1. Storage Devices

2. 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, CD-ROM, microfilm

3. 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 situation.
Random Processing
Each record has its own address allowing direct access
Address can be calculated from a unique key
Or retrieved from a separate index
Typical in Database Management Systems

4. Floppy Disks & Zip Disks
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
Disk Storage
Diskette has two surfaces, each with (typically) 80 concentric circles called tracks.
Tracks are divided into sectors when disk is formatted

5. Hard Disks PC Minis & mainframes
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, giving rise to concept of cylinders.
Cylinder is the set of tracks that multiple read/write heads access in one position.

6. Hard Disk layout

7. 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

8. 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
1 0 1
1 1 1
0 1 1
0 0 0
1 1 0
0 0 1

9. Other Devices CD-ROM CD-R CD-RW 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
CD-R
Similar to CD-ROM, but usually gold in colour
Intended use for firms to store their own data for rapid retrieval
Favoured by ‘pirates’ to produce illicit copies of software
CD-RW

10. Other Devices Magneto-Optical Disks Microfiche
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.
Microfiche
COM (computer output on microfilm) devices create the4” by 6” hard-copy films that are microfiche
Each sheet may contain upto 270 frames each containing a page of information which can be viewed using a special viewer