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Storage Devices

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Primary & Secondary Storage
Primary Storage
­ Volatile storage, such as RAM, is primary storage
­ All contents lost when machine switched off
­ Good practice to make regular back-ups
Secondary Storage
­ Non-volatile, more permanent storage for
software and data files
­ For example:
· Magnetic Tape, Magnetic Disks,…

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File Processing
Method usually depends upon whether every record
needs to be processed.
Sequential Processing
­ Every record has to be read. Typically used in a payroll
Random Processing
­ Each record has its own address allowing direct access
· Address can be calculated from a unique key

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Floppy Disks & Zip Disks
­ 3.5" Floppy Disk
· Thin plastic disk (mylar) covered in metal oxide in a hard
plastic case.
· HD (high-density) disk can store 1.44 Mb
­ Zip Disk
· Slightly larger than floppy but similar construction
· Hold 100Mb or 250Mb of data

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Hard Disks
­ One or more platters in a sealed casing.
­ Typically 5Gb to 30Gb as of Spring 2001.
Minis & mainframes
­ May be fixed or removable
­ Fixed are faster, more reliable, greater storage capacity
­ Similar storage in concentric track, but on multiple platters,

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Hard Disk layout

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Magnetic Tape
Magnetic Tape is obviously a serial medium.
­ Have to read through tape to find a required record
­ Updates require creation of a new tape
· Data cannot be changed in situ
Cheap and convenient for backup
­ Cartridge tape for PCs can store up to 20Gb

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Magnetic Tape

Data record in frames across the tape
One frame represents one byte
Frames form tracks along the tape, typically 9
tracks ­ 8 data tracks and one parity track

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Other Devices
­ Stores up to 700Mb.
­ Various data types
­ Read-only. Read by reflecting a laser beam off the surface
and detecting the presence or absence of tiny pits which will
have been burned into the surface to represent binary digits
­ Similar to CD-ROM, but…

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Other Devices
Magneto-Optical Disks
­ Integrate optical and laser technology for read and write storage.
­ A 5.5" disk can store up to 2.6 Gb.
­ Still under development, but may replace magnetic disks when price
and speed permit.
­ COM (computer output on microfilm) devices create the4" by…


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