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.
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