Information Technology Reference
In-Depth Information
slower than direct storage in CPU registers, but faster than storage in
main memory. Because processing typically uses relatively small
amounts of data at a time, current data can be stored in cache for rel
atively fast access.
The relationship between main memory and file storage on
tapes, disks, and CDs is similar. Access to data on tapes, disks, and
CDs is slow, because these media require physical activity to store
and retrieve data. For example, to obtain data from a tape, the tape
is moved along a device (sometimes called a
read head
) that senses
electronic signals on the tape. Similarly, placing data on a tape re
quires the tape to move over a device (called a
write head
) that
places charges on the tape. Sometimes the same device is used for
both reading and writing, in which case it is called a
read/write
head
. Disks and CDs must spin around until relevant data can be
detected by read/write heads. For disks, those read/write heads also
must move inward or outward from track to track to reach the ap
propriate sector. All of this activity requires something to move
physically, and such motion consumes much more time than activi
ties requiring only electrical processing. On the other hand, tapes,
disks, and CDs have some great advantages: storage is permanent,
the devices are relatively inexpensive, and storage capacity is much
greater than that found in main memory.
Coupled with this contrast between main memory and file stor
age, computer applications have expanded dramatically in size over
the years, and many applications now require huge amounts of
space when they run. Thus, even as technology advances have al
lowed memories to have relatively large capacities (at least in rela
tionship to the past), the demands of applications have increased
even faster. For example, crisp and clear graphical images often con
sume thousands of bytes (or kilobytes) of storage, and such images
are now used commonly in both wordprocessing documents and
Web applications.
Unfortunately, as you have seen in Chapter 1, the CPU controls
all processing in a computer and handles many specific tasks. Thus,
in order to run at all, instructions from computer programs and ap
plications must be placed in the CPU during program execution to
guide the computer in its processing, in the same way that data must
move to and from CPU registers for processing. Because the CPU is
loaded from main memory, main memory must hold program in
structions and data to allow processing to occur.
