Computer System Class 11 Notes

Teachers and Examiners (CBSESkillEduction) collaborated to create the Computer System Class 11 Notes. All the important Information are taken from the NCERT Textbook Computer Science (083) class 11.

Computer System Class 11 Notes

Introduction to Computer System

A computer is an electronic device that can be programmed to accept data (input), process it and generate result (output).

A computer system’s main components include a central input/output devices, memory, and the processor unit (CPU). and storage mechanisms. Each of these parts is functional. together to produce the desired result as a single unit.

components of a computer system
Central Processing Unit (CPU)

All instructions that a computer’s hardware and software send to it are handled by the CPU. The Arithmetic Logic Unit (ALU) and the Control Unit are the two fundamental parts of the CPU. ALU does all necessary arithmetic and logical operations in accordance with programme instructions.

Input Devices

Input devices are the devices used to transmit control signals to a computer. These tools transform the input data into a digital format that the computer system can use. Example – Keyboard, Mouse, Scanner, Microphone etc.

Output Devices

An output device is a device that receives data from a computer system for display to the users. Output device  transforms digital data into forms that people can understand. Example – Monitor, Speaker, Projector, Headphone etc.

Evolution of Computer

  1. 500 BC (Abacus) – Computing is attributed to the invention of ABACUS almost 3000 years ago. It was a mechanical device capable of doing simple arithmetic calculations only.
  2. 1942 (Pasealine) – Blaize Pascal created the Pascaline, a mechanical calculator that does multiplication and division by repeatedly adding and subtracting two numbers, as well as direct addition and subtraction of two numbers.
  3. 1834 (Analytic Engine) – Charles Babbage invented analytical engine, a mechanical computing device for inputting, processing, storing and displaying the output, which is considered to form the basis of modern computers.
  4. 1890 (Tabulating Machine) – A tabulating device was created by Herman Hollerith to summarise the information on the punched card. It is regarded as the beginning of programming.
  5. 1937 (Turing Machine) – Turing’s machine that could solve any problem by running the code written on punched cards was known as a general-purpose, programmable machine (GPPC).
  6. 1945 (EDVAC/ENIAC) – The idea of a stored programme computer, which could store both programmes and data in memory, was first proposed by John Von Neumann. Based on this idea, the EDVAC and later the ENIAC computers were created.
  7. 1947 (Transistor) – Vacuum tubes were replaced by transistors developed at Bell Labs, using semiconductor materials.
  8. 1970 (Integrated Circuit) – A silicon chip called an integrated circuit (IC) has a whole electronic circuit on a relatively small surface. ICs significantly reduced the size of the computer.
Von Neumann architecture for the computer

The central processing unit (CPU), often known as a processor or core, the main memory, and a connection between the memory and the CPU make up the von Neumann architecture. Each of the regions that make up main memory is able to store both data and instructions.

Computer Memory

A computer system needs memory to store the data and instructions for processing.

Units of Memory

Binary numbers are used by computers to store and process data. Bits refer to the fundamental building blocks of memory, the binary digits 0 and 1. These pieces are also combined to create words. A Nibble is an 8-bit word.

  • KB (Kilobyte) 1 KB = 1024 Bytes
  • MB (Megabyte) 1 MB = 1024 KB
  • GB (Gigabyte) 1 GB = 1024 MB
  • TB (Terabyte) 1 TB = 1024 GB
  • PB (Petabyte) 1 PB = 1024 TB
  • EB (Exabyte) 1 EB = 1024 PB
  • ZB (Zettabyte) 1 ZB = 1024 EB
  • YB (Yottabyte) 1 YB = 1024 ZB
Types of Memory

Computers have two types of memory — primary and secondary.

Primary Memory – The area of the computer where current data, programmes, and instructions are stored is referred to as primary memory or main memory. Because the primary memory is located on the motherboard, data from the primary memory may be read and written very quickly. Primary memory have two types

  1. Random Access Memory (RAM) and
  2. Read Only Memory (ROM).

Cache Memory – RAM is faster than secondary storage, but not as fast as a computer processor. So, because of RAM, a CPU may have to slow down. To speed up the operations of the CPU, a very high speed memory is placed between the CPU and the primary memory known as cache.

Secondary Memory – secondary memory to permanently 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. But, it cannot be accessed directly by the CPU.

Data Transfer between Memory and CPU

Both the CPU and primary memory, as well as primary and secondary memory, require data transfers. Data are transferred between different components of a computer system using physical wires called bus.

Bus is of three types —

  1. Data bus – to transfer data between different components,
  2. Address bus – to transfer addresses between CPU and main memory.
  3. Control bus – to communicate control signals between different components of a computer.
system bus


A microprocessor is a small electronic component within a computer that performs a variety of arithmetic, logical, and data processing-related activities. A modern microprocessor is constructed on an integrated circuit that contains millions of small parts, including resistors, transistors, and diodes.

memory size
First1971-73LSI4 / 8 bit1 KB108 KHz200 KHzSingleIntel 8080
Second1974-78LSI8 bit1 MBUpto 2 MHzSingleMotorola 6800 Intel 8085
Third1979-80VLSI16 bit16 MB4 MHz – 6 MHzSingleIntel 8086
Fourth1981-95VLSI32 bit4 GBUpto 133 MHzSingleIntel 80386 Motorola 68030
Fifth1995 till
VLSI64 bit64 GB533 MHz – 34 GHzMulticorePentium, Celeron, Xeon
Generations of Microprocessor
Microprocessor Specifications

Microprocessors are classified on the basis of different features which include chip type, word size, memory
size, clock speed, etc. These features are briefly explained below:

Word Size – Word size is the maximum number of bits that a microprocessor can process at a time. Earlier, a word was of 8 bits and now it is 64 bits.

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. As word size increased to 64 bits.

Clock Speed – The quantity of pulses produced by the clock within a computer every second is known as the clock speed. The clock speed reveals how quickly a computer can carry out instructions. Measurement of clock speed in Hertz (Hz), Kilohertz (kHz), or Gigahertz (GHz).

Cores – The CPU’s core is its fundamental computational unit. Earlier processors could only handle one task at a time since they only had one computing unit. The introduction of multicore processors has made it feasible for the computer to carry out numerous tasks at once, improving system performance.


As opposed to a microprocessor, which has only a CPU on the chip, a microcontroller is a compact computing device that has a CPU, a defined amount of RAM, ROM, and other peripherals all incorporated on a single chip. As an example, a fully automatic washing machine uses its microcontroller to automatically manage the washing cycle.

block diagram of microcontroler

Data and Information

Everything which is meaningful is known as data by a computer system, including instructions, photographs, songs, films, documents, etc. Raw, unorganised facts can also be considered to be data, which is processed to produce valuable information. There are three types of data –

  1. Structured Data
  2. Un-Structured Data
  3. Simi Structured Data

Structed Data – Data that is simple and complies to a specific record structure structured data is data that is easy to understand. Such information along a data file may contain tabular information in a predetermined format.

structured data

Un-Structured Data – Unstructured data refers to data that are not organised into a predetermined record format. Examples include text papers, pictures, social media posts, satellite photographs, audio and video files, and more.

unstructured data

Simi Structured Data – Semi-structured data refers to data that lacks a clear structure but nevertheless maintains internal tags or markers to distinguish data items. Examples include email, HTML pages, CSV files, and documents with comma-separated values.

semi-structured data

What is Software?

The software consists of a set of instructions that produce the desired result. In other words, every piece of software is created with computation in mind. Operating systems like Ubuntu or Windows 7/10, word processors like LibreOffice or Microsoft Word all are software.

Depending on the mode of interaction with hardware and functions to be performed, the software can be broadly classified into three categories viz.

  1. System software,
  2. Programming tools
  3. Application software.

System Software – System software is the software that directly interacts with a computer’s hardware to offer the fundamental functionality needed to run the device. A system software programme can use and operate various computer hardware parts. for example operating systems, system utilities, device drivers, etc.

  • Operating System – The operating system is a system software that operates the computer for example, Windows, Linux, Macintosh, Ubuntu, Fedora, Android, iOS, etc.
  • System Utilities – Software used for maintenance and configuration of the computer system is called system utility. for example, anti-virus software, disk cleaner tool, disk compression software, etc.
  • Device Drivers – The device driver acts as an interface between the device and the operating system for example, language translator, a device driver acts as a mediator between the operating system and the
    attached device. 

Programming Tools – In order to complete some work we have to provide the computer with  guidelines that are applied to the computer to achieve the desired result.  Languages for computers are created  for creating these guidelines. for example, Compilers, Interpreter, assemblers, and code editors.

  • Classification of Programming Languages – Assembly language or High level language was created to make writing code easier by allowing the use of English-like phrases and symbols in place of 1s and 0s. 
  • Language Translators – A translator is required to translate programmes written in assembly or high level language to machine language which helps to understand easily.
  • Program Development Tools – A text editor is necessary every time we decide to build a programme. A text editor is a piece of software that enables us to write instructions in a text file and save it as the source code.

Application Software – Application software is a type of computer programme that carries out a specified task for the user. for example, Microsoft Suite. Office, Excel, Word, PowerPoint, chrome, safari etc. There are again two broad categories of application software — general purpose and customised application software.

  • General Purpose Software – General purpose software refers to application software created for generalist applications in order to serve a larger audience generally. Such ready-made application software can be used by end users as per their requirements. For example, spreadsheet tool Calc of LibreOffice.
  • Customised Software – These are specialised applications that have been created to satisfy the needs of a particular business or person. School management software, accounting software, etc. are a few examples of customised software.

Proprietary or Free and Open Source Software

Open source software is a free software and is made available to anyone who requests it, and programmers are free to view or modify it without permission of developer. For example, the source code of operating system Ubuntu is freely accessible for anyone with the required knowledge to improve or add new functionality. More examples of Open Source Software are Python, Libreoffice, Openoffice, Mozilla Firefox, etc.

Operating System

A platform between the user and the computer is the operating system. An operating system (OS) is a piece of software that controls all other application programmes in a computer after being originally loaded by a boot programme. Examples of popular OS are Windows, Linux, Android, Macintosh and so on. 

OS User Interface

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

  • Command-based Interface – A user using a command-based interface must type the commands to carry out various activities, such as creating, opening, editing, or deleting a file. MS-DOS and Unix are two examples of operating systems with a command-based interface.
  • Graphical User Interface – Through the use of icons, menus, and other visual options, the Graphical User Interface (GUI) enables users to run programmes or provide commands to the computer. Microsoft Windows, Ubuntu, Fedora, Macintosh, and other operating systems are examples of systems with graphical user interfaces.
  • Touch-based Interface – The two most well-known operating systems featuring touch-based user interfaces are iOS and Android. Touch-based interfaces on touchscreen devices are likewise supported by Windows 8.1 and 10.
  • Voice-based Interface – The user having a special needs and those who want to use computers or cellphones while completing other tasks, they can use voice – based interface software. like iOS (Siri), Android (OK Google or Google Now), Microsoft Windows 10 (Cortana), and so forth.
  • Gesture-based Interface – Some Android- and iOS-based smartphones, as well as some laptops, give the facilities to interact with the devices via gestures including shaking, tilting, and eye movements.
Functions of Operating System

Essential functions and duties that an operating system does for the management of the computer system are –

Process Management – Process management includes numerous activities such as process creation, scheduling, process termination, and a deadlock. A process is an active programme, and they play an essential role in modern operating systems. The OS must allot resources to allow processes to communicate and share data.

Memory Management – Memory management involves controlling how much of the system’s primary memory is used by various programmes while monitoring whether or not each and every location in the memory is free or occupied.

File Management – In a computer system’s secondary storage, data and applications are kept as files. In the secondary memory, file management entails the creation, updating, deletion, and protection of these files.

Device Management – There are numerous hardware and I/O devices attached to a computer system. These interdependent heterogeneous devices are managed by the operating system. The device driver and associated software for a specific device are in communication with the operating system. Additionally, the operating system must offer options for setting a specific device so that it can be used by a user or another device.

Logical Gate

Logic gate is used to design electronic circuit, Logic gate operation are based on one or more input and produce a single output. The most common types of logic gates are AND gate, OR gate, NOT gate, NAND gate, NOR gate, XOR gate, and XNOR gate. These gates are help to build digital circuits and make decisions making on the input signals.

  1. AND gate: It outputs “1” if and only if all of its inputs are “1”.
  2. OR gate: It outputs “1” if any of its inputs are “1”.
  3. NOT gate: It inverts the input signal. If the input is “1”, the output is “0”, and vice versa.
  4. NAND gate: It is the negation of AND gate. It outputs “0” if and only if all of its inputs are “1”.
  5. NOR gate: It is the negation of OR gate. It outputs “0” if any of its inputs are “1”.
  6. XOR gate: It outputs “1” if exactly one of its inputs is “1”.
  7. XNOR gate: It is the negation of XOR gate. It outputs “1” if all of its inputs are the same.

De Morgan’s laws

De Morgan’s laws are two fundamental boolean algebraic rules that describe how conjunction (AND) and negation (NOT) relate to disjunction (OR). These principles, which bear the name Augustus De Morgan after the British mathematician, offer a means to simplify boolean expressions.

According to the first law, the disjunction (OR) of two propositions’ negations is equal to the negation of a conjunction (AND) of two propositions. This can be written mathematically as:

According to the second law, the conjunction (AND) of the negations of two propositions is equal to the negation of an OR between them. This can be written mathematically as:

Logic Circuits

A logic circuit is a digital circuit that generates an output by carrying out one or more logical operations on binary inputs. Basic logic gates like AND gate, OR gate, NOT gate, NAND gate, NOR gate, XOR gate, and XNOR gate are used in various combinations to achieve more complicated functions in logic circuits.

Many different functions, including data processing, information storing and decision-making can be carried out by logic circuits. Logic circuits are employed in digital electronics to regulate data flow and carry out actions on binary signals.

A key component of computer engineering and electronics, logic circuit design and implementation is utilised in a wide range of contemporary technologies, including control systems, computer hardware and telecommunications .

Number System

Different bases or radix points are used in number systems to represent numbers. The four most widely used number systems are hexadecimal, octal, binary, and binary.

  • Binary: Only the digits 0 and 1 are used in the base-2 number system known as binary to represent numbers. Electronics that use digital frequently use it.
  • Octal: It is a base-8 number system that represents numbers with eight digits, ranging from 0 to 7.
  • Decimal is a base-10 number system that represents numbers using ten digits, ranging from 0 to 9. It is the number system that is most frequently utilised in daily life.
  • Hexadecimal is a base-16 number system that represents numbers with sixteen digits (0 to 9) and letters (A to F). It is frequently employed when programming computers.

Encoding schemes

In computers and other electronic devices, characters, symbols, and words are represented using encoding systems. The encoding systems available are ASCII, ISCII, and UNICODE.

  • ASCII: The American Standard Code for Information Interchange is a 7-bit encoding technique. The 26 letters of the alphabet, 10 numerals, as well as additional punctuation and control characters, total 128 characters.
  • ISCII: Indian Standard Code for Information Interchange, or ISCII, is an 8-bit encoding system. It symbolises the alphabetic symbols found in Indian languages, such as Bengali and Devanagari.
  • UNICODE: Characters from practically all writing systems throughout the world are represented by the multi-byte encoding standard known as UNICODE. UTF-8 and UTF-32 are just two of the implementations that are included.
  • UTF-8: A minimum of 8 bits are used in the variable-width encoding technique known as UTF-8. Both ASCII and the entire UNICODE character set are supported, and it is backwards compatible with ASCII.
  • UTF-32: A fixed-width encoding method called UTF-32 employs 32 bits per character. Although it supports the entire UNICODE character set, UTF-8 is more effective in terms of storage and transmission.
Computer Science Class 11 Notes
Computer Science Class 11 MCQ
Computer Science Class 11 NCERT Solutions