Comprehensive Computer System Class 11 Notes Chapter 1

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Computer System Class 11 Notes Computer Science Chapter 1

Chapter 1 : Computer System

Introduction to Computer System

A computer is an electronic device that can be programmed to accept data (input), process it and generate result (output). Following figure shows the block diagram of a computer system.

Computer system class 11 notes
Computer System Class 11 Notes

Computer System Class 11 Notes

Components of Computer System

1. Central Processing Unit (CPU) : It is the component of a computer that carries out the actual processing and usually referred as the brain of the computer. CPU has two main components — Arithmetic Logic Unit (ALU) and Control Unit (CU). ALU performs all the arithmetic and logical operations while CU controls sequential instruction execution.

2. Input Devices : The devices through which signals are sent to a computer are termed as input devices. for example Keyboard, Mouse, Scanner etc. Braille keyboards are used to help the visually impaired for entering data into a computer.

3. Output Devices : The devices that receives data from a computer system for display, physical production, etc. are called output device. for example Monitor, Printer, Speaker etc. Three types of commonly used printers are inkjet, laserjet and dot matrix. Now-a-days, there is a new type of printer called 3D-printer, which is used to build physical replica of a digital 3D design.

Computer System Class 11 Notes

Computer Memory

A computer system needs memory to store the data and instructions for processing. There are two types of memory

1. Primary memory : It is also called main memory. Program and data are loaded into the primary memory before processing. It is of two types viz. (i) Random Access Memory (RAM) and (ii) Read Only Memory (ROM).

It stands for Random Access MemoryIt stands for Read Only Memory
It is temporary memory.It is permanent memory.
Data will be wiped out as the power
supply is turned off.
Its contents are not lost even when the power is
turned off.
Computer System Class 11 Notes

2. Secondary Memory : This memory is used to store the data or instructions for future use. The secondary memory is non-volatile and has larger storage capacity than primary memory. It is slower and cheaper than the main memory. Examples of secondary memory devices include Hard Disk Drive (HDD), CD/DVD, Memory Card, etc.

Computer System Class 11 Notes

Units of Memory

A computer system uses binary numbers to store and process data. The binary digits 0 and 1, which are the basic units of memory, are called bits. A 4-bit word is called a Nibble. Examples of nibble are 1001, 1010. A two nibble word, i.e., 8-bit word is called a byte, for example, 01000110, 01111100, 10000001, etc.

Computer System Class 11 Notes Memory Units
Computer System Class 11 Notes Memory Units

Computer System Class 11 Notes

Data Transfer between Memory and CPU

Data are transferred between different components of a computer system using physical wires called bus. Bus is of three types— (i) Data bus to transfer data between different components, (ii) Address bus to transfer addresses between CPU and main memory. (iii) Control bus to communicate control signals between different components of a computer. As the CPU may require to read data from main memory or write data to main memory, a data bus is bidirectional. But the control bus and address bus are unidirectional.

Microprocessors :

A processor (CPU) which is implemented on a single microchip is called microprocessor. Microprocessor is a small-sized electronic component inside a computer that carries out various tasks involved in data processing as well as arithmetic and logical operations. Microprocessors are classified on the basis of different features which include chip type, word size, memory size, clock speed etc

(A) Word Size : Word size is the maximum number of bits that a microprocessor can process at a time.

(B) Memory Size : Depending upon the word size, the size of RAM varies. Initially, RAM was very small (4MB) due to 4/8 bits word size but now it become feasible to use RAM of size upto 16 Exabytes (EB).

(C) Clock Speed : The clock speed indicates the speed at which the computer can execute instructions. It is measured in Hertz (Hz).

(D) Cores : Core is a basic computation unit of the CPU. CPU with two, four, and eight cores is called dual-core, quad-core and octa-core processor, respectively.

Computer System Class 11 Notes


The microcontroller is a small computing device which has a CPU, a fixed amount of RAM, ROM and other
peripherals all embedded on a single chip as compared to microprocessor that has only a CPU on the chip. washing machine, digital camera, pendrive, remote controller, microwave are few examples of microcontrollers. The simple use of microcontroller has permitted repetitive execution of tedious tasks automatically without any human intervention, thereby saving precious time.

Data and Information :

Raw fact and figures is called data. Processed data is called information. So we can say that Data are raw and unorganised facts that are processed to get meaningful information.

Computer System Class 11 Notes

Data and Its Types :

Data can be input to a computer in the text form consisting of English alphabets A–Z, a–z, numerals 0–9, and special symbols like @, #, etc. For example, a fee receipt is made of numeric and non-numeric characters. Primarily, there are three types of data

(A) Structured Data : Data which is stored in pre specified tabular format and easy to understand is called structured data. Structured data may be arranged in ascending or descending order, for example the following table shows marks of students in Term1 and Term2

Roll NoNameTerm1Term2
Computer System Class 11 Notes

(B) Unstructured Data : Data which are not organised in a pre-defined format is called unstructured data. Examples graphics, text documents, social media posts, satellite images, etc

(C) Semi-structured Data : Data which have no well-defined structure but maintains internal tags or markings to separate data elements are called semi-structured data. Examples include email document, HTML page, comma separated values (csv file), etc.

Computer System Class 11 Notes

Data Capturing, Storage and Retrieval :

(A) Data Capturing : It involves the process of gathering data from different sources in the digital form. This capturing may be done from keyboard, barcode readers etc.

(B) Data Storage : It is the process of storing the captured data for processing later. Data keeps on increasing with time. In large organisations, computers with larger and faster storage called data servers are deployed to store vast amount of data.

(C) Data Retrieval : It involves fetching data from the storage devices, for its processing as per the user requirement. Minimising data access time is crucial for faster data processing.

Computer System Class 11 Notes

Data Deletion and Recovery :

One of the biggest threats associated with digital data is its deletion. The storage devices can malfunction or crash down resulting in the deletion of data stored. Deleting digitally stored data means changing the details of data at bit level.

Data recovery is a process of retrieving deleted, corrupted and lost data from secondary storage devices.

Steps to protect our data :

  1. Don’t allow any stranger to work on your system.
  2. Use password to protect your important/confidential document.
  3. Dispose off old or damaged storage devices carefully.

Computer System Class 11 Notes

Software :

The software comprises a set of instructions which on execution deliver the desired outcome. It is that component of a computer system, which we cannot touch or view physically. The software can be broadly classified into three categories viz. (i) System software, (ii) Programming tools and (iii) Application software.

(i) System Software : The software that provides the basic functionality to operate a computer by interacting directly with its constituent hardware is termed as system software. Examples of system software are operating systems, system utilities, device drivers, etc.

(A) Operating System : As the name implies, the operating system is a system software that operates the computer. An operating system is the most basic system software. It is an interface between the user and hardware. Some of the popular operating systems are Windows, Linux, Macintosh, Ubuntu etc.

Functions of Operating System :

  1. It manages all the resources of a computer, i.e., its hardware including CPU, RAM, Disk, Network.
  2. It act as an interface between user and computer.
  3. It provide services for applications software.

Computer System Class 11 Notes

(B) System Utilities : Software used for maintenance and configuration of the computer system is called system utility. Some system utilities are with the operating system for example disk defragmentation tool, formatting utility, system restore utility.

(C) Device Drivers : As the name signifies, the purpose of a device driver is to ensure proper functioning of a particular device. The device driver acts as an interface between the device and the operating system. Just
like a language translator, a device driver acts as a mediator between the operating system and the attached device.

(ii) Programming Tools : In order to get some work done by the computer, we need to give instructions. Computer languages are developed for writing these instructions. Humans are able to write programs in high-level language while computers understand machine language. There is a continuous need for conversion from high level to machine level language, for which translators are needed.

(A) Classification of Programming Languages : It is very difficult for a human being to write instructions in the form of 1s and 0s. So different types of computer programming languages are developed to simplify the coding. Two major categories of computer programming languages are low-level languages and high-level languages.

Low Level LanguageHigh Level Language
machine dependent languagesmachine independent
Execution is fasterExecution is slower
Do not need language translatorNeeds Language translator
Low-level languages include
Machine Language and Assembly Language
high level language include C++,
Java, Python, etc.
Computer System Class 11 Notes

Computer System Class 11 Notes

(B) Language Translators : As the computer can understand only machine language, a translator is needed to convert program written in assembly or high level language to machine language. The program code written in assembly or high-level language is called source code. The source code is converted by a translator into the machine understandable form called object (machine) code.

The three types of translators used in computing systems are assembler, compiler and interpreter.

Assembler : The translator used to convert the code written in assembly language to machine language is called assembler.

Compiler : It converts the high level language to low level language (machine language) in one go.

Interpreter : It converts the high level language to low level language (machine language) line by line

(C) Program Development Tools : Whenever we decide to write a program, we need a text editor where we type instructions and store the file as the source code. Then an appropriate translator is used to get the object code for execution. There are software called Integrated Development Environment (IDE) consisting of text editor, building tools and debugger. Python IDLE, NetBeans, Eclipse, Atom, Lazarus are few other examples of IDEs.

Computer System Class 11 Notes

Application Software :

Those software which are designed to perform specific task for the end users are called application software like Microsoft word, Microsoft excel etc. There are again two broad categories of application software—
general purpose and customised application software.

(A) General Purpose Software : The application software developed for generic applications, to cater to a bigger audience in general are called general purpose software. Adobe Photoshop, GIMP, Mozilla web browser, iTunes, etc.

(B) Customised Software : These are custom or tailor-made application software, that are developed to meet the requirements of a specific organisation or an individual. They are designed as per user requirements. for example school management software, accounting software etc.

Computer System Class 11 Notes

Proprietary or Free and Open Source Software :

Those software whose source code is available with an aim to develop and improve further with each other’s help. Such software is known as Free and Open Source Software (FOSS). examples of FOSS include Python, LibreOffice, Open office, Mozilla Firefox.

Sometimes, software are freely available for use but source code may not be available. Such software are called freeware. Examples of freeware are Skype, Adobe Reader.

When the software to be used has to be purchased from the vendor who has the copyright of the software, then it is called proprietary software. Examples of proprietary software include Microsoft Windows, Tally, Quick heal, etc

Computer System Class 11 Notes

Operating System :

An operating system (OS) can be considered to be a resource manager which manages all the resources of a computer, i.e., its hardware including CPU, RAM, Disk, Network.

OS User Interface :

There are different types of user interfaces each of which provides a different functionality. Some commonly used interfaces are :

(A) Command-based Interface : Command-based interface requires a user to enter the commands to perform different tasks like creating, opening, editing or deleting a file, etc Examples of operating systems with command-based interface are MS-DOS and Unix.

(B) Graphical User Interface : Graphical User Interface (GUI) lets users run programs or give instructions to the computer in the form of icons, menus and other visual options. GUI interfaces include Microsoft Windows, Ubuntu, Fedora and Macintosh etc.

(C) Touch-based Interface : Using the touchscreen, a user provides inputs to the operating system, which are interpreted by the OS as commands like opening an app, closing an app, dialling a number, scrolling across apps, etc. Examples of popular operating systems with touch based interfaces are Android and iOS.

(D) Voice-based Interface : Now a days user can use voice-based commands to make a computer work in the desired way. Some operating systems which provide voice-based control to users include iOS (Siri), Android (Google Now or “OK Google”), Microsoft Windows 10 (Cortana).

(E) Gesture-based Interface : Some smartphones based on Android and iOS as well as laptops let users interact with the devices using gestures like waving, tilting, eye motion and shaking.

Computer System Class 11 Notes

Disclaimer : I tried to give you the appropriate and to the point handouts of ” Computer System Class 11 Notes ” , but if you feel that there is/are mistakes in the handouts of “Computer System Class 11 Notes given above, you can directly contact me at Above article on “Computer Science Class 11 Notes” are taken from NCERT Computer Science Book

Computer System Class 11 Notes

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