No matter how large a computer a person or business has, there is always the problem of having useful programs that require more information than can be stored in internal memory. Adding more internal memory is not always the best solution because the larger the memory, the longer it takes to find each byte. Also, the cost per bit of storage is much greater for RAM than for disk drives, the most common external memory devices. Therefore computer systems usually have some kind of external memory.

External memories differ from internal memories in three important ways. First, external memories do not provide random-access to a single byte of information. Rather, information is organized as blocks of hundreds of bytes. Second, the access time, that I, the time needed to locate information, is many times longer for external memory than for internal memory. However, once access to a location in external memory is established, the information-transfer rate can be as fast as for internal memory. Third, external memories provide permanent storage of information, even when power is lost.

As a computer program runs, it transfers information between internal and external memory. Good programs usually transfer many blocks of information at one time to minimize the total access time. Disk drives are the most common type of external memory devices. The access time is the time necessary to rotate the disk and move the head into position to read or write the information.

Archival Memory

Some information is important for historical purposes but is rarely needed in data processing. For example, many businesses have payroll programs that they run once a month and tax programs that they run once a year. Information that is too important to lose but is needed only rarely is stored in archival memory. Magnetic tape is the most common medium for archival storage of computer information. Magnetic tape drives transfer information between tape and other forms of memory.

Archival memories provide inexpensive and secure storage of important information. Storing such information on disks, especially hard disks that cannot be removed from their drives, would soon exhaust the available external memory space.

Another important use or archival memory devices is to save duplicate copies (back-ups) of information. Computer systems managers usually perform back-ups on a daily basis because the failure of a disk drive would lead to the permanent loss of all the information on the disk. Daily backups provide a means to recover most of the lost information.