NCERT Textbook: Computer System | Computer Science for Class 11 - Humanities/Arts PDF Download

 Page 1


1.1 Introduct Ion to c omputer Sy Stem A computer is an electronic device that can be 
programmed to accept data (input), process it and 
generate result (output). A computer along with 
additional hardware and software together is called a 
computer system.  
A computer system primarily comprises a central 
processing unit (CPU), memory, input/output devices 
and storage devices. All these components function 
together as a single unit to deliver the desired output. 
A computer system comes in various forms and sizes. 
It can vary from a high-end server to personal desktop, 
laptop, tablet computer, or a smartphone. 
Figure 1.1 shows the block diagram of a computer 
system. The directed lines represent the flow of data  
and signal between the components. 
1.1.1 Central Processing Unit (CPU)
It is the electronic circuitry of a computer that carries 
out the actual processing and usually referred as the 
brain of the computer. It is commonly called processor 
also. Physically, a CPU can be placed on one or more 
microchips called integrated circuits (IC). The ICs 
comprise semiconductor materials. 
“A computer would deserve 
to be called intelligent if it 
could deceive a human into 
believing that it was human.” 
–Alan Turing
Chapter 1 
Computer System
In this chapter
 » Introduction to 
Computer System
 » Evolution of 
Computer 
 » Computer Memory 
 » Data Transfer 
between Memory 
and CPU
 » Data and 
Information
 » Microprocessors
 » Software
 » Operating System
Figure 1.1: Components of a computer system
Secondary 
Storage Devices
Input 
Device
Control Unit 
(CU)
Arithmetic Logic 
Unit (ALU)
Primary 
Memory
Output 
Device
Central Processing 
Unit (CPU)
Ch 1.indd   1 08-Apr-19   11:36:15 AM
2024-25
Page 2


1.1 Introduct Ion to c omputer Sy Stem A computer is an electronic device that can be 
programmed to accept data (input), process it and 
generate result (output). A computer along with 
additional hardware and software together is called a 
computer system.  
A computer system primarily comprises a central 
processing unit (CPU), memory, input/output devices 
and storage devices. All these components function 
together as a single unit to deliver the desired output. 
A computer system comes in various forms and sizes. 
It can vary from a high-end server to personal desktop, 
laptop, tablet computer, or a smartphone. 
Figure 1.1 shows the block diagram of a computer 
system. The directed lines represent the flow of data  
and signal between the components. 
1.1.1 Central Processing Unit (CPU)
It is the electronic circuitry of a computer that carries 
out the actual processing and usually referred as the 
brain of the computer. It is commonly called processor 
also. Physically, a CPU can be placed on one or more 
microchips called integrated circuits (IC). The ICs 
comprise semiconductor materials. 
“A computer would deserve 
to be called intelligent if it 
could deceive a human into 
believing that it was human.” 
–Alan Turing
Chapter 1 
Computer System
In this chapter
 » Introduction to 
Computer System
 » Evolution of 
Computer 
 » Computer Memory 
 » Data Transfer 
between Memory 
and CPU
 » Data and 
Information
 » Microprocessors
 » Software
 » Operating System
Figure 1.1: Components of a computer system
Secondary 
Storage Devices
Input 
Device
Control Unit 
(CU)
Arithmetic Logic 
Unit (ALU)
Primary 
Memory
Output 
Device
Central Processing 
Unit (CPU)
Ch 1.indd   1 08-Apr-19   11:36:15 AM
2024-25
Computer SCien Ce – Cla SS xi 2
The CPU is given instructions and data through 
programs. The CPU then fetches the program and data 
from the memory and performs arithmetic and logic 
operations as per the given instructions and stores the 
result back to memory.
While processing, the CPU stores the data as well 
as instructions in its local memory called registers. 
Registers are part of the CPU chip and they are limited 
in size and number. Different registers are used for 
storing data, instructions or intermediate results. 
Other than the registers, the CPU has two main 
components — Arithmetic Logic Unit (ALU) and Control 
Unit (CU). ALU performs all the arithmetic and logic 
operations that need to be done as per the instruction in a 
program. CU controls sequential instruction execution, 
interprets instructions and guides data flow through the 
computer’s memory, ALU and input or output devices. 
CPU is also popularly known as microprocessor. We will 
study more about it in section 1.5. 
1.1.2 Input Devices
The devices through which control signals are sent 
to a computer are termed as input devices. These 
devices convert the input data into a digital form that is 
acceptable by the computer system. Some examples of 
input devices include keyboard, mouse, scanner, touch 
screen, etc., as shown in Figure 1.2. Specially designed 
braille keyboards are also available to help the visually 
impaired for entering data into a computer. Besides, we 
can now enter data through voice, for example, we can 
use Google voice search to search the web where we can 
input the search string through our voice.
Data entered through input device is temporarily 
stored in the main memory (also called RAM) of the 
computer system. For permanent storage and future use, 
the data as well as instructions are stored permanently 
in additional storage locations called secondary memory.
1.1.3 Output Devices
The device that receives data from a computer system 
for display, physical production, etc., is called output 
device. It converts digital information into human-
understandable form. For example, monitor, projector,   
headphone, speaker, printer, etc. Some output devices 
Figure 1.2: Input devices
Scanner
Touch Screen
Keyboard
Mouse
Figure 1.3: Output devices
Speaker
Printer
3D printer
Display monitor
Ch 1.indd   2 08-Apr-19   11:36:15 AM
2024-25
Page 3


1.1 Introduct Ion to c omputer Sy Stem A computer is an electronic device that can be 
programmed to accept data (input), process it and 
generate result (output). A computer along with 
additional hardware and software together is called a 
computer system.  
A computer system primarily comprises a central 
processing unit (CPU), memory, input/output devices 
and storage devices. All these components function 
together as a single unit to deliver the desired output. 
A computer system comes in various forms and sizes. 
It can vary from a high-end server to personal desktop, 
laptop, tablet computer, or a smartphone. 
Figure 1.1 shows the block diagram of a computer 
system. The directed lines represent the flow of data  
and signal between the components. 
1.1.1 Central Processing Unit (CPU)
It is the electronic circuitry of a computer that carries 
out the actual processing and usually referred as the 
brain of the computer. It is commonly called processor 
also. Physically, a CPU can be placed on one or more 
microchips called integrated circuits (IC). The ICs 
comprise semiconductor materials. 
“A computer would deserve 
to be called intelligent if it 
could deceive a human into 
believing that it was human.” 
–Alan Turing
Chapter 1 
Computer System
In this chapter
 » Introduction to 
Computer System
 » Evolution of 
Computer 
 » Computer Memory 
 » Data Transfer 
between Memory 
and CPU
 » Data and 
Information
 » Microprocessors
 » Software
 » Operating System
Figure 1.1: Components of a computer system
Secondary 
Storage Devices
Input 
Device
Control Unit 
(CU)
Arithmetic Logic 
Unit (ALU)
Primary 
Memory
Output 
Device
Central Processing 
Unit (CPU)
Ch 1.indd   1 08-Apr-19   11:36:15 AM
2024-25
Computer SCien Ce – Cla SS xi 2
The CPU is given instructions and data through 
programs. The CPU then fetches the program and data 
from the memory and performs arithmetic and logic 
operations as per the given instructions and stores the 
result back to memory.
While processing, the CPU stores the data as well 
as instructions in its local memory called registers. 
Registers are part of the CPU chip and they are limited 
in size and number. Different registers are used for 
storing data, instructions or intermediate results. 
Other than the registers, the CPU has two main 
components — Arithmetic Logic Unit (ALU) and Control 
Unit (CU). ALU performs all the arithmetic and logic 
operations that need to be done as per the instruction in a 
program. CU controls sequential instruction execution, 
interprets instructions and guides data flow through the 
computer’s memory, ALU and input or output devices. 
CPU is also popularly known as microprocessor. We will 
study more about it in section 1.5. 
1.1.2 Input Devices
The devices through which control signals are sent 
to a computer are termed as input devices. These 
devices convert the input data into a digital form that is 
acceptable by the computer system. Some examples of 
input devices include keyboard, mouse, scanner, touch 
screen, etc., as shown in Figure 1.2. Specially designed 
braille keyboards are also available to help the visually 
impaired for entering data into a computer. Besides, we 
can now enter data through voice, for example, we can 
use Google voice search to search the web where we can 
input the search string through our voice.
Data entered through input device is temporarily 
stored in the main memory (also called RAM) of the 
computer system. For permanent storage and future use, 
the data as well as instructions are stored permanently 
in additional storage locations called secondary memory.
1.1.3 Output Devices
The device that receives data from a computer system 
for display, physical production, etc., is called output 
device. It converts digital information into human-
understandable form. For example, monitor, projector,   
headphone, speaker, printer, etc. Some output devices 
Figure 1.2: Input devices
Scanner
Touch Screen
Keyboard
Mouse
Figure 1.3: Output devices
Speaker
Printer
3D printer
Display monitor
Ch 1.indd   2 08-Apr-19   11:36:15 AM
2024-25
Computer Sy Stem 3
are shown in Figure 1.3. A braille display monitor is 
useful for a visually challenged person to understand 
the textual output generated by computers.
A printer is the most commonly used device to get 
output in physical (hardcopy) form. 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. These printers are being 
used in manufacturing industries to create prototypes 
of products. Their usage is also being explored in the 
medical field, particularly for developing body organs.
1.2 e volut Ion of c omputer From the simple calculator to a modern day powerful 
data processor, computing devices have evolved in a 
relatively short span of time. The evolution of computing 
devices in shown through a timeline in Figure 1.4
Figure 1.4: Timeline showing key inventions in computing technology
A punched card is a 
piece of stiff paper that 
stores digital data in 
the form of holes at 
predefined positions.
 
Abacus
Analytic Engine
Pascaline
EDVAC/ENIAC
Tabulating Machine Integrated Circuit
1970
Transistor
1947
1937 500 BC
1945
1642
1890
1834
The Turing machine concept was a 
general purpose programmable 
machine that was capable of solving 
any problem by executing the 
program stored on the punched cards.
Computing is attributed to 
the invention of ABACUS 
almost 3000 years ago. It 
was a mechanical device 
capable of doing simple 
arithmetic calculations only.
Vacuum tubes were 
replaced by transistors 
developed at Bell Labs, 
using semiconductor 
materials.
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.
Herman Hollerith designed 
a tabulating machine for 
summarising the data stored 
on the punched card. It is 
step towards programming.
Blaize Pascal invented a mechanical 
calculator known as Pascal calculator 
or Pascaline to do addition and 
subtraction of two numbers directly 
and multiplication and division through 
repeated addition and subtraction.
John Von Neumann introduced 
the concept of stored program 
computer which was capable of 
storing data as well as program 
in the memory. The EDVAC and 
then the ENIAC computers were 
developed based on this concept.
An Integrated Circuit (IC) is 
a silicon chip which contains 
entire electronic circuit on a 
very small area. The size of 
computer drastically reduced 
because of ICs.
Turing Machine
Ch 1.indd   3 08-Apr-19   11:36:15 AM
2024-25
Page 4


1.1 Introduct Ion to c omputer Sy Stem A computer is an electronic device that can be 
programmed to accept data (input), process it and 
generate result (output). A computer along with 
additional hardware and software together is called a 
computer system.  
A computer system primarily comprises a central 
processing unit (CPU), memory, input/output devices 
and storage devices. All these components function 
together as a single unit to deliver the desired output. 
A computer system comes in various forms and sizes. 
It can vary from a high-end server to personal desktop, 
laptop, tablet computer, or a smartphone. 
Figure 1.1 shows the block diagram of a computer 
system. The directed lines represent the flow of data  
and signal between the components. 
1.1.1 Central Processing Unit (CPU)
It is the electronic circuitry of a computer that carries 
out the actual processing and usually referred as the 
brain of the computer. It is commonly called processor 
also. Physically, a CPU can be placed on one or more 
microchips called integrated circuits (IC). The ICs 
comprise semiconductor materials. 
“A computer would deserve 
to be called intelligent if it 
could deceive a human into 
believing that it was human.” 
–Alan Turing
Chapter 1 
Computer System
In this chapter
 » Introduction to 
Computer System
 » Evolution of 
Computer 
 » Computer Memory 
 » Data Transfer 
between Memory 
and CPU
 » Data and 
Information
 » Microprocessors
 » Software
 » Operating System
Figure 1.1: Components of a computer system
Secondary 
Storage Devices
Input 
Device
Control Unit 
(CU)
Arithmetic Logic 
Unit (ALU)
Primary 
Memory
Output 
Device
Central Processing 
Unit (CPU)
Ch 1.indd   1 08-Apr-19   11:36:15 AM
2024-25
Computer SCien Ce – Cla SS xi 2
The CPU is given instructions and data through 
programs. The CPU then fetches the program and data 
from the memory and performs arithmetic and logic 
operations as per the given instructions and stores the 
result back to memory.
While processing, the CPU stores the data as well 
as instructions in its local memory called registers. 
Registers are part of the CPU chip and they are limited 
in size and number. Different registers are used for 
storing data, instructions or intermediate results. 
Other than the registers, the CPU has two main 
components — Arithmetic Logic Unit (ALU) and Control 
Unit (CU). ALU performs all the arithmetic and logic 
operations that need to be done as per the instruction in a 
program. CU controls sequential instruction execution, 
interprets instructions and guides data flow through the 
computer’s memory, ALU and input or output devices. 
CPU is also popularly known as microprocessor. We will 
study more about it in section 1.5. 
1.1.2 Input Devices
The devices through which control signals are sent 
to a computer are termed as input devices. These 
devices convert the input data into a digital form that is 
acceptable by the computer system. Some examples of 
input devices include keyboard, mouse, scanner, touch 
screen, etc., as shown in Figure 1.2. Specially designed 
braille keyboards are also available to help the visually 
impaired for entering data into a computer. Besides, we 
can now enter data through voice, for example, we can 
use Google voice search to search the web where we can 
input the search string through our voice.
Data entered through input device is temporarily 
stored in the main memory (also called RAM) of the 
computer system. For permanent storage and future use, 
the data as well as instructions are stored permanently 
in additional storage locations called secondary memory.
1.1.3 Output Devices
The device that receives data from a computer system 
for display, physical production, etc., is called output 
device. It converts digital information into human-
understandable form. For example, monitor, projector,   
headphone, speaker, printer, etc. Some output devices 
Figure 1.2: Input devices
Scanner
Touch Screen
Keyboard
Mouse
Figure 1.3: Output devices
Speaker
Printer
3D printer
Display monitor
Ch 1.indd   2 08-Apr-19   11:36:15 AM
2024-25
Computer Sy Stem 3
are shown in Figure 1.3. A braille display monitor is 
useful for a visually challenged person to understand 
the textual output generated by computers.
A printer is the most commonly used device to get 
output in physical (hardcopy) form. 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. These printers are being 
used in manufacturing industries to create prototypes 
of products. Their usage is also being explored in the 
medical field, particularly for developing body organs.
1.2 e volut Ion of c omputer From the simple calculator to a modern day powerful 
data processor, computing devices have evolved in a 
relatively short span of time. The evolution of computing 
devices in shown through a timeline in Figure 1.4
Figure 1.4: Timeline showing key inventions in computing technology
A punched card is a 
piece of stiff paper that 
stores digital data in 
the form of holes at 
predefined positions.
 
Abacus
Analytic Engine
Pascaline
EDVAC/ENIAC
Tabulating Machine Integrated Circuit
1970
Transistor
1947
1937 500 BC
1945
1642
1890
1834
The Turing machine concept was a 
general purpose programmable 
machine that was capable of solving 
any problem by executing the 
program stored on the punched cards.
Computing is attributed to 
the invention of ABACUS 
almost 3000 years ago. It 
was a mechanical device 
capable of doing simple 
arithmetic calculations only.
Vacuum tubes were 
replaced by transistors 
developed at Bell Labs, 
using semiconductor 
materials.
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.
Herman Hollerith designed 
a tabulating machine for 
summarising the data stored 
on the punched card. It is 
step towards programming.
Blaize Pascal invented a mechanical 
calculator known as Pascal calculator 
or Pascaline to do addition and 
subtraction of two numbers directly 
and multiplication and division through 
repeated addition and subtraction.
John Von Neumann introduced 
the concept of stored program 
computer which was capable of 
storing data as well as program 
in the memory. The EDVAC and 
then the ENIAC computers were 
developed based on this concept.
An Integrated Circuit (IC) is 
a silicon chip which contains 
entire electronic circuit on a 
very small area. The size of 
computer drastically reduced 
because of ICs.
Turing Machine
Ch 1.indd   3 08-Apr-19   11:36:15 AM
2024-25
Computer SCien Ce – Cla SS xi 4
The Von Neumann architecture is shown in Figure 
1.5. It consists of a Central Processing Unit (CPU) 
for processing arithmetic and logical instructions, a 
memory to store data and programs, input and output 
devices and communication 
channels to send or receive the 
output data. Electronic Numerical 
Integrator and Computer (ENIAC) 
is the first binary programmable 
computer based on Von Neumann 
architecture. 
During the 1970s, Large Scale Integration (LSI) of 
electronic circuits allowed integration of complete 
CPU on a single chip, called microprocessor. Moore’s 
Law predicted exponential growth in the number 
of transistors that could be assembled in a single 
microchip. In 1980s, the processing power of computers 
increased exponentially by integrating around 3 million 
components on a small-sized chip termed as Very 
Large Scale Integration (VLSI). Further advancement in 
technology has made it feasible to fabricate high density 
of transistors and other components (approx 106 
components) on a single IC called Super Large Scale 
Integration (SLSI) as shown in Figure 1.6. 
IBM introduced its first personal computer (PC) for 
the home user in 1981 and Apple introduced Macintosh 
Figure 1.5: Von Neumann architecture for 
 the computer 
10,000,000,000
1,000,000,000
100,000,000
10,000,000
1,000,000
100,000
10,000
1,000
100
10
1
1940 1950 1960 1970 1980 1990 2000 2010 2020
Intel Microprocessors
Doubles every 2 years
Invention of the
Transistor
4004
8086
286
386
486
486
Pentium II
Pentium IV
 
Pentium III
Core 2 DUO Core i7
Pentium 
Number of Transistors
per Integrated Circuit
Figure 1.6: Exponential increase in number of  transistors used in ICs over time
In 1965, Intel co-
founder Gordon Moore 
introduced Moore’s 
Law which predicted 
that the number of 
transistors on a chip 
would double every two 
years while the costs 
would be halved. 
Ch 1.indd   4 08-Apr-19   11:36:15 AM
2024-25
Page 5


1.1 Introduct Ion to c omputer Sy Stem A computer is an electronic device that can be 
programmed to accept data (input), process it and 
generate result (output). A computer along with 
additional hardware and software together is called a 
computer system.  
A computer system primarily comprises a central 
processing unit (CPU), memory, input/output devices 
and storage devices. All these components function 
together as a single unit to deliver the desired output. 
A computer system comes in various forms and sizes. 
It can vary from a high-end server to personal desktop, 
laptop, tablet computer, or a smartphone. 
Figure 1.1 shows the block diagram of a computer 
system. The directed lines represent the flow of data  
and signal between the components. 
1.1.1 Central Processing Unit (CPU)
It is the electronic circuitry of a computer that carries 
out the actual processing and usually referred as the 
brain of the computer. It is commonly called processor 
also. Physically, a CPU can be placed on one or more 
microchips called integrated circuits (IC). The ICs 
comprise semiconductor materials. 
“A computer would deserve 
to be called intelligent if it 
could deceive a human into 
believing that it was human.” 
–Alan Turing
Chapter 1 
Computer System
In this chapter
 » Introduction to 
Computer System
 » Evolution of 
Computer 
 » Computer Memory 
 » Data Transfer 
between Memory 
and CPU
 » Data and 
Information
 » Microprocessors
 » Software
 » Operating System
Figure 1.1: Components of a computer system
Secondary 
Storage Devices
Input 
Device
Control Unit 
(CU)
Arithmetic Logic 
Unit (ALU)
Primary 
Memory
Output 
Device
Central Processing 
Unit (CPU)
Ch 1.indd   1 08-Apr-19   11:36:15 AM
2024-25
Computer SCien Ce – Cla SS xi 2
The CPU is given instructions and data through 
programs. The CPU then fetches the program and data 
from the memory and performs arithmetic and logic 
operations as per the given instructions and stores the 
result back to memory.
While processing, the CPU stores the data as well 
as instructions in its local memory called registers. 
Registers are part of the CPU chip and they are limited 
in size and number. Different registers are used for 
storing data, instructions or intermediate results. 
Other than the registers, the CPU has two main 
components — Arithmetic Logic Unit (ALU) and Control 
Unit (CU). ALU performs all the arithmetic and logic 
operations that need to be done as per the instruction in a 
program. CU controls sequential instruction execution, 
interprets instructions and guides data flow through the 
computer’s memory, ALU and input or output devices. 
CPU is also popularly known as microprocessor. We will 
study more about it in section 1.5. 
1.1.2 Input Devices
The devices through which control signals are sent 
to a computer are termed as input devices. These 
devices convert the input data into a digital form that is 
acceptable by the computer system. Some examples of 
input devices include keyboard, mouse, scanner, touch 
screen, etc., as shown in Figure 1.2. Specially designed 
braille keyboards are also available to help the visually 
impaired for entering data into a computer. Besides, we 
can now enter data through voice, for example, we can 
use Google voice search to search the web where we can 
input the search string through our voice.
Data entered through input device is temporarily 
stored in the main memory (also called RAM) of the 
computer system. For permanent storage and future use, 
the data as well as instructions are stored permanently 
in additional storage locations called secondary memory.
1.1.3 Output Devices
The device that receives data from a computer system 
for display, physical production, etc., is called output 
device. It converts digital information into human-
understandable form. For example, monitor, projector,   
headphone, speaker, printer, etc. Some output devices 
Figure 1.2: Input devices
Scanner
Touch Screen
Keyboard
Mouse
Figure 1.3: Output devices
Speaker
Printer
3D printer
Display monitor
Ch 1.indd   2 08-Apr-19   11:36:15 AM
2024-25
Computer Sy Stem 3
are shown in Figure 1.3. A braille display monitor is 
useful for a visually challenged person to understand 
the textual output generated by computers.
A printer is the most commonly used device to get 
output in physical (hardcopy) form. 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. These printers are being 
used in manufacturing industries to create prototypes 
of products. Their usage is also being explored in the 
medical field, particularly for developing body organs.
1.2 e volut Ion of c omputer From the simple calculator to a modern day powerful 
data processor, computing devices have evolved in a 
relatively short span of time. The evolution of computing 
devices in shown through a timeline in Figure 1.4
Figure 1.4: Timeline showing key inventions in computing technology
A punched card is a 
piece of stiff paper that 
stores digital data in 
the form of holes at 
predefined positions.
 
Abacus
Analytic Engine
Pascaline
EDVAC/ENIAC
Tabulating Machine Integrated Circuit
1970
Transistor
1947
1937 500 BC
1945
1642
1890
1834
The Turing machine concept was a 
general purpose programmable 
machine that was capable of solving 
any problem by executing the 
program stored on the punched cards.
Computing is attributed to 
the invention of ABACUS 
almost 3000 years ago. It 
was a mechanical device 
capable of doing simple 
arithmetic calculations only.
Vacuum tubes were 
replaced by transistors 
developed at Bell Labs, 
using semiconductor 
materials.
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.
Herman Hollerith designed 
a tabulating machine for 
summarising the data stored 
on the punched card. It is 
step towards programming.
Blaize Pascal invented a mechanical 
calculator known as Pascal calculator 
or Pascaline to do addition and 
subtraction of two numbers directly 
and multiplication and division through 
repeated addition and subtraction.
John Von Neumann introduced 
the concept of stored program 
computer which was capable of 
storing data as well as program 
in the memory. The EDVAC and 
then the ENIAC computers were 
developed based on this concept.
An Integrated Circuit (IC) is 
a silicon chip which contains 
entire electronic circuit on a 
very small area. The size of 
computer drastically reduced 
because of ICs.
Turing Machine
Ch 1.indd   3 08-Apr-19   11:36:15 AM
2024-25
Computer SCien Ce – Cla SS xi 4
The Von Neumann architecture is shown in Figure 
1.5. It consists of a Central Processing Unit (CPU) 
for processing arithmetic and logical instructions, a 
memory to store data and programs, input and output 
devices and communication 
channels to send or receive the 
output data. Electronic Numerical 
Integrator and Computer (ENIAC) 
is the first binary programmable 
computer based on Von Neumann 
architecture. 
During the 1970s, Large Scale Integration (LSI) of 
electronic circuits allowed integration of complete 
CPU on a single chip, called microprocessor. Moore’s 
Law predicted exponential growth in the number 
of transistors that could be assembled in a single 
microchip. In 1980s, the processing power of computers 
increased exponentially by integrating around 3 million 
components on a small-sized chip termed as Very 
Large Scale Integration (VLSI). Further advancement in 
technology has made it feasible to fabricate high density 
of transistors and other components (approx 106 
components) on a single IC called Super Large Scale 
Integration (SLSI) as shown in Figure 1.6. 
IBM introduced its first personal computer (PC) for 
the home user in 1981 and Apple introduced Macintosh 
Figure 1.5: Von Neumann architecture for 
 the computer 
10,000,000,000
1,000,000,000
100,000,000
10,000,000
1,000,000
100,000
10,000
1,000
100
10
1
1940 1950 1960 1970 1980 1990 2000 2010 2020
Intel Microprocessors
Doubles every 2 years
Invention of the
Transistor
4004
8086
286
386
486
486
Pentium II
Pentium IV
 
Pentium III
Core 2 DUO Core i7
Pentium 
Number of Transistors
per Integrated Circuit
Figure 1.6: Exponential increase in number of  transistors used in ICs over time
In 1965, Intel co-
founder Gordon Moore 
introduced Moore’s 
Law which predicted 
that the number of 
transistors on a chip 
would double every two 
years while the costs 
would be halved. 
Ch 1.indd   4 08-Apr-19   11:36:15 AM
2024-25
Computer Sy Stem 5
machines in 1984. The popularity of the PC surged 
by the introduction of Graphical User Interface (GUI) 
based operating systems by Microsoft and others in 
place of computers with only command line interface, 
like UNIX or DOS. Around 1990s, the growth of World 
Wide Web (WWW) further accelerated mass usage of 
computers and thereafter computers have become an 
indispensable part of everyday life.
Further, with the introduction of laptops, personal 
computing was made portable to a great extent. This 
was followed by smartphones, tablets and other 
personal digital assistants. These devices have leveraged 
the technological advancements in processor 
miniaturisation, faster memory, high speed data and 
connectivity mechanisms. 
The next wave of computing devices includes 
the wearable gadgets, such as smart watch, lenses, 
headbands, headphones, etc. Further, smart appliances 
are becoming a part of the Internet of Things (IoT), by 
leveraging the power of Artificial Intelligence (AI).
1.3 c omputer m emory A computer system needs memory to store the data and 
instructions for processing. Whenever we talk about the 
‘memory’ of a computer system, we usually talk about the 
main or primary memory. The secondary memory (also 
called storage device) is used to store data, instructions 
and results permanently for future use.
1.3.1 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. Further, these 
bits are grouped together to form words. A 4-bit word 
is called a Nibble. Examples of nibble are 1001, 1010, 
0010, etc. A two nibble word, i.e., 8-bit word is called a 
byte, for example, 01000110, 01111100, 10000001, etc.
Like any other standard unit, bytes are grouped 
together to make bigger chunks or units of memory. 
Table 1.1 shows different measurement units for digital 
data stored in storage devices.
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