LESSON 2: INTRODUCTION TO COMPUTING (2)
WHAT
ARE COMPUTERS ?(2)
RANDOM ACCESS
MEMORY (RAM)
Think of a computer
as a human brain. Your brain is a memory sponge. It contains a lifetime
of memories that cause us to act or react based on inputs. Inputs
come through our 5 senses. If you see and smell hot suya burning on
the grill (inputs) you know how to react based on previous experiences
(memory - " I HAVE TO EAT O!).
A set of miniaturized
circuits which represents the working memory of the computer. This
is where application programs (software) can be loaded from the outside
and then executed. The larger the RAM the better. A typical single
user computer system may contain approximately 8,000,000 bytes of
RAM.
(This is often abbreviated as 8MB RAM.)
The computer's
brain consists of the RAM and the CPU. The CPU and RAM work together
as the computer's "brain". Each day when we start up the
computer one of the first tasks will be to fill RAM with instructions
to give it an ability to do work. This work may be in the creation
of documents or tracking accounting data.
You control which
instructions will go into the computer's brain. You control the sets
of experiences you will provide the computer. Once in RAM, the computer
will evaluate inputs from many devices and react. The most typical
input device is a keyboard. As you type commands, the computer evaluates
them. Based on the set of instructions within its RAM, it will follow
some action: print a document, calculate, send information over a
telephone line, etc.
At some later
point you may empty the computer's brain and install a different set
of instructions, thus giving it a different ability.
The RAM is emptied when the computer is turned off - thus it is often
called "volatile" memory. WHAT YOU SEE ON YOUR COMPUTER
MONITOR IS ACTUALLY IN RAM ONLY - a temporary storage location. To
make it permanent we "save" it or "write" it to
a floppy disk or hard drive. These devices are non volatile storage
- they do not require electricity.
Each time the
computer is turned off, all information within RAM is lost. RAM is
called volatile memory because of the electricity requirement.
READ ONLY
MEMORY (ROM)
This is a special
section of memory that contains instructions which are activated each
time the computer is turned on. These instructions are set at the factory
and cannot be changed - thus, they can only be "read", not
written to. ROM
instructions perform equipment checks and initialization of the computer
prior to each use.
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DISKS
Think of disks
as cassettes. You can record information on a cassette that can be
replayed indefinitely and if desired, recorded over. Floppy and Hard
Disks operate in a similar fashion. We record (Save) something we
have created - like a document - onto the disk. Then, hours, days,
or months later we can play back (Retrieve) the document into the
computer to alter or print out.
The magnetic disk used to store information works in a manner similar
to a tape recorder - magnetic impressions are placed on the tape and
can be later replayed. A magnetic computer disk works in the same
fashion but spins in a circle like a music record rather than moving
in a straight line like recording tape.
Magnetic computer
disks are available in two basic types: floppy and hard disks. Just
like cassettes, the Floppy and Hard Disks do not require electricity
to retain their information. Hard Disks and Floppy Disks are similar.
However, Hard Drives have a larger capacity for file storage, are
faster and are less likely to fail due to the protected environment
from within which they operate. Floppy and Hard Disks are nonvolatile
in nature because they will retain their information without the aid
of electricity.
A hard disk can hold considerably more information than a floppy disk
- frequently billions and millions of computer words (or "bytes")
while a floppy disk holds less than a million in many cases. However
what the floppy disk loses in capacity in gains in the advantage of
portability since it can easily be removed from the PC and stored
which is not true of the hard disk.
When you format
a disk you ask the computer to inspect the magnetic surface of the
disk for any errors, prepare it for use by future data and create
an index "file allocation table (FAT)" which is like a card
index for a large library of books. Formatting a disk is a little
like taking a blank piece of paper and using a pencil and ruler to
turn it into graph paper with both horizontal and vertical lines.
What was blank before now has little cells or file drawers which can
hold information.
DISK DRIVE
The port in which
a floppy disk is inserted. This device "reads data from a magnetic
disk, and copies data into the computer's memory (RAM) so it can be
used by the computer, and that "writes" data from the computer's
memory onto a disk so it can be stored for later use. Each Disk Drive
is labeled A,B,C, etc. because we often must tell the computer which
drive has the disk with the information or where to send the information.
A Disk Drive reads and writes on a 5.25 inch or 3.5 inch floppy disk.
FLOPPY DISKS
The most commonly
used mass storage device. Allows entering programs to RAM and saving
data from RAM. Will hold data even after the computer is turned off.
Data on these disks is stored in concentric rings called tracks. The
Disk surface is a thin piece of mylar and is coated with a magnetized
material similar to audio or video tape.
The read/write
heads can magnetize and demagnetize the coated surface repeatedly.
Therefore, the Disk can be used, erased, and reused indefinitely.
Floppy disks are also available as double density and high density
format. A standard floppy diskette is either 5D inches or 3A inches
square. Obviously the high density of 3A" diskette contains more
information than the 3A" double density diskette. A 5D"
Double-sided, Double density disk holds approximately 360k worth of
information (250 double spaced pages of text). The smaller 3.5 inch
Double density disks which hold at least twice as much - 720k.
Working with
floppy diskettes.
To insert a floppy diskette into your computer drive, first remove
it from the paper or plastic slipcover if one protects it. The proper
way to insert a floppy diskette in most drives is as follows.
For larger 5 - 1/4 inch floppies, turn the printed label side up and
locate the TWO VERY TINY notches along one edge. Near the notches
will be a jelly bean shaped hole about one inch long cut into the
plastic surface of the diskette. This oblong hole is the read/write
opening. Insert the diskette into the drive with the label side up
and the two tiny notches FIRST into the drive opening then close the
drive locking handle. Along one edge of the diskette you will also
see a SINGLE square shaped hole which is the write protect notch.
If this write
protect notch is UNCOVERED you can BOTH read and write data to the
diskette. If the write protect notch is covered with a piece of tape,
then you can READ information from the diskette but you CANNOT write
information to the diskette. This is a safeguard feature you may wish
to use from time to time. Keep fragile diskettes away from smoke,
hair, dirt and ESPECIALLY sources of magnetism such as motors, loudspeakers
or even children's magnetic toys which may ERASE your data!
For smaller 3
- 1/2 inch size diskettes, turn the label side up and locate the metal
"shutter". Insert the diskette into the drive with the label
up and the shutter FIRST into the drive. The write protect notch or
opening is a small square hole with a SLIDING PLASTIC TAB which is
slid CLOSED (cannot see an open hole) to enable BOTH reading and writing
to the diskette. The sliding tab is placed OPEN (visible open hole)
to enable reading but NOT writing.
FIXED DISK
DRIVE
Usually named
disk drive C. It is essentially a very large floppy disk. This Fixed
Disk (commonly called a Hard Drive) is secured within the machine
and cannot be seen or transported. The storage capacity is so large
it is measured in megabytes (1M = 1K squared = 1,048,576 bytes). Fixed
Disks are available from 5M on up. The main advantages are that it
has enough space to meet most users' total storage needs, operates
much faster than a floppy (5-10 times faster), and is less likely
to fail since it "lives" within the protected computer.
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INPUT DEVICES
Since we have
covered data storage lets move to data input.
(a) PRIMARY INPUT DEVICES
Two primary input devices are key to getting data into a PC. The keyboard
and the mouse.
(i) KEYBOARD
Input device that lets you enter data into the computer. The layout
is similar to the standard QWERTY typewriter keyboard. However, there
are many extra special keys that are defined by the software you are
running.
(ii) MOUSE
Hand operated pointing and selection device which serves as alternate
input to the keyboard. It is very useful for Graphical User Interface
(GUI) Applications such as Windows, etc., which is rolled or moved
across the desktop to position a cursor or pointer on the computer
screen.
The mouse also contains several buttons to help select items on data
on the monitor screen. A mouse was initially an optional device, but
it is becoming difficult to work without it, with the spread of Windows
based systems.
(b)
SPECIALTY INPUT DEVICES
Lets move on to the specialty input devices like the scanner and the
Digital camera.
(i) SCANNER
A scanner converts text and images to digital information. This text
and images can be from a variety of sources such as magazines, photographs,
articles, scientific diagrams, etc. The scanner creates a digital
image from your photograph or drawing, for use in graphics, DeskTop
Publishing or Presentation applications.
There are different types of scanners like Hand scanner, flatbed scanner
and the multifunction scanner/fax/printer/copier. The flatbed scanner
provides a larger scanning area than the other scanners and is the
usually more expensive.
(ii) DIGITAL
CAMERA
The Digital Camera produces the same result as a Scanner. Any pictures
taken are transferred straight to the computer, i.e. in the form of
a graphics image suitable for image editing or DeskTop Publishing
applications. It eliminates the need for film.
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OUTPUT
DEVICES
Another introductory
topic is that of output devices such as a monitor, printer or plotter.
(i) PLOTTER
A plotter is a
device which uses a motor to move pens or drawing implements in tightly
controlled horizontal and vertical motions on a piece of paper or
film. The computer can control a plotter to combine on one piece of
paper differing pen colors and text and pictures stored within the
computer. Computer plotter can be purchased with flat table or flat
bed configurations or in models which move the pen(s) back and forth
with gears that also drive the paper movement at the same time.
(ii) PRINTER
The printer is
probably the most common and useful output device attached to your
computer. There are many types of modern computer printer with differing
speeds and capabilities. The most common printer is the Dot
matrix printer which provides characters made up from tiny dots of
ink on paper. Line printers (usually with Mainframe computers or Minicomputers)
print entire lines of text in one sweep then move to the next line
and are thus very fast. Ink jet printers produce characters made from
individual dots of ink sprayed onto the paper.
The ink jet printer squirts individual dots of ink onto the paper
to form letters or other characters. A high quality paper is necessary
since the wet ink can smear if not carefully handled. Although with
the most recent models, ordinary paper can also be used. The Colour
print facility is also now standard with most inkjet printer.
Finally, laser printers use a rapidly scanning laser to sensitize
a polished drum with an entire page of information quickly and look
and work roughly like an office copier. The first two printers are
classified as impact printers since something strikes the paper while
the later two are non impact printers.
The laser and
ink jet printers are becoming more popular due to rapid speed of printing
and quiet mode of operation.
The laser printer is used for quickly producing one page of text at
a time. In operation, the laser scans a polished drum with an image
which is then dusted with dark toner particles which stick to the
exposed areas made sensitive by the laser. Paper is then placed in
contact with the drum and the toner is transferred to the page and
is finally fused with heat to "fix" or seal the toner particles
to the page.
Of the Microcomputer printers, the Laser is the most expensive in
terms of purchase price, maintenance cost and consumable cost.
Dot matrix printers
are common and affordable alternatives for many small offices, home
computer hobbyists or organizations with voluminous printing requirements
(e.g. statements of accounts for banks). The Dot matrix is additionally
designed for use with continuous flow paper, as well as typical single
sheet paper.
Dot matrix usually operates in varying modes of draft and letter quality.
In draft mode, the printer speed is faster, with draft quality. Letter
quality is slower with higher quality.
Dot matrix printers produce letters via small pins which strike the
ink ribbon and paper to produce print which can be jagged looking.
Nine pin dot matrix printers produce somewhat rough looking letters
while 24 pin dot matrix printers produce crisper, fully-formed letters.
The Dot matrix printer strikes the paper through a ribbon to transfer
ink to the printed page.
Connecting a printer
via a cable to the computer is always done through one of two plugs
(or interfaces) on the back of the computer. One type of interface
(computer plug) is serial, the other called parallel.
The most commonly used interface for printers today is the parallel
interface but serial interface printers do exist. What is the difference?
Recall that there are eight bits (computer dots and dashes) to a byte
(or computer word). The serial interface has each bit sent one at
a time to the printer - like men in single file at the supermarket
checkout counter. The parallel interface sends all eight bits at once
- like eight men all entering eight supermarket counters at once.
Each interface is different, the printer manufacturer will tell you
which interface to use, i.e. serial or parallel.
Frequently, modems or mouse devices use the serial interface leaving
the printer to the parallel interface.
(iii) MONITORS
We have talked
about output to paper, next let's briefly discuss output to a monitor
or screen. The monitor is a television like device that the computer
uses to communicate with you. The monitor or video display works much
like your television - some older home computers still use a TV. An
old term for a monitor is the cathode ray tube or CRT. Monitors differ
in the sharpness or resolution they can display. On the low end of
the resolution spectrum is the monochrome (single color) monitor frequently
available in either green or amber screens. Next is the color RGB
monitor (RGB stands for Red, Green and Blue) which displays low resolution
color dots to make up an image.
Higher resolution is obtained with an EGA monitor (Enhanced Graphics
Adapter) and still higher with a VGA (Video Graphics Array) Monitor
and even higher with an SVGA (Super Video Graphics Array) Monitor.
Each monitor is matched to work with a circuit card located within
the body of the computer. One way to upgrade a computer is to switch
both the monitor and display/graphics circuit card to produce a sharper,
more colorful image. The dots which make up all images on the monitor
screen are called pixels. The smaller the pixels, the higher and sharper
the image resolution.
Typically the monitor displays 80 columns (characters) by 25 rows
(lines) of information. The initial SVGA cards could only display
16 colours. And then 256 colours. Now some SVGA card can display millions
of colours.
INPUT &
OUTPUT DEVICES
Certain devices can act as both input and output devices to the computer.
Typical devices mentioned earlier are the disks (floppy and hard).
(i) MODEM
Short for Modulator/Demodulator.
A device to send and receive computer output over telephone lines.
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THE COMPUTER DEVICES & RELATIONSHIPS
CASING &
MOTHERBOARD
Where is your CPU kept? Don't look for RAM near your mouse. Most of
the components are internal, and kept inside a casing. This casing
model can be Tower, minitower or desktop. Inside the casing, there
is a power supply unit that takes in the power supplied from the public
power supply and steps it down to supply the computer's needs. Also
inside the casing is the motherboard, which is a large printed circuit
board that all expansion boards plug into.
The motherboard
contains the most essential parts of the computer such as the CPU,
RAM, ROM, keyboard, speaker and power connections, and other assortment
of important parts.
The expansion boards contain special circuits for the monitor (monitor
card), disk drives and mouse (multi Input/Output card) and other options
such as modem and scanner.
TUTORIAL QUESTIONS
(WHAT ARE COMPUTERS? (2))
1. RAM is usually
more powerful than ROM. COMMENT.
2. What are the differences between the floppy disk and the hard disk?
In which situation is the floppy preferable to the hard disk?
3. It is possible to make a Floppy Drive Read Only. COMMENT.
4. What does the message "MEMORY FULL" indicate?
5. Why are certain devices called specialty input devices? What makes
them different from the usual input devices?
6. Can printers and monitors be described as Input/Output devices?
Explain.
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