Data storage

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A computer uses two types of storage. A main store consisting of ROM and RAM, and backing stores which can be internal, eg hard disk, or external, eg a CD or USB flash drive.


Main store (or computermemory) is divided into Read Only Memory (ROM) and Random Access Memory (RAM).


ROMis memory that cannot be changed by aprogramor user. ROM retains its memory even after the computer is turned off. For example, ROM stores the instructions for the computer to start up when it is turned on again.


RAMis a fasttemporarytype of memory in which programs,applicationsanddataare stored. Here are some examples of what's stored in RAM:

  • theoperating system
  • applications
  • the graphical user interface (GUI)

If a computer loses power, all data stored in its RAM is lost.

two RAM modules, green PCB (plastic circuit board) with gold contacts along one of the two longest sides (

Storage capacity and file size

Storage capacities and file sizes are measured from lowest to highest in:

  • bits
  • bytes
  • kilobytes
  • megabytes
  • gigabytes
  • terabytes

Anoperating systemabbreviates these measurements, eg 1 megabyte becomes 1MB (megabyte).

Relationship between measurements

The table below outlines the relationship between bits (smallest) and terabytes (largest). Read each row of the table from left to right:

Relationship between measurement

SizeEqual to 8 bits 1 byte 1024 bytes 1 kilobyte 1024 kilobytes 1 megabyte 1024 megabytes 1 gigabyte 1024 gigabytes 1 terabyte

The size of a file and a storage device's capacity will always be written in its simplest form. For example, an operating system would report a 1 terabytehard disk'ssize as 1TBnotas 1024GB, although both are correct.

Example file sizes

The table below lists files commonly found on a computer and their typical file size (compressed):

FileFile size Photo 3MB Song 5MB Film 700MB

A file’s size can be influenced by a number of factors but ultimately the more information a file stores, the larger it will be.

The hard disk

The main internal backing store is a computer's hard disk.

Hard disk

the top side of a hard disk (

The top side of a hard disk

  1. Back
  2. 1
  3. 2
  4. Next

A hard disk stores:

  • theoperating system
  • softwareapplicationsorprograms
  • the majority of your data files

Hard disks spin at very high speeds (around 7,200 RPM - revolutions per minute) within a sealed unit inside the computer. Hard disks store large amounts of data - 200GB to 1TB is common in desktop computers. The data stored on a hard disk is retained until deleted, but it needs to be loaded into main store RAM before it can be used.

Floppy and Zip disks

bright green floppy disk (

Floppy disks hold very small amounts ofdata, just 1.44MB and a computer needs a floppy drive to read them. They are used to:

  • transfer small files of data from one machine to another
  • backupimportant small files that are stored on yourhard disk
  • store restricted files that you don't want other users of your computer seeing

Zip disk

blue zip disk (

Zip disks…


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