Multitasking OS
Operating systems that allow multiple software processes to be run at the same time is called multitasking OS, such as Microsoft Windows XP and Linux. It means multitasking operating systems allow a user to do more than one thing at the same time. For example, when working on Windows operating system, you can listen to music and work on MS Word simultaneously. This is one of the best examples of multitasking OS.
OS as Resource Manager
Operating system works as a resource manager. It involves the following
Process Management
In computer science process is a program execution. During program execution, a process needs various resources such as CPU time, memory space, files and I/O devices. At a particular instance of time, a computer system normally consists of a collection of processes. The process management module of an operating system involves 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. Job Scheduling is a kind of software application which provides a GUI and a single point of control for monitoring of background executions.
Multiprogramming
The concept of multiprogramming brings new revolution in computer's world, basically multiprogramming is a technique to execute number of programs simultaneously by one processor. In multiprogramming more than one job reside in main memory at a time. The operating system picks the job one by one and executes simultaneously in the main memory. For example, you can work MS Word, MS Excel and Power point simultaneously. Various operating systems, such as Windows XP, Mac and Linux support multiprogramming.
Memory
Management Memory management is one of the most important parts of the operating system. In early days, execution of computer programs was based on existing memory. The concept of virtual memory overcomes this limitation. Basically virtual memory is a technique which enables to run a big program on available memory. For example, you can run 600 MB program on 32 MB RAM. The memory management module of an operating system takes care of the allocation and deal location of memory space to the various programs in need of this resource.
Spooling
Simultaneous Peripheral Operations On Line (Spooling) is a process of transferring data. Program is used to buffer data for the printer and remote batch terminals. This process sends the output to the disk and printer does not interact with CPU during printing. Spooling utilities are used mainly in computer systems with multiuser/networking - environment. In networking/multi - user environments, the input and output devices are generally slow. Due to this the processing of computer is also slowed down. To control such kind of slowed down, the spooling programs are used. 
Structure of OS
The internal structure of operating system is divided into layers. The outermost layer is called user interface and innermost layer is hardware.
The following are the main layers in OS:
Kernel
Kernel is a central controlling part of OS that implements the most primitive of the system's functions. This part cannot be replaced or modified by user. Basically kernel performs basic function for memory management, device management, low level security and process management.
Shell
Shell is a part of software that serves as a user interface it forms the outer layer of an operating system covering the other modules of the operating system. The shell can be a command-line interface or GUI (Graphical User Interface). An operating system can support many shells. UNIX supports three command-line shells known as Bourne shell, C shell and Korn shell.
User Interface
User interface plays an important role to use an OS. Basically there are two types of interface command line interface and GUI. The command line interface is a textual user interface in which the user gives instruction to the computer by typing command. For example, in DOS you need to type whole command to perform some operations. GUI provides user friendly environment to use a computer. In command lie interface you need to write a command to perform any operation but in GUI you just need to click. Microsoft Windows OS is the best example of GUI. Due to windows a simple user who does not know about computer science theory can use computer easily.
Apple is an organization which developed Mac operating system. Who is the founder of Apple?
(A) Bill Gate
(B) Steve Jobs
(C) Paul Alien
(D) All of these
(E) None of these
Answer: (B)
Explanation
Correct Option:
(B) Steve Jobs and Wozniak are the founder of Apple
Incorrect Options:
Rest of the options is incorrect.
The internal structure of OS is divided into three parts including kernel, shell and interface. Which one of the following parts of OS cannot be replaced or modified by user?
(A) Kernel
(B) Shell
(C) Interface
(D) All of these
(E) None of these
Answer: (A)
Explanation
Correct Option:
(A) Kernel cannot be replaced or modified by user.
Incorrect Options:
Rest of the options is incorrect.
Which of the following statements is true?
Statement A: UNIX operating system supports three command-tine shells.
Statement B: C shell is a type of command line shell.
(A) Statement A is correct
(B) Statement B is correct
(C) Both statement A more... 
Operating System
An Operating System is a master control program. Basically it is system software that manages the operation of a computer. Without an operating system computer cannot work. The computer is useless unless it is provided with essential software that makes it ready to use. As we have seen that an operating system is software, which makes the computer ready to use by a process called booting. Basically booting is a process which loads operating system from disk to RAM. About twenty years ago Steve Jobs and Wozniak, the founders of Apple, came up with the very strange idea of selling information processing machines for use at the home. The business took off, and its founders made a lot of money and received the credit they deserved for being daring visionaries. But around the same time, Bill Gates and Paul Alien came up with an idea even stranger and more fantastically: selling computer operating systems. This was much wider than the idea of Jobs and Wozniak. A computer at least had some sort of physical reality to it. It came in a box, you could open it up and plug it in and watch lights blink. An operating system had no tangible incarnation at all. It arrived on a disk, of course, but the disk was, in effect, nothing more than the box that the OS came in. The product itself was a very long string of ones and zeroes that, when properly installed and coddled, gave you the ability to manipulate other very long strings of ones and zeroes. Yet, now the company that Gates and Alien founded is selling operating systems like Hindustan Lever Limited sells detergents. New releases of operating systems were launched as if they were Bollywood blockbusters, with celebrity endorsements, talk show appearances and world tours. The market for them is vast enough that people worry about whether it has been monopolized by one company. Even the least technically minded people in our society now have at least a hazy idea of what operating systems do; what is more, they have strong opinions about their relative merits. To be more elaborative on the topic, every general-purpose computer requires some type of operating system that tells the computer how to operate and how to utilize other software and hardware that are installed on to the computer. All software programs developed today require some type of Operating System to operate properly. MS DOS, UNIX and Windows are all examples of operating systems. Because the history of computer operating systems parallels that of computer hardware, it can be generally divided into five distinct time periods, called generations, that are characterized by hardware component technology, software development and mode of delivery of computer services. 
Storage Media
Magnetic Tape
Magnetic tape is storage medium. Basically it consists of a thin tape with a coating of a fine magnetic material. It is used for recording analogue or digital data, such as audio video. Data is stored in frames and the frames are grouped into blocks which are separated from other blocks by some free spaces. Magnetic tape allows sequential technique for searching records. It similarly works like audio and video cassette. Storing data on tapes is considerably cheaper than storing data on disks. Accessing data on tapes, however, is much slower than accessing data on disks.
The following figure shows magnetic tape:
Hard Disk
The hard disk is another type of permanent storage media that is used to store huge volume of data. It is a reliable storage media. The storage capacity of hard disk is more than 160GB. A hard disk is physically composed of a series of flat, magnetically coated platters stacked on a spindle. A hard disk drive consists of numbers of inflexible magnetic disks mounted on a motor. These disks are spun by high speeds motors. The stored data of hard disk is read and written by read/write heads.
The internal structure of hard disk contains the following parts:
There are four types of hard disk available in market. They are the following:
Optical Disk
It is a storage medium from which Data is read and write by lasers. The storage capacity of optical disk is more than floppy. It stores data up to 6 gigabytes (6 billion bytes).
CD -ROM
It is a more... 
Data Access Methods
In computer science data access method is a hardware mechanism or computer program that moves data between the computer and storage device, such as hard disk, floppy and CD. Basically there are two types of data access methods.
They are the following:
Sequential Access Method
In this method records must be read in the same order as they were written to the storage media. The computer begins searching for a record in the file and then sequentially examining the next record and so on until the required record is located. The best example of these kinds of storage media is magnetic tape. The magnetic tape allows only sequential access to data. While searching a record on a reel of magnetic tape, the tape must be read sequentially, beginning with the first record.
Direct Access Method
This method allows immediate direct access to individual records in a file. There are several techniques used to accomplish direct-access file organization. Magnetic disks, such as CD is the most commonly used devices for storing direct-access files. Direct access file organization must be used whenever the application requires immediate access to individual records. It is widely used today whenever the computer configuration includes CRT (Cathode Ray Tube) terminals that display management information on demand. 
Number system
The number of digits in the system determines the base of any number system, such as decimal and binary. Basically binary is a base -2 number system as it uses two basic digits. Thus it means that the whole Binary number system depends on the two basic digits. Whereas Decimal is a base-10 system since it uses ten digits and these are 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.
Decimal Number System
A system with base - 10 is a decimal number system. Thus it means that there are ten basic digits on which the decimal number system depends. The digits are 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. By using these ten digits all the numbers in decimal number system are written. Thus the place value of a digit in a number increases the power from right to left.
The following are the place value of each digit of number 5471 :
Binary Number System
A number system with a base-2 is known as binary number system. The whole binary number system depends on two digits these are 0 and 1, respectively. By using these two digits the number in binary number system are written. Thus the place value of a digit in a number increases in the power of 2 from right to left.
The following example shows how to convert binary number 1010101 into decimal number:
| Power of | 2 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Binary number | 1 | 0 | 1 | 0 | 1 | 0 | 1 |
Decimal to Binary Conversion
While converting decimal to binary there are two methods. These are:
The following example shows all the steps
\[{{156}_{10}}\]
Classification of Computers
Micro Computers
Micro Computer is a computer whose CPU (Central Processing Unit) is a microprocessor. All the components of a microprocessor are on a single integrated circuit chip. Micro computer can be categorized I as the desktop, programmable and workstation. The microprocessor based computers are called third generation computers. They are the backbone of the modern computer era. The first and second generation computers are based on vacuum tubes and bipolar junction transistors. In the early 1970s, minicomputers carried the work, based on integrated circuits but too large to be called microcomputers. In 1974, Intel released the Intel 8080. It was the first usable microprocessor. In 1980s microcomputer began to slowly edge out. (See Figure. 9.1.1)
Mini Computers
Minicomputers were introduced in early 1960s. They were faster than micro computers. They were cheap and small. Basically these computers were mainly multiuser systems, where many users work on the systems. Generally these types of computers had larger memories and greater storage capacity. Initial minicomputers were 8 bit and 12 bit machines. In late 1970s almost all minicomputers were 16 bit machines. They had large instruction set and address field. These kinds of computers have efficient Storage for handling of text, in comparison to lower bit machines. Due to more efficient processor, speed and memory size, minicomputer was used in variety of applications and could support business applications along with the scientific applications. It was also used for various stand alone or dedicated applications. Minicomputer was a multi - user system which means more than one user could use this system simultaneously. (See Figure. 9.1.2)
Mainframe Computers
Mainframe computers are large and expensive machines. The word length of mainframe computers may be 48.60 or 64 bits, memory capacity being in some megabytes and storage capacity in some terabytes. Generally they handle huge volumes of information and data. In terms of speed, they are having significant processing capacity. More than hundred users can use mainframe at a time. The ICL39 and IBM 3090/4300 are the few examples of mainframe computers. They are used in research organizations, large industries, airlines reservation where a large database has to be maintained. (See Figure. 9.1.3)
Super Computers
Super Computers are the fastest computer in current era. The processing capabilities of super computer lies in the range of GIPS2, word length 64-128 or may be in 256 or so. The memory capacity of super computer is in some gigabytes or in terabytes. The storage capacity of this type of computer is in pixabytes. (See Figure. 9.1.4)
The parallel procession of super computer is very fast more... 
While writing a formal letter
Letters
Introduction: Letters can be broadly classified into formal letters and informal letters. Formal letters include business letters, official letters, applications, complaints, letter to editors, and letters written to people whom we want to convey certain important information. While, informal letters include personal letters, letters that are written to our friends and family. Letter writing is a skill that needs to be honed over time with practice.
1. You are Shekhar and study in a residential school in Darjeeling. Write a letter to your father in Kolkata informing him about your result in Half-yearly Examination.
St. Vivekanand
Residential School
Darjeeling
25 June, 2012
My dear Father,
I have not heard from you for about a fortnight. Please let me know as early as possible how you are all doing.
The result of the Half-yearly Examination is just announced. You will be glad to learn that I have stood first in the Half-yearly Examination. I will try to score the same and retain my position in the Annual Examination too.
Nothing more for the present. I am all right. With best regards for yourself and mother and love for my younger sister, Puttu. more... 
Write a paragraph on each the following topics in not more than 250 words.
1. A cell phone
2. A treasured belonging
3. Your memory of a place that you visited as a child
4. An embarrassing experience
5. The breakup of a friendship
6. The act of heroism or cowardice
7. A memorable journey
8. An experience that taught you a lesson
9. How to develop self-confidence?
10. The values of science
The three essential parts of a paragraph are:
1. The Topic Sentence
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