Information Technology Reference
In-Depth Information
turned off. These are examples of
transitory memory
, which does
not retain data values after processing is completed or when electric
power is discontinued. Data held in RAM or the CPU is not re
tained from one run of a program to the next, so this type of mem
ory cannot be used for longterm storage of information.
In contrast, data stored on disks, tapes, and CDs can be retained
and retrieved over long periods of time—at least with reasonable
care—and these storage media are called
permanent memory
. Even
though this name suggests unlimited storage times, one must be a bit
careful. Disks and tapes can be erased by magnetic fields—be careful
to not carry them in a case that also contains a refrigerator magnet.
Tapes can become brittle and break or distort when subjected to
heat. CDs can become scratched. And, for any of these media, it
would be unwise to tack them to a bulletin board in a lostandfound
area. However, with reasonable care and handling, disks, tapes, and
CDs will hold data for many years without trouble.
Files and Their Organization: From the discussion in Chapter 2
of how various types of data are stored, we know that all informa
tion for any application is stored within a computer as a sequence
of electrical voltages, which correspond to a series of 0's and 1's. At
this basic level, work for any application has a similar look, and bi
nary information for an application typically is stored as a logical
collection, called a
file
. According to the needs of the application,
such a file may range from only a few bytes to many thousands of
bytes. Thus, conceptually, a file is simply a sequence of bits stored
on a permanent memory device, such as a disk, tape, or CD.
When using tapes to store data from memory, computers nor
mally begin reading the tape at one end and continue reading until
the other end. With this simple approach for processing, file han
dling is fairly straightforward. To store a file on a tape, the tape first
is examined until empty space is found. Then, the computer writes
a file header, including a file designator or a name provided by the
user, and the file material itself. To retrieve the file, the computer
reads the tape from the beginning, until the start of the file is found.
Then the material is retrieved in sequence. Although the examina
tion of tapes from their beginning can take some time, such storage
and retrieval is quite simple to implement.
Although information could, in principle, be stored on disks in
much the same way as on tapes, such a linear organization does not
