Storage
Main Memory
Secondary Memory
Different speeds
Magnetic Media
Optical Media
Access Modes
A Computer System

1.
2.
3.
4.
As we already know a computer system is
made up of different components such as;
the CPU,
input devices,
output devices,
storage devices and others.
What is Storage?

Storage devices are used to store data
used by the computer.
Two main types of storage devices are;
1. Primary Storage/Memory: used to
store essential data or data that is
currently being used by the computer
2. Secondary Storage/Memory: are
used to store data permanently

Primary/Main
Memory
ROM – Read Only Memory

ROM is a primary memory

It is a non-volatile computer chip; this
means that the data saved in ROM is not
lost once the computer is turned off

The programs stored on a ROM cannot
be changed or deleted since it is read
only
Other types of ROM
1.
PROM (programmable read only memory) – a blank
chip which can be programmed only once using a special
device called a programmer. Once it’s programmed its
contents cannot be modified or erased.
2.
EPROM (erasable programmable read only
memory) –can be programmed multiple times. Its
contents can be erased by using UV (ultraviolet) light.
Exposure to the UV light will erase all contents.
3.
EEPROM (electrically erasable programmable
read-only memory) – similar to EPROM but its contents
can be electrically erased and re-written without having to
remove it from the computer.
4.
Flash Memory – a modern type of EEPROM, it can be
erased and re-written to electrically. It is much faster than
EEPROM. It is ideal for electronic devices such as mobile
phones and digital cameras.
BIOS – Basic Input Output System

In a PC, the ROM is found on the motherboard

The ROM stores the programs required to start up the
computer, this program is called the BIOS (basic input
output system)

The main task of the BIOS is to load and start the operating
system

The BIOS identifies system devices such as the hard-disk, CD,
video card, mouse, keyboard and other hardware devices.

BIOS can also be used to modify some computer settings, for
instance configuring hardware, setting the time, select the
boot devices, set passwords and others.
RAM – Random Access Memory

RAM (random access memory) is another
type of primary memory.

Unlike ROM, it is a volatile computer chip;
which means once the computer is
turned off all the data is lost

RAM is a read and write memory hence
data stored on the RAM can be modified
Where is RAM found?

In a PC, the RAM (also called DRAM –
dynamic RAM) is also plugged in the
motherboard.

The size of the RAM is measured in GB
(gigabytes).

Modern PCs or Laptops have around 3GB of
RAM.

RAM in a computer system is used for fast
processing of data.
Example of why is RAM needed?

Charlie buys a PC game from a shop; this
game is stored on a DVD.

Before James is able to use it, he has to
install the game on the hard-disk.

The first step is to insert the DVD into the
DVD Drive and follow the on-screen wizard
to install the game. This process will copy
the data from the DVD to the hard-disk for
permanent storage.
…
The game cannot be run directly from the DVD
for two reasons:
1. DVD is slower than the hard-disk, therefore
this will affect the performance of the game
2. Charlie cannot save progress to DVD since
it’s read only


Once the game is copied on to the hard-disk,
Charlie opens the game by double-clicking on its
icon.

Now, when the game is being executed, the files
related to the game are copied to the RAM. This
happens because the RAM is much faster than the
HD, so the game will benefit from this
performance!
Cache Memory/SRAM

Although RAM is very fast, sometimes the CPU
might require data to be processed even faster.

This is when the cache memory is used; this is
just another type of RAM which is also called
SRAM – static RAM.

This is not plugged in the motherboard but
it is found inside the CPU.

This is faster than DRAM but it is limited in size;
CPUs have around 2MB, because it’s very
expensive.
Diagram
CPU
Control Unit,
Arithmetic
Logic Unit,
Main Memory
Cache
Memory
RAM
Cache
Memory
Faster Access
ROM
Fast Access
Magnetic Storage
Magnetic storage uses magnetic charges to store data.
The presence of a magnetic spot represents a 1,
whilst the absence represents a 0.
1
0
Floppy Disk

When the floppy disk was first invented, it was
consider to be large and could store only a few
kilobytes of data.

With improvements, the floppy disk evolved to
store 1.44MB of data.

At the time, the main advantage of the floppy disk
was its portability.

The floppy disk was a very convenient device
which could store a number of files and can be
transferred from one computer to another very
easily
Disadvantages
1.
Too small – the floppy disk can only store
1.44MB which when compared to other devices
such as the CD (700MB) or the DVD (4.7GB) is
very little.
2.
Relatively expensive – once again compared
to much larger devices, the floppy disk is more
expensive.
3.
Reliability – data stored on a floppy disk can
be corrupted easily due to temperature,
condensation, magnetism, dust and scratches.
4.
Slow – data read and written to a floppy takes
a long time to be accessed.
Write Protect Tab

The floppy disk has a write-protect tab,
situated in the upper left corner.

When locked the contents of the floppy
cannot be modified or deleted.

The tab must be moved in order to be
able to write content to the disk.
Write Protect Tab
Floppy Disk Structure
A floppy disk stores data on a magnetic
disc which is protected by fabric, which
is then enclosed in a plastic cover.
 It has a metal flap which protects the
magnetic disk from being scratched.

Plastic Cover
Fabric
Magnetic Disk
Formatting the Floppy Disk

Before it can be used for the first time, a floppy disk has to
be formatted.
When formatting two things occur;
1.
all data stored on the floppy is erased
2.
a filing system (FAT) is created.


In the case of the floppy
disk its surface is divided
into tracks and sectors
Track-Sector
Sector
Track
Hard Disk

The hard disk was first invented by IBM in
the late 1950s, at that time is was big and
very expensive and had limited storage

Nowadays, the hard disk is an essential part
of the computer system

It stores all the applications, operating
system and user’s files.

A hard disk works on the same way a floppy
disk works but it is more reliable
Hard Disk Structure

It’s made up of a number of platters stacked on top of
each in a metal case and all air is removed.

Each platter has a read/write head attached to an arm
which moves in and out to reach data stored on these
platters. The arms are then attached to a motor which
allows movement.

When the hard-disk is switched on the platters will
start to rotate, the head moves to the required location

Once switched on the hard-disk platters will constantly
rotate so that whenever the computer requires data, it
can be accessed very fast
Diagram

The heads will never touch the platter, since if
this happens the hard disk will crash and data
would be lost.
Formatting the Hard Disk

When a hard disk is formatted (just like the
floppy disk) tracks and sectors are created.

However, since the hard disk has more than
one surface, the tracks on top of each other
form a track.
Track
Cylinder
Hard Disk Characteristics
Capacity
240GB
The capacity of the hard disk. The larger it is, the
more files it can store.
Average
<9ms
platters to the correct location and move the heads
Seek Time
Rotational Speed
This is the time the hard disk takes to rotate the
at the correct position.
7200RPM In a minute the platters can rotate 7200 times.
Data Transfer Rate 130MB/s
In one second the hard-disk can read 130MB.
Magnetic Tape

A magnetic tape reel system stores its data
on meters of magnetic tape.

The surface of the magnetic tape is split into
9 tracks; 8 of them to store the data and an
additional one for parity checking/error
correction.

A character can be written on the tape by
using magnetic spots to represent binary
patterns (so 9 bits). A group of 9 bits is
called a frame
Diagram
Storage Method

Data on the tape is stored in data blocks.

The blocks are then separated by an interblock gap.

In order to access a particular block all the
previous blocks have to be accessed (serial
access)
Tape Streamer

A tape streamer is a smaller version of
the magnetic tape reel.

This can be used with a normal PC; it’s
also a serial device so it’s mostly ideal for
backups.
Data Access Methods
1.
2.
Serial Access
Direct Access
Serial Access

In serial data access the items are organized
one after each other,

In order to access a particular item all the
previous items have to be accessed.
Item 1 Item 2 Item 3 Item 4 Item 5 Item 6 Item 7 Item 8 Item 9 ...

In this example in order to access Item 6 all
the previous items (from 1 to 5) must be
processed.
Why Serial Access?

Serial access is useful when all the items
stored are required, so they must be
loaded.

In other cases it would be very slow since
unwanted items must still be accessed

This type of access is mostly used for
backing up storage
Direct Access

In case of direct access the required files
can be accessed immediately without
having to go through the other files.

This can be done because the device
stores the location of all the data stored

Modern storage devices such as disks and
electronic media are all direct access
Optical Storage
Optical devices use laser beams to store data. Data is
stored on these devices by using laser beams and
their reflective properties.
CD – Compact Disk

The CD stores 700MB of data
It became very popular due to its
portability and also its ability to store
various media such as
1. videos,
2. text files
3. images
4. and sound

CD Structure
A CD is made up of different layers;
A. A polycarbonate plastic which is one of
th e thickest layers where the bumps are
created,
B. A sheet of aluminum on top of the
plastic which reflects light
C. A lacquer layer to protect the aluminum
D. Art work
E. A laser beam used to read the data

Diagram
How is data Stored?

Data is stored in pits and lands which are
used to reflect ones and zeros;

If there is a pit the laser beam is not
reflected back, whilst if there is a land the
laser beam is reflected.

This change of light reflection allows data
to be stored as binary.
Diagram
Laser
Pick Up
Since there is a land the
laser is reflected and
received by the laser picksup data
Laser
Pick Up
In this case since there is a
pit, the laser is not
reflected back so no data is
picked up.
How it works

Although the CD is slower than the harddisk it has a very good speed.

The CD starts to spin as soon as data is
required from it. So it will only spin when
data is being read from it (or written to it).

Data is stored spirally on a CD
starting from the center moving
outward
The different CD types

1.
2.
3.
There are different types of CDs,
CD-ROM: CD read only memory which
has data stored on it and it cannot be
modified or deleted.
CD-R: CD recordable which can be used
to write data on it but then it cannot be
deleted. If the CD is not full more data can
be added.
CD-RW: CD rewritable allows the user
to write data and erase it.
DVD - Digital Video Disc

The DVD is an improvement over the CD

It allows more data to be stored on it; on
average 4.2GB. This is achieved by
reducing the size of the lands and pits.
The different DVD types

1.
2.
3.
There are different types of DVDs,
DVD-ROM: DVD read only memory
which has data stored on it and it cannot
be modified or deleted.
DVD-R: DVD recordable which can be
used to write data on it but then it cannot
be deleted. If the DVD is not full more data
can be added.
DVD-RW: DVD rewritable allows the
user to write data and erase it.