A storage device stores programs and data either temporarily or permanently.
The capacity of a storage device is described in terms of bits, bytes, kilobytes, megabytes and gigabytes.
| Unit | Equivalent To |
| 1 Bit | A single storage location which can hold either a 0 or a 1. |
| 1 Byte | 8 Bits |
| 1 Kilobyte (Kb) | 1024 Bytes |
| 1 Megabyte (Mb) | 1024 Kilobytes |
| 1 Gigabyte (Gb) | 1024 Megabytes |
- Immediate access store holds programs and data that the user is currently working with.
- Backing store keeps data and programs when the
computer is turned off.
5. Immediate access store is also known as main store or primary store. Backing store is also known as secondary store.
6. Immediate access store can be either ROM or RAM : - Read Only Memory (ROM) : Contents are kept when computer is turned off (non-volatile) but can not be altered.
- Random Access Memory (RAM) : Contents are lost when
computer is turned off (volatile) and can be altered by the user.
7. ROM is used to store the BIOS program in a microcomputer and the programs that run embedded systems. RAM is used to store programs and data the user is currently working with.
8. There are many different types of backing storage device. The main ones are :
Types of Backing Storage Device Type Examples Common Applications Magnetic Disk Floppy Disk Transporting files, small backups. 
Hard Disk Running software, storing data files.
Zip/Jaz Transporting big files, medium size backups.
Optical Disk CD-ROM Distributing software, multimedia products.
CD-R Permanent backups, small volume distribution.
CD-RW Temporary backups.
DVD-ROM High quality digital films.
DVD-RAM Backup of moderate volumes of data.
Magnetic Tape High volume backups, batch processing.
Flash Memory Transferring files, storage in compact devices.
9. To select an appropriate storage device for a task several different criteria should be considered : - What the maximum storage capacity of the device is.
- How much the device costs per megabyte (Mb) of storage space.
- How quickly the data stored on the device can be accessed.
- Whether the device can store data whilst the computer is turned off.
- Whether the data stored on the device can be changed or not.
- Whether the storage device can be used to transfer data from one computer to another or not (is it portable?).
- The method used to store the data e.g. electronically, magnetically, optically.
- Direct Access : The drive head can move directly to any record/file on the device so locating data is very fast. All types of disk are direct access. Used for transaction processing.
- Serial Access : The drive head must read through all of the data that comes before the record/file to be loaded so locating data is slow. Only magnetic tapes are serial access. Used for batch processing.
- Sequential Access : Identical to
serial access, but records/files are stored in a particular order.
14. Data can be compressed before it is stored so that it will use up less storage space. 15. Compression is the act of reducing the size of a file without losing any of the data it contains. Decompression is the act of returning a compressed file to its original size and format. A file that has been compressed must be decompressed before it can be used. 16. How much a file size is reduced by when it is compressed will depend upon the type of data that is stored in the file and the method used to compress the file. Important applications of compression are distributing software, hard disk compression and data transmission. 17. Common file processing techniques include sorting and merging :
11. The table below compares the main types of storage device.
Device Capacity Cost per Mb Transfer Speed (capacity per second) Direct or Serial Access Stores Data when Computer Switched Off Can Have Data Written to It Portable Immediate Access Store RAM 32Mb to 1024Mb 35p 500Mb to 1500Mb Direct No Yes No
ROM 1Kb to 64Mb 35p 500Mb to 1500Mb Direct Yes No No
Backing Store Floppy Disk 1.44Mb 17p 0.2Mb Direct Yes Yes Yes
Hard Disk 40Gb to 300Gb 0.05p 40Mb to 100Mb Direct Yes Yes No
Zip / Jaz 100Mb to 2Gb 1p 0.5Mb to 8Mb Direct Yes Yes Yes
Optical Disk 650Mb to 17Gb 0.02p 3Mb to 22Mb Direct Yes Depends on Type Yes
Magnetic Tape 20Gb to 400Gb 0.03p 5Mb to 60Mb Serial Yes Yes Yes
Flash Memory 16Mb to 1024Mb 20p 0.4Mb to 10Mb Direct Yes Yes Yes
12. Caching is a technique used to speed up access to data stored on a backing storage device. A cache is a part of a computer's RAM that is set aside to store copies of frequently used sectors on a backing storage device such as a hard disk drive so that they can be accessed more quickly.
13. Backing storage devices can be accessed using one of three access methods :