or its content.
Secondary Storage
Secondary Storage
Page 1
� 1997-1998 No-Moa Publishers
Monday, June 12, 2000
www.tongatapu.net.to
Monday, June 12,
Secondary Storage
Goals and Objectives
The course module provides a summary of secondary storage devices. Students
are introduced to the mechanisms and use of some of the secondary storage de-
vices available today for microcomputers.
Hard Disk.
Technically microcomputer hard disks are called Winchester disks (from the ori-
gins when the first hard disks were termed Winchester) or fixed disks as they are a
pack of disks permanently sealed inside the drive.
A hard disk is composed of
platters which have a surface
coated with metallic oxides
spinning constantly. Read/
Write heads move over the
platters similar to the old vi-
nyl records reading and re-
cording data on the surface of
the disk platter. The read/
write heads do not actually
touch the platters but float
above the surface and mag-
netically read and write data
to the platter.
Because of the microscopic
size of elements being dealt with, the Hard Disk is sealed in a case to minimise in-
trusions of dust and the effects atmospheric changes have on the internal compo-
nents. The improved materials, sealing of hard disks contributes to their higher
reliability than floppy diskettes.
The faster the platter spins, and the read/write heads move in and out, the more
data can be read and written to/from the disk platter. Increased volume of data
and the higher speed of retrieving information from the hard disk gives hard disks
an advantage of floppy diskettes.
Hard Disk Advantages:
Speed, Size, Security/Integrity.
Hard Disk Disadvantages:
Higher Cost
The disadvantage of storing everything on a hard (fixed) disk is the need to share
data/programs. A network allows for sharing files within the network but transfer-
ring data/programs between unconnected machines is difficult if it requires you to
take the hard disk out and put it onto the other machine.
A Name - by any other name
Each disk drive on an IBM PC Compatible computer is assigned (or given) a letter
(drive letter). Historically the first two letters (A and B) have been used for the
only disk drives available for IBM PC microcomputers at the time, floppy disk
Read/Write Head
One for each platter
Access arm
movement
Direction of rotation
Platters
Sectors
Tracks Secondary Storage
Page 2
� 1997-1998 No-Moa Publishers
Monday, June 12, 2000
drives. Drive letter C, later became the first letter used by the Hard Disk Drive.
Additional Hard Disk drives added to a microcomputer gain, or get the next drive
letter, and so on.
Floppy Disks.
Diskettes, often called floppy disks, are single platter plastic disks in a plastic
jacket. The plastic jacket is quite hard on modern 3�" diskettes, but they are nev-
ertheless still classified as floppy disks, not hard disks as some people think.
Floppy diskettes are usually inserted into a floppy disk drive with the label face-
up. For 3.5 floppy diskettes this side of the floppy diskette is usually marked
with a HD symbol.
Floppy disks are characterised by several parameters:
Size, the diameter of the disk.
Floppy diskettes were originally developed along with the USA
computer industry and the US influence continues in the impe-
rial measurements for floppy disks.
Early machines using floppy diskettes for file storage used 8 disks but there were
so many different 8 disks, and different ways of recording onto the disks that few
machines having 8 disk drives could read disks used by another manufacturers
computer.
5�" floppy diskettes.
The first widely used disk size was the 5�" diskette. The drives and controllers
for 5�" diskettes were cheaper than the 8 drives and more reliable so the fledg-
ling (young) microcomputer industry standardised on 5�" disks.
Initially each manufacturer developed their own standard for how information is
layed out, or stored on the floppy disk. Unfortunately this meant that disks used
by different computers had the same problem that 8 drives had, not being com-
patible between different machines.
When IBM released their IBM PC they popularised PC-DOS an operating system
licensed from Microsoft. Microsoft had created their own disk layout which they
licensed to other computer manufacturers and subsequently became the standard
format for laying out information on floppy disks. As companies such as Compaq,
Digital, Philips developed their own computers they licensed Microsofts operat-
ing system and in so doing they adopted Microsofts design for using floppy disks.
3� micro-floppies
In 1987 IBM standardised its new IBM PS/2 range of computers on the newer 3�
micro-floppy developed by Sony Corporation. The floppy disk and disk drive
were originally introduced into the microcomputer market by Apples Lisa and
Macintosh computers, Atari STs and Commodore Amigas, but it was IBMs in-
troduction of machines with 3�" disks that sent software suppliers to providing
programs on 3� disks.
The standard MSDOS format was now available on 3� disks and the manu-
facturers copying IBMs design also used the MS-DOS disk layout. The prolif- Secondary Storage
Page 3
� 1997-1998 No-Moa Publishers
Monday, June 12, 2000
eration and importance of the MS-DOS format is evident with most microcom-
puter operating systems supporting 3� also providing direct support for MS-DOS
3� disk format.
The 3�" floppy disks higher capacity (than the 5�) and the greater durability of
the 3�" casing was adopted by hardware and software suppliers. Today, the stan-
dard size of floppies is the 3�".
Single or double-sided.
Floppy disk drives available today read and write data on both sides of the diskette
with read/write heads on each surface of the disk. The first designs of disk drives
only supported reading one side of a disk at a time.
Density - the number of bytes that can be stored on each track.
As technologies improved, more and more magnetic particles could be packed
onto the same area of space on the floppy diskette and correspondingly read by the
disk drive. This increase in density changed the amount of data a diskette could
contain.
The density of floppy diskettes for IBM PC
Compatibles allowed 1.25 kilobytes of data
to be stored per inch of track. As disk
drives and disk technologies were improved
disks that could hold 2.5 kilobytes per inch
were released and called (logically) double
density disks.
Technology improvements continued and soon quad density, later called high den-
sity disks and drives were released. The original low density disks have now dis-
appeared, leaving only double and high density disks. This results in some confu-
sion for new users as the name double density seems to imply it holds more than
the other current type - high density.
High density disks are usually labelled HD. 3�" high-density disks and can hold
up to 2.00MB of data but are formatted on IBM Compatible computers to store
1.44MB of data. Continuing developments in floppy diskettes have released
newer, higher density floppies and disk drives, such as the 2.88MB floppy disk,
but the lack of support from a major hardware supplier and its relative small incre-
mental value to customers and suppliers has seen it languish.
The total capacity of a floppy disk is a multiplication of surfaces by tracks by sec-
tors. So, a 3�" high density disk can hold 2 surfaces x 80 tracks x 18 sectors x 512
bytes = 1,474,560 bytes (1,024 = 1,440K).
Densities are an issue when using older computers that only have double density
disk drives. High density disk drives will read and use lower density disk, but un-
fortunately older computers with only double density disk drives cannot read high
density diskettes.
Diskettes rotate within their permanent jacket at either 300 or 600rpm. The jacket
is lined with a fabric to clean the disk as it spins and to prevent the friction of the
plastic platter rubbing against plastic casing.
Size
Density Tracks
per Side
Sectors K per Inch
of Track
Total K
5�"
Double
40
9
2.5
360
5�"
High
80
15
5
1,200
3�"
Double
80
9
2.5
720
3�"
High
80
18
5
1,440 Secondary Storage
Page 4
� 1997-1998 No-Moa Publishers
Monday, June 12, 2000
Access slots for read/write heads are located on either side and allow touching the
surfaces. On 5�" disks the slots are protected when the disk is not in use by stor-
ing the disks in paper envelopes. 3�" disks have a spring-loaded metal shutter that
closes and protects the access slots (the disk drive opens this shutter automatically
as the disk is inserted).
All PC disk drives have indicator lights that show when the disk is actually being
used, either being written to or being read from. Removing a diskette while the
computer is writing data to the diskette may cause file information to be partially
written which may corrupt the data and other files on the diskette.
A diskette should never be removed while this light is on. You should always look
at this light before removing a diskette. Apple computers have a software eject
facility for removing diskettes to ensure data is saved properly before the floppy
disk is ejected.
Advantages of using floppy disks: copy files from one computer to another; Easy to move floppies from place to place (machine to machine) cheap.
Disadvantages of using floppy disks: limited capacity relatively slow; less reliable than hard disks so proper care should be taken to prevent loss of
data.
It is important to take proper care of your di