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External memory which is sometimes called backing store or secondary memory, allows the permanent storage of large quantities of data. Some method of magnetic recording on magnetic disks or tapes is most commonly used. More recently optical methods which rely upon marks etched by a laser beam on the surface of a disc (CD-ROM) have become popular, although they remain more expensive than magnetic media. The capacity of external memory is high, usually measured in hundreds of megabytes or even in gigabytes (thousand million bytes) at present. External memory has the important property that the information stored is not lost when the computer is switched off.
The most common form of external memory is a hard disc which is permanently installed in the computer and will typically have a capacity of hundreds of megabytes. A hard disc is a flat, circular oxide-coated disc which rotates continuously. Information is recorded on the disc by magnetising spots of the oxide coating on concentric circular tracks. An access arm in the disc drive positions a read/write head over the appropriate track to read and write data from and to the track. This means that before accessing or modifying data the read/write head must be positioned over the correct track. This time is called the seek time and is measured in milliseconds. There is also a small delay waiting for the appropriate section of the track to rotate under the head. This latency is much smaller than the seek time. Once the correct section of the track is under the head, successive bytes of information can be transferred to the main memory at rates of several megabytes per second. This discrepancy between the speed of access to the first byte required, and subsequent bytes on the same track means that it is not economic to transfer small numbers of bytes. Transfers are usually of blocks of several hundred bytes or even more. Notice that the access time to data stored in secondary storage will depend on its location.
The hard disc will hold all the software that is required to run the computer, from the operating system to packages like word-processing and spreadsheet programs. All the user's data and programs will also be stored on the hard disc. In addition most computers have some form of removable storage device which can be used to save copies of important files etc. The most common device for this purpose is a floppy disc which has a very limited capacity. Various magnetic tape devices can be used for storing larger quantities of data and more recently removable optical discs have been used.
It is important to note that the CPU can only directly access data that is in main memory. To process data that resides in external memory the CPU must first transfer it to main memory. Accessing external memory to find the appropriate data is slow (milliseconds) in relation to CPU speeds but the rate of transfer of data to main memory is reasonably fast once it has been located.