Storage : Backing Store

If data needs to be kept whilst a computer is turned off then it must be stored on backing store. Any programs or data that are not currently being used by a computer will be kept on backing store. When programs or data are used they are copied (loaded) into immediate access store for faster access.

Information stored on backing store is placed on a storage medium. The most common media which are used for backing store are :

• Magnetic Disks

• Magnetic Tapes

• Optical Disks such as CD-ROMs and DVDs

• Flash Memory Devices

The data is read from or written to the storage medium by a piece of hardware known as a drive or a storage device.

It takes much longer to access data which is on backing store than data which is in immediate access store, typically 100 to 1000 times as long. This is because most backing storage devices operate mechanically. Computers have much more backing store than immediate access store for two reasons :

1. IAS only needs to store programs and data that are currently being used whereas the backing store needs to hold all of the programs and data that can be used on the computer.

2. Backing store is much cheaper per Mb than IAS.

Programs and data can not be used directly from backing store. They must be copied (loaded) into immediate access store before they can be used. Any data which needs to be kept must be transferred back to the backing store from the immediate access store before the computer is switched off. This is called saving.

Sometimes data is compressed before it is stored. Compressing data reduces the storage space that a file uses without losing any of its contents.

GCSE ICT Companion 04 - (C) P Meakin 2004