Sharing

Personal Computer



Real Time

1.3.1 Simple Batch Processing System

Earlier, computing machines ran from console manner ; through character user interface. In these systems, the user was required to supply bids to system to put to death assorted undertaking and plans. Besides, user was non able to straight entree different I/O devices. Users made a occupation dwelling of plans, informations and control information which was so submitted to an operator for executing on the computing machine system. Generating the end product took clip based on the type of occupation submitted. The terminal user had to roll up the end product from the operator. The OS was really simple and its chief undertaking was to reassign control from one occupation to another. The OS resided in the memory of the computing machine system as illustrated in Figure 1.2.

Figure 1.2: Memory Layout of a Simple Batch Processing System

To rush up the executing, occupations of similar sorts were batched together for group executing. For illustration, all the plans written in COBOL or FORTRAN were processed as a batch. All the occupations were jointly executed.

In these systems, the velocity for treating occupation increased. However, the CPU was frequently idle because of the incompatibility which existed between the operating velocities of CPU and I/O devices. CPU maps in microsecond/nanosecond whereas I/O devices work with velocity in second/minutes. To work out this issue, which was decelerating down the processing velocity, spooling mechanism was devised.

Spooling mechanism means directing informations or procedures to a impermanent storage country. This impermanent work country can be a storage disc. While executing a occupation, the information is read straight from the disc. Spooling mechanism utilizes the disc as a big buffer to read in front as an input device and besides for hive awaying the end product till the clip devices are available to treat them.

For illustration, when a print bid is given by a user, the end product is first written in the buffer on a disc and so printed subsequently. Particularly, this applies to printing of big files. Spooling overlaps calculation of other undertakings with I/O of one occupation. Spooler may read the input from one occupation, print the end product of 2nd occupation and at the same time treat a 3rd occupation as illustrated in Figure 1.3.

Figure 1.3: Spooling Mechanism

By leting faster CPU and decelerate I/O devices to work at a higher operating rate, spooling velocities up the public presentation of a system.

1.3.2 Multiprogrammed Batched OS

With spooling another facet of processing was introduced, called occupation programming. If several occupations are ready and are to be allocated to memory for executing, the system analyses the handiness of infinite. If adequate infinite is non available, it organises and schedules the occupations to be executed. This procedure is called occupation programming. The most of import characteristic of occupation programming is the ability to multiprogram. In general, it is non possible for a individual user to maintain either the I/O devices or the CPU busy at all the clip. Multiprogramming increases the CPU use by organizing assorted occupations so that the CPU ever has one undertaking to put to death.

Figure 1.4: Multiprogrammed System Memory Layout

As illustrated in Figure 1.4, the OS queues up several occupations in memory at the same time. Then, the OS selects and executes one of the occupations in the memory ; this occupation sometimes has to wait till the I/O operation gets completed. CPU, alternatively of being in idle manner switches to another occupation in the memory. This is in contrast to non-multiprogrammed systems where the CPU remains idle. Again, when this occupation needs to wait, the CPU switches to another occupation and so on. Finally, the first occupation gets executed and gets back the CPU for farther processing. Choosing one out of the several occupations ready for executing in memory by the CPU is called CPU programming.

In Multiprogramming, an OS is required to take determination for all the users. All occupations meant for processing are kept in a occupation pool. When the OS selects a occupation from the occupation pool, it loads that occupation into the memory for executing. With multiple occupations put to deathing at the same time, it is besides indispensable to restrict their ability to impact one another.


1. A occupation pool is an country on disc where all the procedures are queued for allotment to chief memory for executing.

2. Multiprogrammed batch systems are suited for put to deathing big occupations that require minimal or no user interaction.

1.3.3 Time Sharing Systems

A multiprogrammed batch system provides an environment where assorted system resources are utilized efficaciously. However, a user can non interact straight with the system, which means that instructions can non be given straight to either the OS or a plan. Time sharing or multitasking is a logical extension of concurrent execution, which provides synergistic usage of the system and users are able to give instructions to the OS or to the plan straight by utilizing input devices.

A time-shared OS allows many users to utilize and portion a computing machine, at the same time. For illustration, the OS of a waiter may let many users to utilize a peculiar plan at the same time. In a clip sharing system, less CPU clip is required by each user for put to deathing actions or bids. The OS utilises CPU programming and concurrent execution to supply each user a clip slot besides known as clip piece for executing of plans.

A plan in executing is referred to as a procedure. A procedure may necessitate more than one clip piece for executing before it either coatings or demands to execute synergistic I/O operations. Normally these operations are dependent on the velocity of a user, for illustration, the input may be bound by the typing velocity of a user which may be fast for user but decelerate for the system. When this synergistic input takes topographic point, alternatively of allowing the CPU stay idle, the OS quickly switches the CPU to a plan of another user. As the system switches quickly from one user to another, each user is given an feeling that the full system is dedicated to single usage.

Time sharing OSs are more complex than the multiprogrammed OS because several occupations are kept at the same time in the memory. The occupations are interchanged in and out of the memory to the disc to obtain a sensible response clip. The disc serves as a backup storage for the chief memory. However, there could be memory storage restrictions. To get the better of this, a technique called practical memory is used. This technique allows the executing of a occupation that may non be wholly in the memory. The chief logic behind this technique is that plans can be larger than the physical memory. This agreement frees the coder from concerns over memory-storage restrictions. Multiprogramming and clip sharing are the cardinal construct of all modern OSs.

1.3.4 Personal Computers OSs

Earlier, Personal Computers ( Personal computers ) besides referred as desktop system had CPUs which lacked the characteristics needed to procure an Os from user plans. Personal computers were designed for individual user and individual undertaking OS presuming that merely one user uses the machine and runs merely individual plan at a clip.

The OS of a desktop system consists of two parts, Basic Input Output system ( BIOS ) , which is stored in Read Only Memory ( ROM ) and Disk Operating System ( DOS ) . An illustration of such a system is MS DOS.

The BIOS performs a power-on ego trial when the power is turned on. This trial checks the position of the memory whether it is working and besides operation of all other relevant units. After this, it reads a little part of the OS known as boot from the disc and loads it into the chief memory. This boot plan so calls the remainder of the OS and shops it in the chief memory. This procedure is known as the booting procedure.

However, the ends of these OSs have changed with the clip. Along with maximizing peripheral and CPU use, the OS maximises user convenience and reactivity. Some illustrations of these systems are Microsoft Windows and Apple Macintosh.

Personal computers have benefitted from following engineerings developed for mainframe systems such as concurrent execution, multitasking and clip sharing.

1.3.5 Multiprocessor Operating System

Most systems are by and large individual processor system, which means that they have merely one CPU. However, multiprocessor OSs have more than one processor. The processors portion the same computing machine coach, the clock and sometimes memory and peripheral devices. These systems are besides known as parallel systems or tightly coupled systems. Multiprocessor systems have the undermentioned chief advantages:

Increased Throughput: Addition in figure of processors besides increases the sum of occupations done in a peculiar clip.

Economy of Scale: Multiprocessor systems save cost because they portion peripherals, mass storage and power supplies.

Increased Dependability: With multiprocessor system, the maps are distributed among several processors. Therefore, failure of one of the processors does non hold the system but merely slows it down. Therefore, the full system runs with slower velocity instead than neglecting wholly. This ability to supply service even if there is failure in one of the processors is known as mistake tolerance.

The most widely used multiprocessor system uses Symmetric Multiprocessing ( SMP ) . In SMP, the system runs an indistinguishable transcript of OS and these transcripts communicate with one another as needed. Some systems use asymmetric parallel processing in which each processor is assigned a specific undertaking. The chief processor controls the system and all the other processors either take direction from the chief processor or have predefined undertakings. This defines a master-slave relationship. The maestro processor agendas and allocates work to the slave processors. This is achieved by planing hardware or package to execute the master-slave technique. Particular hardware differentiates the multiprocessor or package is written to let merely one maestro and many slaves. For illustration, Sun OS version4 uses asymmetric parallel processing.

1.3.6 Distributed Systems

In distributed systems, all calculation is distributed among several processors. Distributed system depends on networking for its functionality. Networking provides the channel to pass on and be able to portion computational undertakings and to supply assorted characteristics to the users. A web, in simple footings is a communicating way between several systems.

The processors in a distributed system vary in size, map and are referred to as sites, nodes and computing machines. The following are the chief grounds for planing a distributed system:

Calculation: A peculiar calculation is partitioned into a figure of sub calculations that can put to death at the same time. In add-on to this, a distributed system allows us to administer the calculations among assorted nodes. This is known as burden sharing.

Resource Sharing: Assorted users can portion the resources.

Dependability: These systems offer dependability because if one site fails, the staying node portions the work.

Communication: When many sites are connected to one another by a communicating web, the procedures at assorted sites can interchange information among them. For illustration, a user may direct an electronic mail to another user at the same site or at a different web.

Distributed systems are besides referred as slackly conjugate systems as the processors do non portion memory or clock, alternatively processors communicate with each other through assorted communicating lines, such as coach, web overseas telegrams or even telephone lines.

1.3.7 Real Time OSs

Real clip systems are considered to be particular purpose OSs as they are used when stiff clip demands are placed on the operation of a processor or flow of informations. Therefore, these are frequently used as a control device in dedicated applications. For illustration, detectors send informations to a computing machine which so analyses the informations and consequently instructs the system to take appropriate action. Systems that control scientific experiments, industrial control systems like fume and fire detectors, medical imagination system utilize existent clip systems.

Real clip systems are of two sorts, difficult and soft. A difficult existent clip system ensures that critical undertakings are completed on clip. Existing general purpose OS does non back up hard existent clip functionality. A less restrictive type is soft existent clip system. In this system, critical existent clip undertakings get precedence over other less of import undertakings and retain that precedence till its completion. Several countries where soft existent clip systems are used are multimedia, practical world and progress scientific undertakings such as geographic expedition. Most current OSs, such as major version of UNIX and different versions of Window Server OS system usage soft existent clip functionality.

1.4 Operating System Structure

An OS can be designed as a monolithic, disorderly aggregation of plans and procedures without any construction. Any procedure can name any other procedure to bespeak a service from it. A series of procedures can be activated by executing of user bids. However, this sort of execution may be suited merely for little OSs. It is non suited for big OS because the deficiency of construction makes it highly hard to stipulate codification, trial and debug the system.

The design of a new big OS is a really critical undertaking. The end of the system has to be defined decently before the design begins. It is really of import to see the services that an OS provides along with the interface that is made available to the users and coders. It is every bit of import to see its constituents and interconnectednesss. In this subdivision, we will larn assorted design attacks needed to manage elaboratenesss of an OS.

1.4.1 Layered Approach

As per the superimposed attack, an OS is divided into several beds and the maps of an OS are divided among these beds. Each bed has a good defined functionality and I/O with two bordering beds. Modern Unix OS has a superimposed construction. This is illustrated in Figure 1.5.

By and large, the bottom bed is concerned with hardware and top bed is concerned with users. The overall functionality and characteristics are determined and are separated into constituents that allow the coders to conceal information doing the system more secure.

The modularisation or the superimposed attack of the system is designed so that each bed uses maps and services of merely the low-level beds. This attack simplifies debugging and system confirmation. The first bed can be debugged without impacting the other beds and the system. Each bed in the superimposed construction can be designed, coded and tested independently. Therefore, it simplifies the design, specification and execution of an OS. However, it is critical to that OS maps are assigned carefully to assorted beds because a bed can utilize merely the maps provided by the bed beneath it.

1.4.2 Kernel-based Approach

Brinch Hansen suggested the design and construction for a meat based system. A meat is a aggregation of services over which the remainder of the OS is built. This construction is illustrated in Figure 1.6.

Figure 1.6: Structure of a Kernel-based Os

The meat provides an environment to construct an OS where interior decorators have considerable flexibleness. It is the nucleus or the cardinal constituent of the system. In this attack, the policy and optimization determination are non made at meats degree. These are left to the outer bed. OS is an orderly growing of package over the meats where all determination sing procedure programming, resource allotment and executing environment are made. Kernel allows dynamic creative activity and control of procedures along with communicating between them.

Note: Including excessively much functionality in a meat can ensue in low flexibleness at a higher degree at the same clip excessively small functionality can ensue in low functional support at higher degree.

Microkernel: Carnegie Mellon University developed an OS called Mach that consisted of modular meats utilizing microkernel attack as shown in Figure 1.7.

Figure 1.7: Microkernel Approach

This method structures the OS by taking all non indispensable constituents from the meat and implementing them as system and user degree plans. As a consequence a little meat is designed. Microkernels allow typical services to run in user infinite. It provides minimum services along with communicating among assorted procedures. The chief undertaking of the microkernel is to supply communicating installation between client plan and the assorted services that are running at user degree. The client plan and the service interact indirectly by interchanging messages with the microkernel.

One of the benefits of this attack includes the easiness of spread outing the OS. All new services are added to user infinite and do non necessitate any alteration of the meat. When the meat has to be modified, alterations tend to be fewer because microkernel is a smaller meat. It is easier to reassign it from one hardware design to another. The microkernel besides provides more dependability and security since most of the services are running as user instead than kernel procedures. If a service fails, the remainder of OS remains untasted. Some illustrations of OS based on microkernel attack are, Minix, MERT and QNX.

1.4.3 Virtual Machines

Conceptually, a computing machine system consists of beds, hardware being the lowest bed. The meat running at following degree uses the hardware instructions to make a set of system calls to be used by outer beds. The system programmes above the meats are able to utilize either system calls or hardware instructions and these plans do no differentiate between the two. Therefore, they are accessed otherwise. They both provide functionality that the plan can utilize to make even more advanced maps. Systems plans in bend dainty both hardware instructions and system call plans every bit one as if they were both at the same degree.

Some systems carry this construct a measure farther by leting the systems plan to be called easy by the application plans. Although the systems plans are at higher degree, the application plans may see everything under the hierarchy as if they were portion of the system itself. This superimposed attack is taken into logical construct of a Virtual Machine ( VM ) as shown in Figure 1.8.

Figure 1.8: Structure of a VM System

A user can run a individual OS on this VM. The VM construct provides higher flexibleness wherein it allows different OSs to run on different VMs, for illustration, IBM 370 system. In this system, the VM package provides VM to each user for processing. When a user logs in, VM 370 creates a new VM for the user.

There are two primary advantages of VM. First, it wholly protects the system resources. It provides a robust degree of security. Second, it allows system development to be done without interrupting the normal system operation because each VM is isolated from the other 1. The disadvantage of a VM system is that there is no direct sharing of the resources. Two attacks have been implemented for sharing the resource. First attack is sharing a minidisk which is accessed by the VM. With this technique files can be shared. The other attack is the web VM, wherein a system can direct information over the VM communicating web.

VMs are progressively going popular as a agency for work outing compatibility jobs. VMs now exist that allow Windows applications to run on Linux-based computing machines. The VM runs both the Windows application and the Windows OS. Java is one of the applications that runs on a VM there by leting a Java plan to run on any computing machine system that has a Java practical machine.

1.5 Drumhead

An OS is a aggregation of plan that acts as an intermediary between user and the computing machine hardware.

An OS performs assorted maps such as procedure direction, I/O direction, pull offing usage of chief memory and supplying security to user occupations and files.

In a simple batch OS, to rush up processing, occupations with the same demands are batched together and executed as a group.

The primary aim of a multi-programmed OS is to maximise the figure of plans executed by a computing machine in a specified period and maintain CPU of the computing machine at the same time busy.

A time-shared OS allows a figure of users to at the same time utilize a computing machine.

The primary aim of a Distributed System is to increase the throughput by acquiring more occupations done in less clip.

A existent clip system is used in the control of physical systems. The response clip of such system should fit the demands of the physical system.

An OS is big. Therefore, it is critical to see modularity while planing a system.

The layered-approach considers design of the systems as a sequence of beds.

The practical machine construct takes the superimposed attack and treats the meat of the OS and hardware as though they were all hardware.

1.6 Chapter Review Questions

The OS of a computing machine serves as a package interface between the user and the ________ .





OSs that are used for scientific experiments, industrial control systems and medical imagination systems are:

Real Time Systems

Batch Processing Systems

Time Sharing Systems

All of the above

An OS manages ________ .



Disk and I/O devices

All of the above

Which technique was introduced because a individual occupation could non maintain both CPU and I/O devices busy?

Time sharing



CPU programming

Real clip systems are ________ .

Chiefly used on mainframe computing machines

Used for supervising events as they occur

Used for plan development

Used for existent clip synergistic users

Which of the followers is non the bed of an OS?



Application plan

Critical Section

A procedure is defined as a:

plan in executing.

a coincident plan.

any consecutive plan.

something which prevents dead end.

In ______ OS, the response clip is really critical.





In which sort of system indistinguishable transcript of OS tallies?

Asymmetric parallel processing system

Symmetric parallel processing system

Distributed systems

Time sharing systems

Which systems are referred to as slackly coupled systems?

Time sharing

Real clip

Simple Batch Processing


1.6.1 Answers

1. a

2. a

3. vitamin D

4. degree Celsiuss

5. B

6. vitamin D

7. a

8. vitamin D

9. B