CPU PRIMARY AND SECONDARY
The CPU contains circuits that control and execute
instructions by using some type of MEMORY. Memory
is referred to by size, such as 16K, 32K, 64K, and so on.
The "K" represents the value of 1,000. Therefore
16,000 is 16K.
Semiconductor memory consists of hundreds of
thousands of tiny electronic circuits etched on a silicon
chip. Each of these electronic circuits is called a BIT
CELL and can be in either an OFF or ON state to
represent a 0 or 1 bit. This state depends on whether or
not current is flowing in that cell. Another name used
for semiconductor memory chips is integrated circuits
(ICs). Developments in technology have led to
large-scale integration (LSI) that allows more and more
circuits to be squeezed onto the same silicon chip.
Some of the advantages of semiconductor storage
are fast internal processing speeds, high reliability, low
power consumption, high density (many circuits), and
low cost. However, there is a drawback to this type of
storage. It may be VOLATILE, which means it requires
a constant power source. When the power for your
system fails and you have no backup power supply, all
of the stored data is lost.
Two classifications of primary storage with which
you should become familiar are read-only memory
(ROM) and random-access memory (RAM).
READ-ONLY MEMORY (ROM).In computers,
it is useful to have instructions that are used often,
permanently stored inside the computer. ROM enables
us to do this without loosing the programs and data
when the computer is powered down. Only the
computer manufacturer can provide these programs in
ROM; once done, you cannot change it. Consequently,
you cannot put any of your own data or programs in
ROM. Many complex functions, such as translators for
high-level languages, and operating systems are placed
in ROM memory.
Since these instructions are hardwired, they can be
performed quickly and accurately. Another advantage
of ROM is that your imaging facility can order
programs tailored for its specific needs and have them
installed permanently in ROM. Such programs are
called microprograms or firmware.
RANDOM-ACCESS MEMORY (RAM).RAM
is another type of memory found inside computers. It
may be compared to a chalkboard on which you can
scribble down notes, read them, and erase them when
finished. In the computer, RAM is the working
memory. Data can be read (retrieved) or written
(stored) in RAM by providing the computer with an
address location where the data is stored or where you
want it to be stored. When the data is no longer
requited, you may simply write over it. Thus you can
use the storage location again for something else.
Secondary storage, or auxiliary storage, is memory
external to the main body of the computer (CPU) where
programs and data can be stored for future use. When
the computer is ready to use these programs, the data is
read into primary storage. Secondary storage media
extends the storage capabilities of the computer system.
Secondary storage is required for two reasons. First, the
working memory of the CPU is limited in size and
cannot always hold the amount of data required.
Second, data and programs in secondary programs do
not disappear when the power is turned off. Secondary
storage is nonvolatile memory. This information is lost
only when you erase it. Magnetic disks are the most
common type of secondary storage. They may be either
floppy disks or hard disks (hard drives).
Peripheral devices include all the input and output
devices used with a computer system. When these
devices are under control of the CPU, they are said to
be on line. When they perform their function
independently, not under direct control of the CPU, they
are said to be off line. The following peripheral devices
are used commonly for input and output. Those that
perform only input are marked (I), those that perform
only output are marked (O), and those that perform both
input and output are marked (I/O).
Optical Character Reader (I)
An optical character reader reads printed data
(characters) and translates it to machine code.
The keyboard is used by a computer operator to
communicate with a computer system.