Final Version
Computer Storage
Section 1
What is Computer Storage?
Computer storage refers to all units of computer equipment that can be used to store data and instructions either
permanently or temporarily.
Different types of Computer Storage.
A typical computer system has different types of storage and these can be divided into two groups; Internal Storage
and External Storage.
Figure 1.1
Computer Storage
Computer storage is sometimes
referred to as computer memory.
However, the term memory generally
refers to internal storage while the term
storage usually
Internal Storage
External Storage
storage devices.
Brain Teaser
Can you mention some units of computer equipment that are able to store data
either permanently or temporarily?
HINT: mp3 files, word processing files and photos are considered as data. On
which media do you store this data.
Internal Storage
Section 1.1
What is Internal Storage?
Every computer has some kind of Internal Storage, which consists of the CPU Registers, Cache and Main Memory
(RAM and ROM), see figure 1.2. These are usually referred to as memory units.
Figure 1.2
Internal Storage
CPU Registers
Main Memory
Internal Storage stores data and instructions that are being processed by the Central Processing Unit (CPU). All these
storage units can be randomly accessed.
Random Access
memory location
Random Access means that all memory locations in a particular
memory unit (example RAM or Cache) can be accessed directly in
the same amount of time. Therefore, the access time will be the
same for any read or write operation. So we can say that if the
Central Processing Unit is accessing data from location 1 in RAM
it will take the same amount of time as if the data was accessed
from location 100 in RAM.
Internal Storage
Section 1.1
What is Access Time?
The access time (performance) of an internal memory unit is the time taken for the CPU to write or to read data from that
particular internal storage unit.
Figure 1.3
Note that the main memory, cache and
Main Memory
levels have the fastest access time,
therefore they are more expensive and
their capacity is smaller.
In this storage hierarchy the topmost
CPU registers have different access
Backing Storage
Did You Know?
The access time of internal memory units is
measured in nanoseconds (ns).
Nanosecond = A billionth of a second.
Internal Storage
Section 1.1
Figure 1.4
Main Memory
This diagram shows that data and instructions are exchanged between the CPU, Cache and Main Memory. In fact,
each time the CPU needs data and instructions to process, it first checks the Cache and if the required information
is not there it then fetches it from the Main Memory.
Why do we need three types of Internal Memory?
CPU Registers - These are tiny storage units which are placed right inside the processor core, and they are the fastest
type of internal storage (lowest access time). Registers are able to hold few bits of data that the CPU is currently
working with. Typically, registers can hold 16 or 32 bits.
Cache Memory is the second fastest type of memory. It is capable of delivering data at or near the speed of the CPU.
The purpose of Cache is to allow the computer to operate faster, by storing the most recently used data so that the CPU
would not have to wait while each item is fetched from the slower Main Memory. However, the capacity of cache
memory is usually very small because it is expensive, examples of typical cache sizes are 2Mb, 1Mb, and 512K.
Main Memory consists of both RAM and ROM (figure 1.2), and it contains data and instructions that need to be
accessed by the CPU. The Main Memory Unit is also called the Central Memory, Immediate Access Memory or
Primary Memory. The CPU can only process data that is in main memory. Therefore, every program you execute and
every file you process must first be copied from a backing storage device into main memory. The amount of main
memory on a computer is crucial because it determines how many programs can be executed at one time and how
much data can be readily available to a program. The purpose of main memory is :
To store the program that is being executed.
To hold data processed by the running program.
Brain Teaser
A student decided to install a cache that is faster than the CPU, because he believes that this will speed up his
computer. Do you agree with this student, Yes or No?
Answer: no
Section 1.2
What is RAM (Random Access Memory)?
RAM is used to temporarily store all the information being processed by the CPU. The RAM is a read-write memory
thus information can be read and written to it. When you first turn your computer on, the RAM is empty since it is
volatile. Volatile means that everything stored in RAM will be lost each time the computer is switched off. RAM will
contain no data and instructions until you load a program. These data and instructions are originally permanently stored
in the external storage devices, like CD ROM, hard disk or floppy disk. When you load the program, part of it is copied
from the external device it is located on to the RAM, which is directly connected to the CPU. This is necessary because
the CPU can only fetch data and instructions from RAM. The RAM is useful because it can feed data and instructions to
the central processor at extremely high speeds. However, programs cannot be permanently stored on the RAM because
RAM, unlike hard drives, is not big enough to store all the programs a computer system contains.
RAM is expensive.
RAM is volatile.
Application of RAM outside the CPU
In a printer some RAM is needed to store the next set of data to be printed.
Did You Know?
performance of your computer.
computers with 256MB, 512MB, or 1G RAM.
Two different types of RAM are SRAM
(Static RAM) and DRAM (Dynamic RAM).
Static RAM provides faster access to data
and is more expensive than DRAM.
Section 1.3
What is ROM (Read Only Memory)?
The ROM is a Read-Only Memory. This means that the CPU can only read its contents, so it cannot change or alter it in
any way. ROM is used to store those programs essential for the normal running of a computer system such as the
Bootstrap Loader. The Bootstrap Loader is a small program that is executed after switching on the computer. Its
function is to load Windows and run it, thereafter the user can start using the computer system. ROM is nonvolatile
which means that all data and instructions stored on ROM will not be lost once the computer is switched off. Note that
any data and instructions stored on ROM are permanently fixed during the manufacture of the ROM integrated circuit.
ROMs are very cheap and reliable, but unfortunately, they are read-only.
Application of ROM outside the Computer.
In a modern washing machine, the processor needs ROM to permanently store the control program and RAM to
temporary store data created by the program.
Different types of ROM?
PROM (Programming Random-Access Memory) is a memory IC which is manufactured blank so that the user is then
able to store the desired programs on it. The disadvantage of PROM is that it can only be programmed once.
EPROM stands for Erasable and Programmable ROM. Like PROM, EPROM is also manufactured blank so that the
user can write programs on it, but it also has the additional feature of being erasable. To erase an EPROM, UV light is
used. Thus the advantage of EPROM is that this kind of ROM can be re-programmed.
The EEPROM is similar to the EPROM, except that you don't have to erase the entire chip to change some information.
Also, another advantage is that you do not need any special equipment, like UV light, to erase the chip. Pen drives and
flash memories are example of EEPROM (see page 20)
Brain Teaser
Can ROM, like RAM, be directly accessed by the Central Processing Unit. Yes or No?
Answer: no
Answer: yes
External Storage
Section 1.4
What is External Storage?
External Storage Devices store data and instructions that are not currently being processed by the central processing
unit. It is important to note that External Storage Devices are also referred to as Backing Storage Devices and
Secondary Storage Devices. They are an essential part of a computer system due to the two major constraints of the
main memory unit.
Volatile - Whatever stored in RAM is lost when the computer is switched off. Thus
we need a place to store our programs and data so that they will not be lost when
the computer is switched off.
Limited Size - Even on computers with large internal memories, the amount of
storage needed on a typical computer system is far greater than the storage
available in memory. Since internal storage is expensive, we need a relatively
cheaper form of backing storage which can be used to store vast quantities of
The main advantage of every secondary storage device is that it is non-volatile thus everything stored on a secondary
storage device is not lost once the computer is switched off. The main disadvantage, is that the central processing unit
is not able to directly access the data and instructions stored on secondary storage devices so a copy of the program
must first be transferred into main memory.
Most secondary storage systems consist of the drive and the medium. The medium is the material on which data and
instructions are stored. The drive/device is the mechanical part of the system that is able to place data on the
dium and to fetch it from the medium. Since, most backing storage devices operate mechanically it takes much longer
to access data which is on backing storage devices than data which is in main memory.
Brain Teaser
1. In the adjacent diagram which internal
storage unit is not listed?
2. How many backing storage devices
are listed?
Answer: Cache , 3
External Storage
Section 1.4
The need for Backups
It is very important to keep a backup of all the data and programs stored on your hard disk, in case it gets lost or
damaged. Backups will be saved on some form of backing storage.
Main Differences between Internal and External Storage.
Internal Storage stores data and instructions that are being processed by the Central Processing Unit.
External Storage Devices (Backing Storage) store data and instructions that are not being used by the Central
Processing Unit.
The Central Processing Unit can only directly access the data stored on Internal Storages.
Internal Storage is much faster than External Storage (figure 1.3).
Internal Storage is much more expensive (cost per megabyte) than External Storage (figure 1.3).
Internal Storage is much more expensive than External Storage. For this reason, Internal Storage Units are
manufactured with small storage capacity because otherwise the computer will become too expensive.
Types of Backing Storage
Section 1.5
Classification of Backing Storage Devices.
Secondary storage devices can be split into two major types, Magnetic and Optical Storage Devices. Lately, however,
there is a newer type of secondary storage which is totally different and is like permanent RAM. These are known as
Electronic Storage Devices. Note that each type of storage device uses different principles for reading and writing the
0s and 1s stored on the medium.
Figure 1.5
Backing Storage
Magnetic Storage Devices
Electronic Storage Devices
Optical Storage Devices
Magnetic Storage Devices - All magnetic storage devices store the logic 0s and 1s as magnetic spots, where logic 1
and logic 0 are represented by a particular polarity (North or South).
Since magnetic storage devices store data in the form of magnetic spots, it is important not to
leave any magnets near the storage medium because this might alter the data stored on the
Optical Storage Devices - Note that all optical storage devices make use of different techniques in order to store data,
but data is read using the same principle. A laser beam is shone over the surface of the medium and the amount of light
reflected will be detected by the photo detector, which then converts this information into logic 1s and logic 0s.
Electronic Storage Devices - Electronic Storage Devices store the logic 0 and logic 1 as electrical charge.
Data Access Methods
Section 1.6
Data Access Methods
One of the characteristics of a backing storage device is the method used to access the information stored on it. There
are two main ways in which data can be accessed: Direct Access and Serial Access.
Serial or Sequential Access
storage location
Any type of backing storage device that allows sequential access
requires to start reading data items from the first storage location.
This means that if data item 25 has to be accessed by the
computer, all previous data items (from 1 to 24) have to be
searched. Serial access is not effective in terms of time when only
one particular item is required because all previous items have still
to be searched.
Direct Access
storage location
Any type of backing storage device that allows direct access is
able to directly access any storage location on the medium.
However, the amount of time the device takes to access a data
item is dependent to some extent on the mechanical process
Random Access
There are also some type of backing storage which are very
similar to Main Memory, where any storage location can be
accessed directly and at the same amount of time.
Hard Disk
Section 1.7
A hard disk drive is the main storage device within a computer. It is
where the operating system, application programs and data are kept.
Nowadays, there is at least one hard disk within every desktop computer
but it is possible to add more. Hard disks are capable of storing large
amounts of information thus the storage capacity of a hard disk is
measured in Gigabytes (GB).
A hard disk consists of a stack of permanently
fixed (non exchangeable) rigid circular disks
called platters.
Each platter is made of
aluminum or glass and coated on both sides with
illustration on the left shows four platters forming
a disk pack. These platters are connected to an
axes (spindle) and they all rotate together around
this same axes.
Platter—note that each platter has two surfaces.
Data on a hard disk is stored magnetically, as magnetized spots arranged in tracks and sectors (see section 1.8). Each
magnetized spot represents one bit of information (binary 1 or 0).
Hard Disk
Section 1.7
Surface O
In order to perform the reading and writing operations
Arm for Head 1
we need the read/write heads. There is one read/
write head for every side of each platter (see the
diagram on the left). While the platters rotate, these
heads move in or out (from the centre to the edge)
over the surface to write or read data on the various
Head O
Head 2
Arm for Head 3
tracks. Therefore, all data found on a hard disk can
be directly accessed.
Whether it is a read or write operation, the read/write head hovers on a cushion of air, only nanometres above the disk
surface. While performing a write operation the platter is exposed to a magnetic field, where it becomes instantly and
permanently magnetized.
magnetic field.
Note that it can be re-magnetised in the opposite direction by exposure to a different
When performing a reading operation the head will detect the magnetic field around the specific
location on the platters, and then convert this pattern into binary digit.
Did You Know?
The first disk drive was the IBM 350 Disk File,
and it was introduced in September 13th 1956
with the 350 RAMAC computer. It had 50, 24”
platters which held 4.4 MB of data.
platters are marked with the red arrow in the
adjacent picture. Its cost was $50,000. Before
the 350 RAMAC, magnetic computer storage
had consisted of core memory, tape, and drums.
Hard Disk Formatting
Section 1.8
How is data organized on a Hard Disk?
Data and instructions stored on a hard disk need to be organized in some way. This is why platters are virtually divided
into several tracks, where each track is further divided into sectors. These tracks and sectors are formed during hard
disk formatting. This process prepares the disk to hold data and instructions. In fact, all backing storage devices must
be formatted before any data can be stored on them.
A sector is the smallest unit of data that can be read or written from a disk.
Typically, sectors are 512 bytes in size, but other sizes including 1024 and
2048 are common. These sectors have a fixed size, that is all sectors on
the same disk are of the same size. However, these sizes may vary from
one disk to another.
A cluster is the smallest unit of data that can be allocated to a file.
consists of a number of sectors.
It is important to realize that the larger the sectors, the more disk space
will be wasted. This occurs, because if for example a sector is 32K and
we need to store a 5K file, 27K of disk space will be wasted.
A track is a concentric ring of sectors on a platter. A read/write head can
read all the data from a certain track by moving the read/write head to any
position on that track and then rotate the stack of platters.
A cylinder is made up of a group of tracks, where each track is found on a
different platter and all tracks need to be on top of each other. When data is
stored on the same cylinder it will reduce the access time since there will be
no need to move the read/write in and out.
Filing System
Section 1.9
Why do we need a Filing System?
When formatting a storage device, a file allocation table is
created which stores the location of all files on disk. The
Operating System needs this file allocation table in order to
know which sectors are storing which files, so that it would be
able to load your files when you need to access them later on.
Usually files are too large to fit in one sector, therefore a file is
typically stored into a number of sectors. Many filing systems
make use of clusters. This means that files are organized into
clusters and not into sectors.
File Allocation Table Example
Consider a file that is stored in locations 0003, 1003 and 2003. The file allocation table will store something like this.
File X
Pointer to next location
File X only directly points to the first sector allocated to it, that is 0003. Then that sector will point to the next sector, and
so on until the null signal is met which means that the file ends there and no more sectors are allocated to it.
There are different allocation tables, including FAT, NTFS
FAT (File Allocation Table) is one type of allocation table that allocates 12, 16 or 32 bits for each address. This system
is the only allocation table that can be used by MsDos, Windows 95, 98 and ME. Although nowadays Windows still
support the FAT system, the NTFS is taking over. FAT is nowadays mostly used for floppy disks and pen drives.
NTFS (New Technology File System) is a more recent filing system, which supports several features that FAT does not,
such as high security, larger disks and Unicode file naming. Systems that use NTFS include Windows NT 4.0, 2000,
Server 2003 and XP. These systems, however, support FAT as well.
Did You Know?
Q. When a hard disk is formatted all data is lost, but is the data really erased from the hard disk?
A. The answer is no. What happens is that the file allocation table is deleted so the Operating System would not
know where the files are stored on the hard disk.
Floppy Disk
Section 2.0
IBM introduced the first floppy disk in 1971, but it was a bit different
from the one we use today. In fact, before the 3½” floppy disks
were introduced, there were the 8” and 5¼” floppy disks. The older
ones were flexible, bendable, and fairly big. Nowadays, the 3½”
floppy disk is fairly small and the medium is contained in different
coloured plastic cases.
Different types (3½”, 5¼” and 8” ) of floppy
Components of a Floppy Disk
A 3½” floppy disk basically consists of 5 different components.
1. Protective Case - the purpose of this plastic case is to protect the
magnetic disk from dust, water, etc and from being scratched.
2. Felt Ring - its purpose is to protect the magnetic medium from dust.
3. Magnetic Disk with metal hub in the middle. This is the medium on
which data is stored.
4. Shutter
5. Label found on the front of the protective case.
Floppy disks have a safety feature, in order to protect data
from being accidentally erased. This is possible because
of the write protect switch. Note that 5¼” floppy disks had
a write protect notch, which used to be covered by a sticky
paper tab in order to prevent the computer from writing
data to the disk. Nowadays, 3½” floppy disks, have a
built-in slideable plastic tab to implement write-protection.
Floppy Disk
Section 2.0
The floppy disk is a magnetic secondary storage device, which uses direct access to retrieve and write data.
Despite the size difference, floppy disks are still very similar to hard disks and have many of the same components.
Like the hard disk, floppy disks also have read/write heads. These heads are used both to read information stored on
the disk and to write information into the disk. There are two read/write heads on a floppy disk - one on either side of the
disk. This means that a floppy disk is double sided and is capable of storing information on both sides. The read/write
head in the floppy disk drive touches the disk surface and therefore to reduce the wearing way of the magnetiseable
coating, the disk only rotates during reading/writing operations. Although, floppy disks store and retrieve data in the
same way hard disks do, their access time is much higher.
Formatting a floppy disk
Like hard disks, floppy disks need to be formatted in order to be able
to store data and instructions. Most floppy disks you buy today are
already formatted by the manufacturer (see adjacent picture).
Floppy Disks are formatted using the same principles explained in
pages 10 to 11. The 3½”, HD format, floppy disks uses 512 bytes per
sector, 18 sectors per track, 80 tracks per side and two sides, for a
total of 1,474,560 bytes (1.44Mb) per disk.
It is important to know that if the write protect notch is set on it would
not be possible to format the floppy disk.
Did You Know?
Floppy Disk Chronology
• In 1971, IBM introduced the first floppy disk. This was the 8" floppy disk.
• In 1976, the 5¼” floppy disk was developed by Alan Shugart for Wang Laboratories.
• In 1981, Sony introduced the first 3½” floppy disk.
Floppy Disk
Section 2.0
Protect the Data stored on Floppy Disks
Protect your floppy disk from high or low temperatures.
Keep your floppy disk away from devices that produce magnetic fields.
Put your floppy disk in a hard plastic cover to protect it from dust and water.
Do not bend your floppy disk because you might break the protective case.
Do not touch the magnetic disk because this might corrupt the data stored on your floppy disk.
Do not put heavy objects on your floppy disk.
Finally, if a floppy disk starts showing problems, copy the data to a new disk and throw
away the old disk.
Did You Know?
For more than two decades, the floppy disk has been
the most usable portable storage device. Nowadays,
floppy disks are being replaced by CD-ROMs, DVDs
manufacturers like Apple have removed the floppy
drive from their machine. In 1998, the iMac were
produced without floppy drives.
CDs & DVDs
Section 2.1
Another way of storing data is by using optics. This is the method used by CDs (Compact Disks) and DVDs (Digital
Versatile Disk). CDs and DVDs have become very popular because they can store much more data than floppy disks.
In fact, they have replaced floppy disks due to their high speeds and large storage capacity. 1 CD can store the
equivalent of about 400 floppy disks.
CDs and DVDs are flat, plastic discs whose undersides are coated with a thin layer of
light-reflecting aluminum. Data is encoded on them in a spiral form, therefore CDs
and DVDs have only one track (see the diagram on the right). In order to read the
data stored on CDs and DVDs we need a specific devices, these are the CD-ROM
drive and DVD drive or DVD player. However, nowadays we find drives that are able
to read both CDs and DVDs. These drives can move the reading mechanism (the
laser beam) to any area of the disk surface for direct access. Note that the disk only
rotates during a reading operation.
Data is encoded in a
spiral form
Different types of CDs and DVDs
CD-ROM (Compact Disk - Read Only Memory) - This is by far the most widely used type of optical disk. A
CD-ROM disk can store between 700Mb to 800Mb of data. The data is written onto the CD-ROM disk before it is sold
and can not be changed by the user. Because of this CD-ROMs are often referred to as WORM (Write Once Read
Many). Data is stored as a sequence of surface variations called lands (flat surface) and pits (bumps or holes). These
lands and pits cannot be seen by the naked eye. The laser beam is shone onto the surface of the disk and light is
reflected back. The amount of light reflected varies according whether it hits a land or a pit, this is how logic1 and logic
0 are represented.
CD-R (Compact Disk - Recordable) - A CD-R disk is blank when bought. The user can write data to it just
once. After data has been written to the disk it cannot be changed. The user must have a CD Writer, which is a device
that writes data on a blank CD.
CD-RW (Compact Disk - Rewriteable) - CD-RW disks are like CD-R disk with the only difference that they can
be written more than once. Here the user also needs to have a CD Write.
Did You Know?
The first data CD-ROM was developed in 1985, by Sony and Philips.
CDs & DVDs
Section 2.1
DVD (Digital Versatile Disk - Read Only Memory) - DVDs are of the same diameter and thickness as CDs,
and they are made using some of the same materials and manufacturing methods. However, DVDs are able to store
more data than CD-ROMs. Like CD-ROMs, the data on a DVDs is encoded in the form of pits and lands. There are
different types of DVDs, each vary in the amount of data they can store. The reason is that DVDs may have one or two
sides written with data, and one or two layers of data per side.
Single Sided, Single Layer DVD
Single Sided, Dual Layer DVD
Double Sided, Single Layer DVD
Double Sided, Dual Layer DVD - have the largest capacity that of 17GB
By looking at the DVD one can understand whether they are single or double sided. Double sided DVDs have both
sides shiny, that is both sides of the DVD will look like the bottom side of a CD-ROM.
DVD-R (Digital Versatile Disk—Recordable) - Like CD-R, DVD-R are bought blank and the user is able to
write data on them. The user needs to have a DVD Writer, which is a device that writes data on blank DVDs.
Protect the Data stored on CDs and DVDs
• Always put your CD or DVD back in its case after using it.
• If there is an emergency, and you have to leave your CD or DVD out, put
it face down, so the aluminum is facing up.
• Don’t stack CD’s or DVD’s on top of each other.
• Clean your CD or DVD regularly, important to clean it outwards and not
in a circular movement.
• Whey you hand a CD or DVD, hold it on the sides, and do not touch the
shiny bottom.
• Keep CD’s and DVD’s away from hot places and from direct light.
• Make sure not to scratch your CD or DVD.
• If you are going to write on a CD or DVD, in order to label it, make sure
you use a CD/DVD Pen and that you write on the surface.
Flash Memory
Section 2.2
What is Flash Memory?
Flash Memory is a form of EEPROM and this type of memory is
commonly used in memory cards, USB flash drives, MP3
players, digital cameras and mobile phones. Since flash
memory is a type of EEPROM it implies that it can be randomly
accessed and that it is non-volatile.
Memory Cards and Flash Drives are the latest technology of
secondary storage devices.
In fact, they are still quite
Pen Drive
Memory Card
expensive. These electronic secondary storage devices are
very reliable and consequently they are becoming more
Memory Card
There are different types of memory cards such as Smart Media
(SM) Card, Secure Digital (SD) Card, MultiMedia Card (MMC)
and more. Each type of memory card is used for a specific
purpose. In order to be able to read the data from a memory
card we need a device know as Memory Card Reader.
Pen Drive
Unlike any magnetic or optical storage device, pen drives need
no drive to read or write their data. Pen drives use the USB
port to connect to the computer. USB ports are found on every
modern computer thus pen drives are very portable.
External Flash Memory Card Reader.
This type of card reader is connected to
the computer through USB.
Did You Know?
The flash drive was first invented in 1998 at IBM as a floppy drive replacement for the ThinkPad line of products.
Section 2.3
While Peter was cleaning one of his DVDs, he wondered why computers need to have so many different types of
storage devices. He decided to go and search the Internet to find an answer to his question and he found the
following answer.
Well, if computer systems did not have so many different types of storage, it would either be extremely slow, or
extremely expensive. The fast memories that can be accessed by the CPU directly are extremely expensive, while
cheaper storage devices are extremely slow. By using a combination of both, both the price and the performance
of a computer system are optimized!
Helpful Hints
The following websites contain some useful and well explained information on the topics covered in this chapter.
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