This course is intended to 2nd Year university students

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CHAPTER TWO
ORGANIZATION OF COMPUTER SYSTEM
GALANA B.

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2.1 COMPUTER SYSTEM
A Computer system is a group of hardware components and associated software,
designed and assembled to perform a specific function or group of functions.
System is an organized relationship among functioning units or components. The
purpose of computer system is to solve a problem by processing data into
information.
In general, a complete computer system includes four distinct parts: Data, Users,
Hardware and Software.
GALANA B.

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DATA
Data are raw facts which the computer can manipulate and process into information that is useful
to people. Computer data is digital, or in the form of digits or numbers. The computer reads and
stores all data as numbers.
Data can be semi information that need to be processed more and generates complete information.
Information is the output of the computer that can be used by human beings to do some decisions.
There are many types of data, some of them are:
Text data: consists of standard alphabets (A – Z, a – z), numbers (0 -9), special characters (%, $, &,
+, * - -).
Graphic data: consists of stick pictures such as drawings, graphs, photographs etc.
Audio data: Include any music & voice.
Video data: consists of motion pictures such as moving clip, pictures of a conference etc.
GALANA B.

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CONT.…
Users
The users are the people operating the computers.
Hardware
The term hardware embraces the physical components of the system that you can touch and fill:
Software
Software comprises the programs that tell the hardware what to do. A program is a sequence of
instructions stored in the memory of the computer system. The central processing unit fetches an
instruction, decodes it and then executes the required operation (e.g. to add two numbers).
When an instruction has been executed the next instruction is fetched, decoded and executed,
etc. A program may be very simple, for example, to calculate the average of ten numbers, or very
complex, as would be required to draw a television quality picture on a display screen.

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Computer Hardware
Hardware: is the physical part of a computer that you can see and touch with our hand. A hard ware
composed of different units. These are: Input device, Central Processing Unit (CPU), Output device and
Storage unit
ALU Control Unit
Registers
Primary Memory
ROM
INPUT UNIT OUTPUT UNIT
SECONDARY STORAGE DEVICE
CENTRAL PROCESSING UNIT

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INPUT UNIT:
An input device can be defined as an electromechanical device that allows the user to feed
information (data) into the computer for analysis, storage, and to give commands to the computer.
Data and instruction are entered into the computer’s main memory through an input device.
Input device captures information and translates it into a form that can be processed and used by
other parts of the computer. After processing the input data, the computer provides the result with the
help of output devices.
Input devices play a major role in the processing of any data via the computer system because the
output of the computer is always based on the given input.

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CONT.
The commonly used input devices are: Keyboard, Mouse, Scanner and Digital
camera
Key board: is the most common data entry device, and has different shapes and size.
The keys on the keyboard are often classified as follows
 Functional key: assigned specific commands by the current application. E.g. F1-help, F2-Setup.
 Alphanumeric keys (typing keys): Letters and numbers.
 Punctuation keys: comma, period, semicolon, etc.
 Special keys: Caps Lock, Spacebar, Shift, and Enter etc.
 Control keys: these keys provide cursor and screen control. E.g. Arrow keys, Home, Page down etc.

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CONT.
In detail the following are descriptions of the keyboard.
Escape key - Generally used to abort some actions.
Function keys - functions vary from program to program.
Enter key - used to enter commands and to create blank lines in a document.
Backspace key - used to move the cursor backwards, deleting backward.
Tab key - used to access tab stop.
Caps lock key - works like the shift lock. - Key on a typewriter.
Shift key - used to produce upper case letters and sometimes to invoke commands
Control key & Alt key - commonly used in combination with other keys to enter commands e g. Control home.
Insert key - used to enter characters.
Arrow keys - used to move the cursor around the display screen.
Delete keys - used to delete characters.
Number lock keys - used to activate the numeric keypad.
Print screen keys - used to print the contents of the screen.

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MOUSE:
Mouse: is a small hand-held pointing device, which is rectangular-shaped with a rubber ball
embedded at its lower side and buttons on the top. The rubber ball used to move the cursor and the
button used to initiate to do some action, and it has two buttons, the right button and left button.
Right click - used to produce a ‘pop-up’ menu.
Left click - carries out an action, such as starting an application.
Types of mouse
 Mechanical - has a rubber or metal ball on its underside that can roll in all directions.
 Mechanical sensors within the mouse detect the direction the ball is rolling and move the screen pointer
accordingly.
 Opto-mechanical - same as mechanical mouse, but uses optical sensors to detect motion of the
ball.
 Optical: it doesn’t have mechanical moving part and uses a laser to detect the mouse’s
movement.

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SCANNER
Scanner: is a device that used to convert hard copy in to soft copy form.
Used to scan or read text and picture and converted them to computer usable form.
Save the scanned image as a graphic file in the computer.
Digital Camera: Stores images digitally rather than recording them on a film.
Once a picture has been taken, it can be downloaded to a computer system and then
manipulated with an image editing software.

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2. CENTRAL PROCESSING UNIT (CPU)
The CPU or the microprocessor (or simply processor) is referred as the brain of computer system. It
controls all internal and external devices, and performs arithmetic and logical operations. The processor
operates only on binary data that is composed of 1’s and 0’s corresponding to electrical switches ON or
OFF. It performs, supervises, and controls the arithmetic operations (Addition, subtraction,
multiplication, division) and logical functions (such as ‘is A greater than B’?), primary storage, or main
memory, provides the temporary locations inside the computer where the data and instructions are
stored while processed.
 The functions of the processor can be summarizing as:
Carrying out arithmetic and logical functions
Controlling the use of main storage (memory) to store data and instructions.
Controlling the sequence of operations
Controlling all the parts of the computer system. BY: GALANA B..

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CONT.

The CPU consists of three main subsystems. These are: Control unit, Arithmetic and Logical unit
and Registers.
These three sub systems work together to provide the operational capabilities of the computer. As buses
carry people from one place to another place, the system bus carries information from one unit to another.
System bus is a collection of wires through which data is transmitted from one unit to another in
computer system (CPU, memory, and I/O devices). It is further divided in to three logical units, namely
the address bus, data bus, and control bus.
Data Bus: it transfers the actual data between the processor, memory, and the I/O devices.
Address Bus: It informs the CPU about the location of the data residing in the memory.
Control Bus: It is responsible for making CPU, memory, and I/O devices work together as a functional
system, carrying signals that report the status (ready, not ready) of various units.

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CONTROL UNIT
The control unit can be thought of as the heart of the CPU. It controls the I/O
devices and transfer of data to and from the primary storage. It reads and interprets
instructions that retrieve from the main memory. It controls the flow of instructions
from memory to CPU or from ALU to registers. The control unit repeats a set of four
basic operations: fetching, decoding, executing, and storing.
Fetching: is the process of obtaining a program instruction or data item from memory.
Decoding: is the process of translating the instruction in to commands the computer
can execute.
Executing: is the process of carrying out the commands.
Storing: is the process of writing the result to memory.

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CONT.
Generally, it performs all the control functions of the computer.
It retrieves the instruction from memory.
Translates those instructions into computer functions and sends signals to other
computer hardware units to carry out those functions.
It is also responsible for determining the next instruction to be executed by the
computer.
In general, it serves as the .
computer traffic cope

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ARITHMETIC LOGICAL UNIT
As the name suggests, the arithmetic logical unit carries out arithmetic and
logical operations on the data made available to it. For simple
understanding, the ALU can be divided in to arithmetic unit and logical unit.
Arithmetic Unit: contains the circuitry that is responsible for performing the
actual computing and carrying out the arithmetic calculations, such as
addition, subtraction, multiplication, and division. It can perform these
operations at a very high speed.
Logical Unit: the importance of the logical unit is the ability it provides to the
CPU to make logical operations based on the instructions provided to it.
Logical unit uses statements such as AND, OR, and NOT. This is useful when
you have a set of instructions to execute only if certain conditions are true.

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REGISTERS:
The registers are special purpose, high speed temporary memory unit. They hold
varies types of information such as data, instructions, addresses, and the intermediate
results of calculations. Essentially, they hold the information that the CPU is
currently working on. Registers can be thought of as CPU’s working memory. As
the size of the registers increase, the computer processing activities also
increase. To execute an instruction, the control unit receives it from the main
memory and places in to the register.
 Registers are paths or conduits that connect the Arithmetic Logical Unit to the main memory.
 When an instruction loaded from main memory, it is placed first in the register to wait instructions
from the control unit.
 Data are also stored in registers prior to execution in the ALU

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3. OUTPUT DEVICES(UNITS):
Peripheral devices which used to convert machine readable information in to human
readable form. Display the processed data in hard copy form/soft copy form.
Hard copy is the physical form of output (paper form). But soft copy is the electronic
version of an output, which usually resides in computer memory or on disk. Soft copy
is not tangible (cannot be touched).
 The most common used output devices are: Printers, Plotters, Monitors, Projectors,
and Speakers
Printer: prints information and data from the computer on to paper. Printers are
divided in to impact printers and non-impact printers.

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CONT.
Plotters: is a pen based output device that is attached to a computer for making vector graphics.
It is used to draw high –resolution charts, graphs, maps, circuit diagrams.
They are mainly used for computer aided design (CAD) and computer aided manufacturing (CAM)
applications such as printing out plans of houses or car parts.
These are also used with programs like AUTO CAD (Computer Assisted Drafting) to give graphic
outputs.
Monitor: is the most frequently used soft copy output device. It used to display the processed
data.
Projectors: are output devices, which are used to project information from a computer on to a large
screen. There are two types of projectors, LCD (Liquid crystal display) and DLP (digital light
processing)
Speaker: is an output device, which is used to magnify sound

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4. STORAGE DEVICE
Devices used to store data or program files. There are two types of storage devices.
Primary storage devices
Secondary storage devices

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CONT.
Primary storage devices: are used to store data that are processed and executed immediately. An example of primary storage device is a Memory.
Memory is the electronic holding place for instructions and data where the computer’s microprocessor can reach quickly. CPU requires memory to handle the intermediately
results and to store the final output. The primary memory (storage device) also implemented by two types of memory technologies. Random Access Memory (RAM) and Read
Only Memory (ROM).
Random Access Memory (RAM):- directly provides the required information to the processor. RAM can be defined as a block of sequential memory locations, each of which has a
unique address determining the location and those locations contain a data element. It stores programs and data that are in active use. It is volatile in nature, which means the
information stored in it remains as long as the power is switched ON. As soon as the power is switched OFF, the information contained in it vanishes (lost). In RAM we can write
data on it and read data from it. RAM also divides further into two.
o Dynamic Random Access Memory: It holds the data in dynamic (keeping on refreshing) manner with the help of a refresh circuitry.
o Static Random Access Memory: Along with DRAM is essential for a system to run optimally, because it is very fast as compared to DRAM. It retains the data as long as power is
provided to the memory chips. It does not need to be ‘refreshed’ periodically.
Features of RAM
 We can write data on it and read data from it.
 It is volatile in nature, means the information stored in it lost, if a power suddenly failure.
 It stores data and instruction temporarily.

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CONT.
Read Only Memory (ROM): This type of primary memory can only be read, not written. In other words, CPU can only read
from any location in the ROM but cannot write.
Features of ROM
The contents of ROM are not lost even in case of a sudden power failure, making it non-volatile in
nature.
It is also random access in nature.
It reads only, but cannot write.
It contains permanent recorded instruction.
Cache Memory: is a piece of very fast memory, made from high speed static RAM that reduces
the access time of the data. It is very expensive and generally incorporated in the processor, where
valuable data and program segments are kept. This enables the processor to access data quickly
whenever it is needed. The cache facilitates the system to catch up with the processors speed.

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CONT..
B. Secondary storage device: are devices used to store data file and program files
permanently.
Secondary storage devices are designed as a backup to the main storage, and they
store data on a long-term basis.
Benefits of secondary storage devices are:
Non-volatile - it does not lose its content even when its power is turn of.
Capacity – they can store large amount of data.
Cost - it is less expensive than Primary storage device.
Portability – they can be easily ported from one computer to another.

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CONT.
Example:
Floppy disk- is a direct access secondary device that used to store information. It is cheap, small in size,
portable and compact. It read and written by a floppy disk drive.
Hard disk- a smooth metal plate coated on both sides with a thin film of magnetic material. Permanently fixed
within a system unit. It has high storage capacity than floppy disk, and it is a random access storage media.
CD/DVD: these types of storage media are removable optical disks used to store data. The first one (CD) is
that in which the data once written cannot be erased. It is stored permanently over it. While the second type
of the CD’s or the DVD’s are called as the rewritable; where in the data that is once written can be erased
completely and the same storage device can be used again for storing the different data.
Flash disk: is a secondary storage device used to read and write on it like floppy disk. It has higher storage
capacity, and more reliable than floppy, but it is more expensive.
The internal component of the computer that used to control, integrate, and manage the hardware components
and to accomplish specific tasks. There are two components of software.
BY: GALANA B..

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Computer Software:
The computer software is collection of set of instructions that instructs the
computer hardware. These set of instructions perform different functions; some deals
with handling the computer hardware and the others perform user specific tasks such
as managing a database. Therefore, the computer software is divided into two major
categories depending upon the nature of work this software does: -
1. System software
2. Application Software

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THE FOLLOWING FIGURE SHOWS THE CLASSIFICATION OF THE COMPUTER SOFTWARE: -

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2.2.1 SYSTEM SOFTWARE
The system software handles the computer hardware so that the user can operate the
computer with little knowledge about the computer hardware. For example, the user is
not concerned where the file is saved on the hard disk. This is the responsibility of the
system software to check the hard disk and space on the free space of the hard disk.
The system software is further divided as operating system software and language
software and Utility software.

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OPERATING SYSTEM SOFTWARE
An operating system is a program that acts as the link between you, the computer’s
software and hardware resources. It is the foundation software onto which all other
application programs are loaded. It is the master program of your system that allows
you, the user, to run application programs (Word Processors, Spreadsheets, Gamed, etc.)
Without the operating system the application software would be unable to function.
Operating System software manages most of your computer’s activities, including the
allocation of computer resources, maintenance of files, and running of application
software. As a file manager the operating system is used to name, save, retrieve and
maintain program and data files you create and use on your computer. As a resource
manager, the operating system controls the flow of information through your computer.
Data accepted by the keyboard, seen on the display, or sent to a printer are all under the
control of the operating system.

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CONT.
To list some of the general things that the operating system does in the computer, we
can come up with the following:
Managing storage media
Booting
User Interface
Managing computer resources
Managing files
Managing tasks.

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PROGRAMMING LANGUAGE SOFTWARE (DEVELOPMENT
SW)
 Programming languages – are used to write programs. Programming languages can generally be classified as low-level
languages and high-level languages.
Low Level Programming Languages
Under low-level language, we have machine languages and assembly languages.
Machine language is the natural language of the computer. A program written in machine language is
ready for immediate execution. Machine language is very difficult to understand. A program written in
machine language takes the form of strings of zeros and ones. Even the simplest program is complex and
lengthy. Moreover, each computer type has its own machine language and thus a program written in
machine language for one computer may not run in another computer, i.e. machine language programs
are not portable.
Assembly language consists of English-like abbreviations. It is easier to understand than machine
language. Programs written in assembly language need to be translated into machine language before
they can be executed. The program that translates assembly language programs to machine language is
called an assembler.

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HIGH LEVEL PROGRAMMING LANGUAGES
High-level languages are English-like instructions. They are easier to learn than
low-level languages. Programs written in high-level language are called source
codes and they must be translated into machine language before they can run on a
computer. After the translation, the program in machine language is called object
code.
There are three types of translators; assembler, interpreter, and compiler.
Compiler - A set of programs that translate the source code of higher-level software
languages into machine language, or object code
 Advantages - fast
 Disadvantages - harder to write and debug
 Examples - Pascal, C, FORTRAN, and open script

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CONT.
2. Assembler - An assembler is a software tool for translating assembly language into machine language
3. Interpreter - An interpreter translates source code instructions, one instruction at a time, from a higher level
language such as BASIC into machine language
Advantages - fairly easy to write, and easy to debug
Disadvantage - SLOW
Examples - basic, lisp

Utility Software: is a program used to support, enhance, expand, and secure existing programs and data in the
computer system. Most common functions of system utilities include:
Back up: sometimes data files can get corrupt, or get accidentally deleted. In such a case, data backups become very
useful. A backup system utility is essential for those organizations that want to keep their data intact.
Data recovery: used to recover data. Since, disk drives or other hardware may fail, these utilities are essential to
recover data.
Virus protection: Antivirus scans for the hard disk for any kind of virus.
Disk management: includes defragmenting disks, data compression software, and formatting disk tools.

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2.2.2 APPLICATION SOFTWARE:
Software refers to the detailed set of instructions that direct the computer to perform a certain
task. Application software is software developed to do user specific tasks such as word processing,
spreadsheet and database management. It is developed using the language software. Application
software is a set of related programs designed to carry out certain tasks that fulfill users’ specific
needs.
Examples of application software are:
Microsoft-Word (used for writing letters, reports, or other documents).
Microsoft-Excel (used to perform calculations)
Microsoft-Access (used for storing information)
Internet Explorer (enables a user to access an internet) …. etc.

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THANK YOU.
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