Anatomy of a Digital Computer

1.1 Introduction
We should keep in mind that a computer is a programmable machine. The two
main characteristics of a computer are: (i) it responds to a specific set of
instructions in a well-defined manner. (ii) It can execute a pre-recorded list of
instructions (a program). Modern computers are electronic and digital. The
actual machinery - wires, transistors and circuits is called hardware; the
instructions and data are called software. All general purpose computers require
the following hardware components:
(i) Central Processing Unit (CPU): The ‘brain’ of the computer, the
component that actually executes instructions.
(ii) Memory : It enables a computer to store, at least temporarily, data and
programs.
(iii) Input device : Usually a keyboard or mouse is used to read data and
programs into the computer.
(iv) Output device: A display screen, printer, etc. that lets you see what the
computer has accomplished.
(v) Mass storage device: It allows a computer to permanently store large
amounts of data. Common mass storage devices include disk drive and
tape drive.
In addition to these components, many others make it possible for the basic
components of a computer to work together efficiently.
1.2 Objectives
After going through this lesson, you will be in a position to:
l identify the basic components of a computer and their working
l explain the importance of various units of a computer
l learn how these units work together to accomplish a given job
1.3 Functions and Components of a Computer
To function properly, the computer needs both hardware and software. Hardware
consists of the mechanical and electronic devices, which we can see and touch.
The software consists of programs, the operating system and the data that reside
in the memory and storage devices.
A computer does mainly the following four functions:
l Receive input —Accept data/information from outside through various
input devices like the keyboard, mouse, scanner, etc.
l Process information—Perform arithmetic or logical operations on data/
information.
l Produce output—Communicate information to the outside world through
output devices like monitor, printer, etc.
l Store information—Store the information in storage devices like hard
disk, floppy disks, CD, etc.
These four basic functions are responsible for everything that computers do.
The hardware components of the computer specialize in any one of these
functions.
Computer hardware falls into two categories: processing hardware and the
peripheral devices. The Processing hardware consists of the Central Processing
Unit (CPU), and as its name implies, is where the data processing is done.
Peripheral devices allow people to interact with the CPU. Together, they make
it possible to use the computer for a variety of tasks.

Central Processing Unit (CPU)
This part of the computer that executes program instructions is known as the
processor or Central Processing Unit (CPU). In a microcomputer, the CPU is
based on a single electronic component, the microprocessor chip, within the
system unit or system cabinet. The system unit also includes circuit boards,
memory chips, ports and other components. A microcomputer’s system cabinet
will also house disk drives, hard disks, etc., but these are considered separate
from the CPU.
The CPU has two parts —The Control Unit (CU) and the Arithmetic Logic Unit
(ALU). In a microcomputer, both are on a single microprocessor chip.
Control Unit (CU)
The control unit tells the rest of the computer system how to carry out a program’s
instructions. It directs the movement of electronic signals between memory -
which temporarily holds data, instructions and processes information - and the
ALU. It also directs these control signals between the CPU and input/output
devices.
Arithmetic - Logic Unit (ALU)
Arithmetic Logic Unit, usually called the ALU, performs two types of operations
- arithmetical and logical. Arithmetical operations are the fundamentalmathematical operations consisting of addition, subtraction, multiplication and
division. Logical operations consist of comparisons. That is two pieces of data
are compared to see whether one is equal to, less than, or greater than the other.
Memory
Memory - also known as the primary storage or main memory - is a part of the
microcomputer that holds data and instructions.
Part of the contents of the memory is held only temporarily, that is, it is stored
only as long as the microcomputer is turned on. When you turn the machine off,
the contents are lost. The capacity of the memory to hold data and program
instructions varies in different computers. The original IBM PC could hold
approximately several thousand characters of data or instructions only. But
modern microcomputers can hold millions or even billions of characters in their
memory.
Registers
Computers also have several additional storage locations called registers. These
appear in the Control Unit and ALU and make processing more efficient.
Registers are a sort of special hi-speed storage areas that hold data and
instructions temporarily during processing. They are parts of the Control Unit
and ALU rather than the memory. Their contents can, therefore be handled much
faster than the contents of the memory.
Addresses
To locate the characters of data or instructions in the main memory, the computer
stores them in locations known as addresses. A unique number designates each
address. Addresses can be compared to post office mailboxes. Their numbers
remain the same, but contents continuously change.
How the CPU and Memory work
Various steps involved for multiplying two numbers is shown in that figure and explained below.
1. The control unit recognizes that the program has been loaded into the
memory. It begins to execute the first step in the program.
2. The program tells the user, "Enter 1st Number".
3. The user types the number 100 on the keyboard. An electronic signal is
sent to the CPU.
4. The control unit recognizes this signal and routes the signal to an address
in memory - say address 7.
5. After completing the above instruction, the next instruction tells the user,
"Enter 2nd Number."
6. The user types the number 4 on the keyboard. An electronic signal is
sent to the CPU.
7. The control unit recognizes this signal and routes it to memory address
8.
8. The next program instruction is executed - "Multiply 1st and 2nd
Numbers."
9. To execute this instruction, the control unit informs the ALU that two
numbers are coming and the ALU is to multiply them. The control unit
next sends to the ALU a copy of the contents of address 7 (100) and
address 8(4).
10. ALU performs the multiplication : 100 × 4 = 400
11. The control unit sends a copy of the multiplied result (400:) back to
memory to store it in address 9.
12. The next program instruction is executed : "Print the Result."
13. To execute this instruction, the control unit sends the contents of the
address 9 (400) to the monitor.
14. Monitor displays the value 400.
15. Final instruction is executed: "End". The program is complete.

1.4 Input devices
An input device presents data to the processing unit in a machine-readable form.
Although the keyboard is a common input device for a small computer, a system
may also support various other input devices such as Optical Character
Recognition (OCR), Magnetic Ink Character Recognition (MICR), mark sense
reader, etc.
1.4.1 Key board
The keyboard is very much like a standard typewriter keyboard with a few
additional keys. The basic QWERTY layout of characters is maintained to make
it easy for the trained typist to use the system. The additional keys are included
to perform certain special functions such as loading a program, edition a text,
etc. These are known as function keys that vary in number from system to system.
1.4.2 Optical Character Recognition
Often abbreviated as OCR, optical character recognition involves reading text
from paper and translating the images into a form that the computer can
manipulate. An OCR system enables you to take a book or a magazine article
and feed it directly into an electronic computer file.
1.4.3 Magnetic Ink Character Recognition (MICR)
An MICR can identify characters printed with a special ink that contain particles
of magnetic material. This device particularly finds applications in banking
industry. Since the MICR system can recognise only certain character styles,
the characters have to be accurately formed.
1.4.4 Optical Mark Recognition (OMR)
Optical mark recognition, also called mark sense reader, is a technology where
an OMR device senses the presence or absence of a mark, such as pencil mark.
OMR is widely used in tests such as aptitude tests.
1.4.5 Bar Code Reader
These devices are generally available in super markets, bookshops, etc. Bar-code
readers are photoelectric scanners that read the bar codes or vertical zebra striped
marks, printed on product containers. Supermarkets use a bar code system called
the Universal Product Code (UPC). The bar code identifies the product to the
supermarket's computer which has a description and the latest price of the
product. The computer automatically tells the Point of Sales (POS) terminal what
the price is.
1.4.6 Digitiging Tablet
This is an input device that enables you to enter drawings and sketches into a
computer. A digitiging tablet consists of an electronic tablet and a cursor or
pen. A cursor (also called a puck) is similar to a mouse, except that it has a
window with cross hairs for pinpoint placement, and it can have as many as 16
buttons. A pen (also called a stylus) looks like a simple ballpoint pen but uses
an electronic head instead of ink. The tablet contains electronic field that enables
it to detect movement of the cursor or pen and translate the movements into
digital signals that it sends to the computer. Digitiging tables are also called
digitizers, graphics tables, touch tables or simply tables.
1.4.7 Scanner
Scanner is an input device that can read text or illustrations printed on paper and
translate the information into a form that the computer can use. A scanner works
by digitizing an image - dividing it into a grid of boxes and representing each
box with either a zero or a one, depending on whether the box is filled in. The
resulting matrix of bits, called a bit map, can then be stored in a file, displayed
on a screen and manipulated by programs. Optical scanners do not distinguish
text from illustrations, they represent all images as bit maps. Therefore, you
cannot directly edit text that has been scanned. To edit text read by an optical
scanner, you need an optical character recognition (OCR) system to translate
the image into ASCII characters. Most optical scanners sold today come with
OCR packages.
1.4.8 Mouse
Mouse is a device that controls the movement of the cursor or pointer on a display
screen. It is a small object you can roll along a hard and flat surface. As you
move the mouse, the pointer on the display screen moves in the same direction.
Mouse contains at least one button and sometimes as many as three, which have
different functions depending on what program is running.
1.4.9 Light Pen
Light pen is an input device that utilizes a light-sensitive detector to select
objects on a display screen.
1.4.10 Speech input devices
Speech or voice input devices convert a person's speech into digital form. These
input devices, when combined with appropriate software, form voice recognition
systems. These systems enable users to operate microcomputers using voice
commands.