This document provides an overview of computer systems. It defines a computer as a device that transforms data into meaningful information by accepting, storing, processing, and retrieving data. The main components of a computer system are described as the central processing unit (CPU), main memory, input/output devices, and bus. The CPU contains a control unit, arithmetic logic unit, and registers and is responsible for executing instructions. Memory is used to store programs and data and comes in various forms like RAM, ROM, disk, and registers. The bus carries instructions, data, addresses, and commands between these components.

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Overview of Computer System:
 What is a Computer?
o Computer is a device that transforms data into meaningful
information. Data can be anything like marks obtained by you in
various subjects. It can also be name, age, sex, weight, height, etc.
of all the students in a class. Computer can also be defined in
terms of functions it can perform. A computer can i) accept data, ii)
store data, iii) process data as desired, and iv) retrieve the stored
data as and when required and v) print the result in desired
format. The major characteristics of a computer are high speed,
accuracy, diligence, versatility and storage.
Figure: Why we need computer?
Type your text
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Figure: Use of Computer
 What does computer do?
o Computer collect data and does processing on the data. After the
processing data is concerted to processing. Information is
meaningful output of processed data.
Figure: Basic task of Computer
Computer
Data
Process
 It is an electronic device used to:
 Store
 Retrieve and process data
 Unprocessed data, figures and stats
 Conversion of raw data into useful information
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 Data processing:
Figure: Data processing
 How does computer work?
o A Computer is a machine which solves problems for people written
as programs.
o A Program is a sequence of statements/steps stating how to
perform a task. For each step an arithmetic and logical operation is
done. For each operation a different set of control signals is
needed – i.e. an instruction. An instruction can be machine
language instructions
o Or assembly language instructions or even high level language
instructions.
o A hardware is something that is tangible. For e.g. CPU, Memory,
I/O devices, Bus etc.
Input (data) Processing Output (Information)
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 Main components of a computer system
Figure: main components of computer system
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Hardware Components :
Figure: hardware components
Computer main components:
o Central Processing Unit (CPU)
o Main Memory
o Input / Output devices
o Bus
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Block diagram of computer is below:
Here we will study about CPU and memory. In the second part we will learn about
I/O system.
o CPU: CPU is the part of the computer that performs the bulk of
the data processing operations. Its main function is to execute
programs stored in the main memory by fetching the instructions,
examining them and executing them. The ALU and the CU of a
computer system are jointly known as the central processing unit
(CPU). You may call CPU as the brain of any computer system.
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 Parts of CPU
o Control Unit ( CU )
o Arithmetic and Logical Unit (ALU)
o Registers
o Control Unit: The process of input, output, processing and storage
is performed under the supervision of a unit called 'Control Unit'. It
decides when to start receiving data, when to stop it, where to
store data, etc. It takes care of step -by-step processing of all
operations inside the computer.
o The control unit fetches program instructions from main memory,
decodes these instructions and sends control signals to other units
of a computer system. The execution of a machine instruction
consists of a sequence of steps by accessing the main memory as
and when necessary to read data.
ALU
o The major operations performed by the ALU are addition,
subtraction, multiplication, division, logic and comparison.
REGISTERS
o The registers store intermediate data used during the execution of
the instructions. We know that memory locations are needed for
storing pointers, counters, return addresses, temporary results,
partial products etc. But having to refer to memory locations for
o Applications is time consuming as memory access is very time
consuming. It is more convenient and more efficient to store these
intermediate values in registers inside the processor. When a large
no. of registers are included in the CPU, it is efficient to connect
them through a common bus system.
Von Neumann architecture - characteristics
o One processor
o Use of stored programs
o Sequential processing of instructions
o Single Instruction, Single Data stream (SISD) mode
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Fetch-decode-execute cycle
o Fetch Phase
o Decode Phase
o Execute Phase
Measures of CPU performance
o MIPS (Million instructions par second)
o Clock Speed(How fast does clock move of processor)
o FLOPS (Floating point operations execution par second)
 Memory
o Place to store programs and data
o Stored as bits/bytes/words.
o A bit is binary digit taking a value of either 0 or 1.
o A byte is collection of 8 bits.
o A word is collection of 16, 32 or 64 bits.
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Categories
o Primary Memory
o Secondary Memory
o Internal Memory
Figure: Memory Types
Primary Memory:
o Implemented in high speed devices located outside CPU
o Costly and volatile
o Holds program contents currently being executed & small amount
of data
Example: RAM
Secondary Memory:
o Low speed
o Non-volatile
o Holds program not currently being executed
o Holds large data
Memory
Seconday
memory
Main
memory
RAM ROM
Cache
memory
Internal External
Registes
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Examples: Disk, Tape
Internal Memory:
o Implemented in very high-speed devices located within CPU
o Holds currently execution instructions and few data related items
o Very expensive
o Examples: Registers
 Bus
o Parallel wires that carry several bits at a time Carries instructions,
data, addresses or commands unidirectional or bi-directional. It is a
ribbon like set of parallel wires that can carry several bits at the
same time.
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Major Categories
o – Data bus
o – Address bus
o – Control bus
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