2The 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.
3? However, where to store this data is the question. We can not change what’s in ROM so that is out of the questionRAM is volatile so it will not preserve anything we place there when the computer is offIn addition both ROM and RAM are much too small anyway.
4Thus 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
5Secondary StorageSometimes 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.
6Medium vs DeviceThere 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 storedDevice – the mechanism used to access the data on the medium
8Distinguishing Characteristics of Secondary Storage AS we examine some secondary storage media, look out for the following :their capacitytheir access speedtheir access method andtheir portability.
9Maximum amount of data which a data storage medium can hold DefinitionsStorage CapacityMaximum amount of data which a data storage medium can hold
10How quickly the required data is retrieved by the storage device DefinitionsAccess SpeedHow quickly the required data is retrieved by the storage device
11Readily and easily transported from one computer system to another. DefinitionsPortabilityReadily and easily transported from one computer system to another.
12Definitions Access Method What particular mechanics/algorithm is used to locate specific data at a particular place on a storage medium.SASD : Sequential Access Storage DeviceDASD : Direct (Random) Access Storage Device
15The Floppy Disk Floppies have come a long way There was the 8” floppy, and the 5¼” floppy, and then the 3½“ floppy.
16The Floppy DiskThe present floppy is a 3.5 inch magnetic disk of flexible material which typically stores 1.44 MB of data maximum
17The Floppy DiskThe 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.
18The 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
19The Floppy DiskBefore 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.
20The Floppy DiskThe 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
22The Floppy DiskFormatting 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.
24Floppies are highly portable and are great for transporting small files between machines They are direct access but rather slowMost desktops still have a floppy drive but newer laptops no longer contain a floppy disk drive
32The Hard DiskThe 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
33The Hard DiskThe hard disk and the hard drive are packaged together
34The Hard DiskEach platter has a pair of read/write heads which are used for writing to and reading from that particular surface of the disk.
35The Hard DiskEach 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.
36The Hard DiskAs 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
37The Hard DiskIt 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.
38The 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.
39The Hard DiskHard disks are direct access devices. The data transfer rate is also much higher than for floppy disksThey also have a FAT which store the addresses of the files stored thereon.
41Magnetic 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
42Magnetic TapeThe data is stored in tracks that run along the entire length of the tape. Usually there are 9 tracks.
43Magnetic TapeMagnetic 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.
45Optical DisksThese 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.
47Optical DisksOptical 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.
48Optical DisksThe earliest optical disks could only be written to once so they became known as write once read many (WORM)disks.
49The CD-ROMThe most common WORM disk is the Compact Disc Read Only Memory[CD-ROM].A typical CD-ROM holds 650MB of data
50The CD-ROMCD-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
51The CD-RCD-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
55The CD-RWCD-RW (CD rewriteable)These became available in 1997
56The CD-RWWhile 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.
57The CD-RWAn 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)
58The DVD Digital video disk Digital versatile diskDigital video diskThese 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
59The DVDDVDs 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.
60The DVD The DVD is a direct access device and is fairly fast. Types DVD-ROMDVD-RDVD-RAM (DVD-RW)
61Optical DisksThe 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
63My 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?
64My QuestionsDescribe the potential impact of optical disk technology on public and university librariesDescribe 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?
65ASSIGNMENTThe ‘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’?
66FLASH MEMORYYou 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
67Reading AssignmentComputers – Information Technology in Perspective : Long and Long 12th edition Pages 192 to 206Understanding Computer Science for Advanced Level : Ray BradleyChapter 12A’Level Computing : Heathcote and Langfield Pages 144 to 148