Secondary Storage
Monday, June 12,
Goals and Objectives
The course module provides a summary of secondary storage devices. Studentsare 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 apack of disks permanently sealed inside the drive.
A hard disk is composed ofplatters which have a surface coated with metallic oxidesspinning constantly. Read/Write heads move over the Platters cording data on the surface of
the disk platter. The read/write heads do not actuallytouch the platters but floatabove the surface and mag-netically read and write datato the platter.
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 higherreliability than floppy diskettes.
The faster the platter spins, and the read/write heads move in and out, the moredata can be read and written to/from the disk platter. Increased volume of dataand the higher speed of retrieving information from the hard disk gives hard disksan 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 sharedata/programs. A network allows for sharing files within the network but transfer-ring data/programs between unconnected machines is difficult if it requires you totake 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 theonly disk drives available for IBM PC microcomputers at the time, floppy disk 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 driveletter, and so on.
Floppy Disks.
Diskettes, often called floppy disks, are single platter plastic disks in a plasticjacket. The plastic jacket is quite hard on modern 3½" 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 markedwith a HD symbol.
Floppy disks are characterised by several parameters:
Size, the diameter of the disk.
Floppy diskettes were originally developed along with the USAcomputer 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 wereso many different 8” disks, and different ways of recording onto the disks that fewmachines having 8” disk drives could read disks used by another manufacturer’scomputer.
5¼" floppy diskettes.
The first widely used disk size was the 5¼" diskette. The drives and controllers
for 5¼" diskettes were cheaper than the 8” drives and more reliable so the fledg-
ling (young) microcomputer industry standardised on 5¼" disks.
Initially each manufacturer developed their own ‘standard’ for how information islayed out, or stored on the floppy disk. Unfortunately this meant that disks usedby 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 systemlicensed from Microsoft. Microsoft had created their own disk layout which theylicensed to other computer manufacturers and subsequently became the standardformat for laying out information on floppy disks. As companies such as Compaq,Digital, Philips developed their own computers they licensed Microsoft’s operat-ing system and in so doing they adopted Microsoft’s design for using floppy disks.
3½” micro-floppies
In 1987 IBM standardised its new IBM PS/2 range of computers on the newer 3½”
micro-floppy developed by Sony Corporation. The floppy disk and disk drive
were originally introduced into the microcomputer market by Apple’s Lisa and
Macintosh computers, Atari ST’s and Commodore Amigas, but it was IBM’s in-
troduction of machines with 3½" disks that sent software suppliers to providing
The standard MSDOS format was now available on 3½” disks and the manu- facturers copying IBM’s design also used the MS-DOS disk layout. The prolif- eration and importance of the MS-DOS format is evident with most microcom-puter operating systems supporting 3½” also providing direct support for MS-DOS3½ disk format.
The 3½" floppy disks higher capacity (than the 5¼”) and the greater durability ofthe 3½" casing was adopted by hardware and software suppliers. Today, the stan-dard size of floppies is the 3½".
Single or double-sided.
Floppy disk drives available today read and write data on both sides of the diskettewith read/write heads on each surface of the disk. The first designs of disk drivesonly 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 packedonto the same area of space on the floppy diskette and correspondingly read by thedisk drive. This increase in density changed the amount of data a diskette couldcontain.
Sectors K per Inch Total K The density of floppy diskettes for IBM PC
Compatibles allowed 1.25 kilobytes of data 5¼"
drives and disk technologies were improved 3½"
disks that could hold 2.5 kilobytes per inchwere released and called (logically) double 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 thanthe other current type - high density.
High density disks are usually labelled HD. 3½" high-density disks and can holdup to 2.00MB of data but are formatted on IBM Compatible computers to store1.44MB of data. Continuing developments in floppy diskettes have releasednewer, 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 3½" high density disk can hold 2 surfaces x 80 tracks x 18 sectors x 512bytes = 1,474,560 bytes (1,024 = 1,440K).
Densities are an issue when using older computers that only have double densitydisk 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 highdensity diskettes.
Diskettes rotate within their permanent jacket at either 300 or 600rpm. The jacketis lined with a fabric to clean the disk as it spins and to prevent the friction of theplastic platter rubbing against plastic casing.
Access slots for read/write heads are located on either side and allow touching thesurfaces. On 5¼" disks the slots are protected when the disk is not in use by stor-ing the disks in paper envelopes. 3½" disks have a spring-loaded metal shutter thatcloses and protects the access slots (the disk drive opens this shutter automaticallyas the disk is inserted).
All PC disk drives have indicator lights that show when the disk is actually beingused, either being written to or being read from. Removing a diskette while thecomputer is writing data to the diskette may cause file information to be partiallywritten 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 lookat this light before removing a diskette. Apple computers have a software ejectfacility for removing diskettes to ensure data is saved properly before the floppydisk is ejected.
Easy to move floppies from place to place (machine to machine) less reliable than hard disks so proper care should be taken to prevent loss ofdata.
It is important to take proper care of your diskettes to minimise the danger of los-ing data.
Looking after that Floppy Diskette
Protecting information on a floppy disk
Floppy disks, like audio and video cassettes, can be delete protected. On diskettesit is called write protection. It protects data from deletion and alteration. If thediskette is never to be altered or contains important data this is a useful facility. Ifyou are attempting to save onto a diskette and the computer responds with a mes-sage “Write Protect Error”, this indicates the diskette has been write protected.
To write protect a 3½" disk, slide the plastic that covers the top right notch so thenotch is open.
To write protect a 5¼" disks, cover the notch in the top right corner with a simpletape over the notch. To remove write protection take the tape off.
Care for the Diskette
Floppy diskettes are magnetic recording devices so when using floppy disks, here magnetic fields (such as televisions, speakers, and computer monitors) Magnetic Fields: The floppy diskette is a piece of plastic carrying tiny particles
which are organised by changes in magnetic polarity by the floppy disk drive.
Since magnetic forces are used to change the location and meaning of the particles
on a floppy diskette then a strong magnetic field, such as sticking a magnet on a
floppy diskette will change the behaviour of the little particles, changing the
meaning of their location and in many cases make it so the computer will no
longer read the diskette.
It is important to avoid, not put, strong magnetic fields. Things at home that gen-erate electro-magnetic fields that can cause problems with floppy diskettes includethe television, and radio.
Dust particles are very big compared to the size of the particles used on floppy
diskettes. Dust on the diskette media can damage the diskette and/or cause errors
in the floppy disk-drive’s attempts to read data. Since a single misplaced 0 or 1
can cause a computer to radically misinterpret information, dust can be a very big
Moisture: High Humidity in Tonga allows mold to develop on floppy diskettes
and pouring water, coffee on floppy diskettes will not do it good either.
Extreme Temperatures: Extremes in temperature effect the floppy diskette
media, which can cause problems with the particles. Extreme heat can also cause
the plastic to deform, change shape, which effects the ability of the diskette drive
to find the correct physical location, since the diskette is now physically changed.
Do not leave floppy diskettes in the car, on the dashboard in the car, or beside a
window where the glass magnifies the heat and rays of the sun.
It is possible to bend the floppy diskettes, but just because you can do it, is not agood reason to do it. Bending a floppy diskette can cause tiny wrinkles in thefloppy media which may not be noticeable to the human eye but cause problemsfor the floppy disk drive. When the problem is big enough for the floppy diskdrive to not read the diskette properly than it has become a big problem.
Labels help computer users identify what they have on the floppy diskette. Likelabels used on books, the more meaningful the label the easier it is to understandwhat is inside the floppy diskette. Place the label on the front of the diskette, atthe top, so that the label does not stick to any exposed areas on the diskette.
Connecting to the Computer
Hard Disks are connected to the microcomputer bus through an interface. Onolder machines a hard disk controller card is often required, while on newer ma- chines (since it is commonly understood that everyone will need a hard-disk) thecontroller interface is built onto the motherboard. The controller card, interface,includes the circuitry that serves as an interface between the motherboard and thehard disk.
Two major standards for Secondary Storage interfaces have been adopted. The
most common is the Integrated Drive Electronics (IDE) with an increasing num-
ber of machines now supporting a second standard due to its greater flexibility (the
Small Computer System Interface, SCSI).
Integrated Drive Electronics
When IBM released its IBM PC AT with an IDE controller card, this defined for
copiers the standard interface for hard disks. At first, the electronics was built
onto an interface card that was plugged onto the motherboard, and with the in-
creasing use of hard disks most new machines now have the IDE electronics built
into the motherboard. Continuing improvements to the IDE interface standard has
seen IDE improve to support increasing capabilities of hard disk drives. These
improvements have meant changing the IDE acronym to EIDE (Enhanced IDE),
IDE Mode 4, etc.
An important limitation in the IDE design is the limit of allowing only two devicesper interface. A microcomputer with one IDE interface can have at most twohard disks. A microcomputer with two IDE interfaces can have at most four harddisks. In general only two IDE interfaces can be used in a microcomputer.
Small Computer System Interface (SCSI)
Small Computer System Interface (SCSI)
is an adaptation of older technology
than IDE but was only previously available on more expensive computer systems.
SCSI’s major advantages over IDE include the ability to transfer data from the
computer to the hard disk at a much faster speed, the ability to connect more de-
vices per interface, and the ability to connect to input devices other than hard disks
(such as scanners).
SCSI interfaces can support up to seven devices each, and as many SCSI inter-faces that can fit on the motherboard can be supported. An important develop-ment for the SCSI interface is the increasing popularity of scanners (input devicesthat can grab images from paper documents) and CD-ROMs. Scanners require aninterface to the computer that can cope with the large amount of data being sentvery quickly. SCSI copes with scanners very well without the limiting constraintsIDE places on the number of devices that can be connected.
SCSI has become an important standard for connecting devices for transferringlarge volumes of data to the computer.
Iomega ZIP Drive
Iomega Corporation released a new disk storage product they called the ZIP drivein 1994. The distinct advantage of the ZIP Drive was its portability, similar tofloppy diskettes, and its storage capacity similar to a hard disk. The ZIP disk in-creased the storage capacity for a removable storage device 100 fold(from 1.44MB to 100MB) while at the same time controllingcosts and maintaining the reliability attributed to hard disks.
The ZIP drive unit is 18 x 13 x 4 cm and weighs about half-a kilo.
has rubber feet to stabilise the unit in either vertical or horizontal positions. Thereare two indicator lights: green for power and amber for disk access. It has an ejectbutton, but no on/off switch.
Three models, a SCSI model, an IDE and a parallel port model, provide customerswith the option for speed (SCSI), cost-effective speed (IDE) and extremeportability. The portability of the SCSI drive model includes theability to be moved between Macintosh and IBM PCs. Dueto the greater transfer rate of SCSI devices, the SCSImodel is about 2 to 5 times faster than the parallel portmodel.
The parallel port model can only be used with a PC and connects to the computer'sparallel port. Since all PCs have printer ports, the parallel port model is ex-tremely transportable between PCs. The drive also offers a printer pass-thoughfeature so you can still use your printer while the ZIP drive is connected.
The IDE model can only be used with a PC and connects to the IDE connector,similar to most PC Hard Disks, with a door similar to the floppy disk for the ZIPto be put into the drive (computer) and taken out with ease.
Jaz Drives.
Iomega also introduced the market to a higher capacity removable storage devicecalled the Jaz drive. The Jaz drive can pack up to 2G on each disk , and there isno limit to the number of disks you can use. An older version is available withsupport for only 1G disks.
Because Jaz drives are only available as SCSI devices they are faster Like Zip drives, Jaz Drives are portable and make transporting large amounts of information easier. It weighs less than 1 kilogram.
Jaz drives come as external (portable) and internal. A Jaz insider drive fits into any available 3.5" drive bay, the same space reserved for addingnew Hard Disks into most computers.
With the Jaz drive's speed and the Jaz disk's capacity and price, the Jaz is a usefullarge system backup device. In environments where large amounts of data are ma-nipulated a Jaz drive is a good supplement to the existing hard disk.
Jaz Disks.
Compact Disks and Digital Versatile Disks (DVD)
Our discussion so far on secondary storage has focussed on magnetic media de-vices. With the increased requirement for larger datafiles by multi-media applica-tions the high cost of magnetic medias has increased the research and availabilityof optical (laser) based storage devices.
CD-ROM Drives are an old technology but their popularity has increased with thecorresponding requirement by programs with animated pictures, video, and soundfor more storage space. Products such as encyclopedias and Windows 95 applica-tions with animated tutorials require far more space than is practical with floppydiskettes. For example, the CD-ROM version of Windows 95 would require over24 floppy diskettes to store; each diskette would cost approximately USD 50¢ toput together for the manufacturer whereas the complete CD-ROM would cost themanufacturer approximately USD$1.00.
With the increasing number of computers with CDROMs, a market has grownfor CD Writers (Drives which can write onto CDs). With a much lower cost of media (the CDs themselves) and a more permanent record of what has been written, CDs have become an important tool for people distributing soft- ware tools, data within an organisation due to the large capacity and ready avail-ability of CDROM readers. For data that does not change, such as the nationalCensus records, or Court Records, Parliamentary Records, CD Writers are an ef-fective tool.
Standard computer CDs suffer from the problem of being slow compared to mag-netic disks and the CDs being read-only.
To help alleviate, solve the speed and read-only problem, a number of opticalproducts have been introduced with little market success. Digital Versatile Disks(DVD), a new optical drive and CD format, is being sponsored by number of the large technology manufacturers to hopefullyprovide a solution for the increasing need for large stor-age, speed, and lower costs.
DVD is promoted as the successor to CD-ROMs, laserdisks, audio CDs, and VHS video cassettes. With an ex-pected capacity of 17 Gigabytes, DVDs should be able tostore full movies, audio CDs, and high quality images while still be-ing the same physical size as today’s CDs. Although the 1997/98 diskdrives and DVDs are read-only the writable variations should appear on the mar-ket by late 1998 or in 1999.
Terms and
Review Questions
1. A disk which is write protected means that:a) the write protect notch is coveredb) existing files cannot be erased from the diskc) new files cannot be written on the disk.
d) all of the abovee) none of the above 2. A computer has two types of storage. Describe at least one difference between pri-mary storage and secondary storage.
3. Examples of Primary storage are RAM and ROM, describe one difference and onesimilarity between them.
a) Similarity:b) Difference: 4. Disks (hard and floppy) are examples of secondary storage devices, and are verypopular. Give 2 reasons why disks are popular.
5. Describe one difference between a floppy disk and a hard disk 6. The way data is stored in main memory and disks differ in many ways. Describe one
difference and give one disadvantage and one disadvantage for each.
a) Memory:
i) method:
ii) advantage
iii) disadvantage
b) Disk:
i) method:
ii) advantage
iii) disadvantage
7. Floppy disks are available in various types and thus differ in capacity. See if you canwrite up the table and capacities relative to the disk size and density.
Sources and References - Computer Studies Course Notes - Tonga on the 'NET is available on all networked computers at Queen Sa-lote College.


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