SECONDARY STORAGE DEVICES Y.Colette LeMard

The ability to store data and programs so that they can be used/reused in the future is one of the most important features of a computer.

? However, where to store this data is the question. We can not change what’s in ROM so that is out of the question RAM is volatile so it will not preserve anything we place there when the computer is off In addition both ROM and RAM are much too small anyway.

Thus the need for : secondary storage a. k. a. auxiliary storage a. k Thus the need for : secondary storage a.k.a. auxiliary storage a.k.a. storage

Secondary Storage Sometimes secondary storage is also called external storage. However, it is important to bear in mind that one of our most important secondary storage media is the hard disk which is not external.

Medium vs Device There is also the tendency when we discuss secondary storage to mix up medium and device. Medium – the actual physical material on which the bits are stored Device – the mechanism used to access the data on the medium

Medium Device Floppy disk Floppy drive

Distinguishing Characteristics of Secondary Storage AS we examine some secondary storage media, look out for the following : their capacity their access speed their access method and their portability.

Maximum amount of data which a data storage medium can hold Definitions Storage Capacity Maximum amount of data which a data storage medium can hold

How quickly the required data is retrieved by the storage device Definitions Access Speed How quickly the required data is retrieved by the storage device

Readily and easily transported from one computer system to another. Definitions Portability Readily and easily transported from one computer system to another.

Definitions Access Method What particular mechanics/algorithm is used to locate specific data at a particular place on a storage medium. SASD : Sequential Access Storage Device DASD : Direct (Random) Access Storage Device

MAGNETIC MEDIA

The Floppy Disk

The Floppy Disk Floppies have come a long way There was the 8” floppy, and the 5¼” floppy, and then the 3½“ floppy.

The Floppy Disk The present floppy is a 3.5 inch magnetic disk of flexible material which typically stores 1.44 MB of data maximum

The Floppy Disk The thin plastic circle is enclosed in rigid plastic to protect it. A metal sliding access shutter opens when the disk is in the drive so that the read/write head can access the disk itself.

The Floppy Disk Data can be written to and read from a floppy disk. A write protect tab can be used to prevent accidental overwriting of data

The Floppy Disk Before data can be written to a disk, it must be formatted. This prepares the surface for storing bits by creating a map of sectors and tracks.

The Floppy Disk The address of a particular file is determined by the track and sector it occupies. Most floppies now come preformatted, so many users are not aware of the requirement for formatting

The Floppy Disk preformatted Micro-floppy disk 2 -sided

The Floppy Disk Formatting also creates a File Allocation Table (FAT) - a directory which stores the names of the files on the disk and their address. A FAT is necessary for management of the storage space on the disk.

Floppies are highly portable and are great for transporting small files between machines They are direct access but rather slow Most desktops still have a floppy drive but newer laptops no longer contain a floppy disk drive

The Zip Disk

The Zip Disk The zip disk comes in different capacities ranging from 100MB to 750MB. This means that each can store the contents of many floppies

The Zip Disk The zip disk is portable and the disk itself operates similarly to a hard disk.

The Zip Disk Zip disks require a zip drive. These are not automatically attached to new systems however, they have to be specially ordered. A zip drive can be external or internal.

The Zip Disk Zip disks are relatively inexpensive. The 750MB version can store the same amount of data as 520 floppy disks. Physically the zip disk is about 1½ times the size of a floppy.

The Zip Disk Zip disks are popular for backing up hard disks and for transferring large files from one computer to another.

The Hard Disk

The Hard Disk The hard disk is made from rigid aluminum coated with a magnetic material. It is normally supplied as a sealed unit and will often consists of a number of disks (platters) on a common spindle

The Hard Disk The hard disk and the hard drive are packaged together

The Hard Disk Each platter has a pair of read/write heads which are used for writing to and reading from that particular surface of the disk.

The Hard Disk Each disk is divided into tracks and sectors. Each sector can store between 256 and 512 bytes. The tracks are immediately above and below each other. A particular set of such tracks is called a cylinder.

The Hard Disk As the spindle turns the platters, the read/write heads move together in and out between the platters. At any point in time they are all positioned over one particular cylinder

The Hard Disk It is now fairly standard for a pc to have a hard disk of at least 60GB. When PCs were first used in the 1980s, the hard disk’s capacity was measured in MB. But modern data and programs take up much more space especially as they are now GUI based.

The Hard Disk Hard disks are pretty reliable However as a standard they are fixed inside the systems unit/computer case. Portable (removable) units are available though, and tend to be used mostly by laptop owners.

The Hard Disk Hard disks are direct access devices. The data transfer rate is also much higher than for floppy disks They also have a FAT which store the addresses of the files stored thereon.

Magnetic Tapes

Magnetic Tape It may range from ¼” cartridges to large reels Tape is flexible reels of plastic with a magnetic coating. It may range from ¼” cartridges to large reels

Magnetic Tape The data is stored in tracks that run along the entire length of the tape. Usually there are 9 tracks.

Magnetic Tape Magnetic tape requires a special drive of course and access is slow as it is a sequential access devices. It is an ideal medium for backup however, because it is fast, inexpensive and has a high storage capacity.

OPTICAL MEDIA

Optical Disks These have a hard plastic disk with a mirrored surface. The data is stored by removing the mirroring from a particular point through the use of a laser. The laser creates a pit on the surface. These pits are usually on the underside of the disk and cannot be seen with the naked eye.

Optical Disks pits

Optical Disks Optical disks have a larger storage capacity than floppies but less than hard disks or tapes. They also fall in the middle in terms of access speed. They are portable but must be transported with care because scratching the recording surface will damage the data.

Optical Disks The earliest optical disks could only be written to once so they became known as write once read many (WORM)disks.

The CD-ROM The most common WORM disk is the Compact Disc Read Only Memory [CD-ROM]. A typical CD-ROM holds 650MB of data

The CD-ROM CD-ROMs are great for distributing programs and other material that is not to be changed. CD-ROM drives became a standard for desktops in the 1990s

The CD-R CD-R (CD-recordable) allows end users to record their own data on CDs. Data can be burned onto these once and thereafter cannot be changed. This makes CD-Rs great for permanent backup

The CD-R The CD-R is write once, read many. The dye layer allows writing of data to this disk

The CD-RW CD-RW (CD rewriteable) These became available in 1997

The CD-RW While a prerecorded CD has its information permanently stamped into its polycarbonate plastic substrate layer, a CD-RW disc contains a phase-change alloy recording layer.

The CD-RW An infra-red laser beam is used to heat (at 400oC) and melt, the crystallized recording layer into a fluid state or to freeze it at a lower temperature back to its crystalline state. In this way previous data is ‘erased’ and new data can be written (burned)

The DVD Digital video disk Digital versatile disk Digital video disk These look similar to CDs but are capable of storing up to 6 times more data principally because the tracks are closer together and because some store data on both surfaces

The DVD DVDs are used mainly for movies as the quality of the sound and the video is superior to that of videotapes. They are also used for software, and data archiving.

The DVD The DVD is a direct access device and is fairly fast. Types DVD-ROM DVD-R DVD-RAM (DVD-RW)

Optical Disks The CD-ROM drive is a standard on PCs, but now CD-R/W drives, DVD-ROM drives, and combo drives are also becoming very popular. DVD writers can also now be seen on sale in computer stores

Any Questions ?

My Questions Magnetic tape has sequential-access capabilities (T/F)? What is the most appropriate application of magnetic tape storage? Why are floppy drives no longer included on many newer machines? What happens during formatting? Why must hard disks and diskettes be formatted?

My Questions Describe the potential impact of optical disk technology on public and university libraries Describe at least two applications where CD-RW would be preferred over hard disk for storage. How many diskettes would you need to backup your Computer Science notes which is 2.8MB large. The DVD is compatible with the CD-ROM. Currently the DVD drive is more expensive than the CD-ROM, but prices are converging. Will the DVD replace the CD?

ASSIGNMENT The ‘THUMB DRIVE’ is now very popular for personal computing. What is a ‘thumb drive’? How does a thumb drive work/what mechanism does it use to store data? What are its advantages? What are the other names for this ‘device’?

FLASH MEMORY You may recall that EEPROM can be erased by using high voltages. There is a type of memory which can be erased with low voltages and then rewritten even while still inside the computer. This type of memory is called flash memory. The thumb drive is a type of flash memory

Reading Assignment Computers – Information Technology in Perspective : Long and Long 12th edition Pages 192 to 206 Understanding Computer Science for Advanced Level : Ray Bradley Chapter 12 A’Level Computing : Heathcote and Langfield Pages 144 to 148

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