l To hold the program statements transferred from
an I/O device. This area is called the program
Please note that the four areas (input, working,
output, and program storage) are NOT fixed in size or
location, but rather are determined by each individual
About now, youre probably wondering how the
control unit is able to find these stored instructions and
data items. To understand this, picture memory as a
wall of post office boxes in a post office. Each box has a
different number (address) and represents a specific
storage location in memory, as shown in figure 1-3.
Like the mail in a post office box, the contents of a
storage location can change, but the number on the post
office box or memory address does not change. In this
manner, a particular program instruction or data item
that is held in primary storage can be located by
knowing its address.
It is the responsibility of the
programmer to assign descriptive names to these data
items. This enables the computer program and the
computer to keep track of the storage location address of
each data item.
Primary storage can be classified by its physical or
Memory Types by Physical Characteristics
Primary storage devices may be classified
according to the type of magnetic or electronic principle
they use to store data. Some of the more common types
are magnetic core storage, semiconductor storage, and
MAGNETIC CORE STORAGE. Magnetic
core storage, although not used as much as it used to be,
provides an easy way to show the general concepts of
memories, including integrated semiconductor and
bubble types of memories. Magnetic core storage is
made up of tiny doughnut-shaped rings made of ferrite
Figure 1-3.Memory locations.
(iron), which are strung on a grid of very thin wires.
Because computers store data in binary form (covered
in chapter 3), a two-state device is needed to represent
the two binary digits (bits), 0 for OFF and 1 for ON. In
core storage, each ferrite ring can represent a 0 bit or a 1
bit, depending on its magnetic state. If magnetized in
one direction, it represents a 1 bit, and if magnetized in
the opposite direction, it represents a 0 bit. These cores
are magnetized by sending an electric current through
the wires on which the core is strung. It is this direction
of current that determines the state of each core. Look at
figure 1-4. Since the cores store data in the form of
magnetic charges, core storage retains the data even
when the power is off.
This is called nonvolatile
storage. An example of nonvolatile storage is ROM.
However, the process of reading from core is
destructive. This means the data must be electronically
regenerated after being read.
SEMICONDUCTOR STORAGE (SILICON
CHIP). Semiconductor memory has hundreds of
thousands of tiny electronic circuits etched on a silicon
Each electronic circuit, called a bit cell, can
represent a 0 bit or a 1 bit, depending on the current flow
in that bit cell. An OFF state represents a 0 bit, and an
ON state represents a 1 bit. Another name youll hear
used for semiconductor memory chips is integrated
circuits (ICs). (See figure 1-5.) Technological
developments have enabled even more circuits to be put
on a single chip, resulting in large-scale integration
(LSI) and very-large-scale integration (VLSI).
Figure 1-4.Two-state principle of magnetic storage.