The document details the basic structure of computers, covering aspects such as computer hardware, instruction set architecture, and computer organization. It describes the five functional units of a computer, including input, memory, arithmetic and logic, output, and control units, as well as different types of memory. Additionally, it distinguishes between system software and application software, explaining the functions and classifications of operating systems.

1. By P Laxmi Devi

2. Basic Structure of Computers
Computer Architecture in general covers three aspects of computer
design namely:
Computer Hardware, Instruction set Architecture and Computer
Organization. Computer hardware consists of electronic circuits,
displays, magnetic and optical storage media and communication
facilities.
Instruction set Architecture is programmer visible machine interface
such as instruction set, registers, memory organization and exception
handling.
Two main approaches are mainly CISC (Complex Instruction Set
Computer) and RISC (Reduced Instruction Set Computer)
Computer Organization includes the high level aspects of a design, such
as memory system, the bus structure and the design of the internal CPU.
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3. Functional Unit
A computer in its simplest form comprises five functional units
namely input unit, output unit memory unit, arithmetic & logic unit and
control unit. Figure 2 depicts the functional units of a computer
system.
1. Input Unit: Computer accepts encoded information through input
unit. The standard input device is a keyboard. Whenever a key is
pressed, keyboard controller sends the code to CPU/Memory.
Examples include Mouse, Joystick, Tracker ball, Light pen, Digitizer,
Scanner etc.
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5. 2. Memory Unit: Memory unit stores the program instructions
(Code), data and results of computations etc.
Memory unit is classified as:
1. Primary /Main Memory
2. Secondary /Auxiliary Memory
Primary memory is a semiconductor memory that provides
access at high speed. Run time program instructions and
operands are stored in the main memory.
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6. Primary Memory
Primary/ Main Memory is classified as :
1. ROM - Read Only Memory and
2. RAM - Random Access Memory
ROM is a non- volatile memory as it never loses its contents
and that can hold data even if computer is turned off and it
holds system programs and firmware routines such as BIOS,
POST, I/O Drivers that are essential to manage the hardware of
a computer.
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7. Some of the types of ROM
1. BIOS-ROM – Basic Input Output System Read Only Memory
2. PROM – Programmable Read Only Memory
3. EPROM – Erasable Programmable Read Only Memory
4. EEPROM – Electrically Erasable Programmable Read Only Memory
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8. RAM is termed as Read/Write memory or user memory that holds run
time program instruction and data. While primary storage is
essential, it is volatile in nature and expensive.
Some of the types of RAM are:
1. DRAM - Dynamic Random-Access Memory
2. EDORAM – Extended Data Out Random-Access Memory
3. VRAM – Video Random-Access Memory
4. SRAM – Static Random-Access Memory
5. SDRAM – Synchronous Dynamic Random-Access Memory
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9. Secondary/ Auxiliary Memory
Secondary memory refers to storage devices, such as hard
drives and solid state drives. It may also refer to removable
storage media, such as USB flash drives, CDs, and DVDs.
Additionally, secondary memory is non-volatile, meaning it
retains its data with or without electrical power.
Secondary memory has four categories like Magnetic Storage,
Optical Storage, Flash storage, and Online Cloud System.
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11. Hard disk
A Hard Drive is the hardware component that stores all of your digital
content. Your documents, pictures, music, videos, programs, application
preferences, and operating system represent digital content stored on a
hard drive. Hard drives can be external or internal.
Function of hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-
mechanical data storage device that stores and retrieves digital data
using magnetic storage and one or more rigid rapidly rotating platters
coated with magnetic material.
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12. Hard disk - 1 TB HDD (SATA)
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13. Magnetic Tape
Magnetic tape data storage is a system for storing digital information
on magnetic tape using digital recording.
The device that performs the writing or reading of data is called a tape
drive.
Magnetic tape is type of non-volatile memory uses thin plastic ribbon is
used for storing data and as data use to be stored on ribbon so data
read/write speed is slower due to which is mainly used for data
backups. On other hand Magnetic Disk is also type of non-volatile
memory uses circular disk used for storing data
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14. Magnetic Tape
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15. Floppy Disk
The Floppy Disk Drive, also known as Diskette, is a
removable magnetic storage medium that allows
recording of data. IBM first introduced it as a 8-in diskette
in 1971
Today, the most commonly used floppy disks are 3.5
inches and have the capacity of 800 KB to 2.8 MB (with a
standard of 1.44 MB).
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16. Floppy Disk
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17. Optical Disk
Optical storage is any storage type in which data is written
and read with a laser. Typically, data is written to optical
media, such as compact discs (CDs) and DVDs.
Optical media is more durable than tape, HDDs and flash
drives and less vulnerable to environmental conditions.
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18. Optical Disk
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19. 3. Arithmetic and logic unit: ALU consist of necessary logic
circuits like adder, comparator etc., to perform operations of
addition, multiplication, comparison of two numbers etc.
4. Output Unit: Computer after computation returns the
computed results, error messages, etc. via output unit.
The standard output device is a video monitor, LCD/TFT
monitor. Other output devices are printers, plotters etc.
5. Control Unit: Control unit co-ordinates activities of all
units by issuing control signals. Control signals issued by
control unit govern the data transfers and then appropriate
operations take place. Control unit interprets or decides the
operation/action to be performed.
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20. 6. Central Processing Unit (CPU):
The computer's central processing unit (CPU) is the portion
of a computer that retrieves and executes instructions.
The CPU is often simply referred to as the processor. The
ALU performs arithmetic operations, logic operations, and
related operations, according to the program instructions.
A central processing unit (CPU), also called a central
processor, main processor or just processor, is the electronic
circuitry that executes instructions comprising a computer
program. The CPU performs basic arithmetic, logic,
controlling, and input/output (I/O) operations specified by
the instructions in the program
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21. Types of CPU
● Single-core CPU. It is the oldest type of CPU which is
available and employed in most of the personal and official
computers.
● Dual-core CPU. It is a single CPU that comprises of two
strong cores and functions like dual CPU acting like one.
● Quad-core CPU.
● Hexa Core processors.
● Octa-core processors.
● Deca-core processor
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22. CPU has four basic functions to perform a task:
● Fetch. The first step of a CPU is to fetch instructions from
the program memory.
● Decode. After fetching information CPU will determine
what to do with that data next, this step is the decode step.
● Execute.
● Arithmetic Logic Unit (ALU)
● Control Unit (CU)
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23. Software
Software is the programs and routines for a computer or the
program material for an electronic device which make it run.
An example of software is Excel or Windows or iTunes.
Computer software is typically classified into two major types of
programs:
1. System software and
2. Application software.
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24. 1. System software is software designed to provide a
platform for other software.
Examples of system software include operating systems like
macOS, Linux, Android and Microsoft Windows, computational
science software, game engines, industrial automation, and
software as a service applications.
2. Application software, or app for short, is software that
performs specific tasks for an end-user.
For example, Microsoft Word or Excel are application software,
as are common web browsers such as Firefox or Google
Chrome.
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25. Operating System (OS)
An Operating System (OS) is an interface between a computer user and
computer hardware. An operating system is a software which performs
all the basic tasks like file management, memory management, process
management, handling input and output, and controlling peripheral
devices such as disk drives and printers.
Every desktop computer, tablet, and smartphone includes an operating
system that provides basic functionality for the device.
Common desktop operating systems include Windows, OS X, and Linux.
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26. Functions of an Operating System
An operating system has three main functions:
(1) manage the computer's resources, such as the central
processing unit, memory, disk drives, and printers,
(2) establish a user interface, and
(3) execute and provide services for applications software.
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27. Classifications of an Operating System
● Multiuser OS: In a multiuser OS, more than one user can
use the same system at a same time through the multi I/O
terminal or through the network.
● Multiprocessing OS
● Multiprogramming OS
● Multitasking OS
● Multithreading
● Batch Processing
● Online Processing
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