In read/write memories, the data can be retrieved
from memory, altered, and written back into memory.
This can be done either independent of a write operation
or as part of the first half of a read/write operation where
the information must be rewritten back into memory to
restore the original data.
Read/write memories are
random access in nature.
They are categorized
according to the materials they are constructed from;
not by their basic operations. Their physical makeup
can be magnetic or semiconductor. Both types have
advantages and disadvantages. Semiconductor
memories cost less, are faster in terms of storage and
access time, and use nondestructive readout. They also
require less space for the same number of bits as a
magnetic memory. Magnetic memories are relatively
low in cost, require less power, and are nonvolatile (they
retain the information after the power is removed).
In our discussion of the two types of memory, you
will study specifics about their architecture, address
selection, and read/write cycles; how address selection
and the read/write cycle are performed; and any
circuitry that is peculiar to that type of memory. First
we discuss two types of magnetic memory (core and
film), then semiconductor memory.
Magnetic memories use magnetic material as a
means of recording binary data. Basically, a magnetic
field is applied to a memory cell (bit); the magnetic field
is generated by passing a current through the conductor.
Magnetic memory is a non volatile form of storage. It
retains its magnetic state (direction of flux lines) in the
absence of current flow through the conductors on
which the core or film is assembled. Only current flow
in the opposite direction of sufficient magnitude to
overcome the magnetic field of the core or film and to
magnetize it in the new direction will change the state
of the core or film. Loss of power should not cause loss
or the data retained in core or film memory.
The major difference between core memory and
film memory technology is the physical structure of the
Mated film memory is easier to
magnetize, which increases the speed of read/write
Also, less power is required for these
operations. Mated film memory is also more compact
and durable, and twice as many mated film memory
cells can be put in the same space as ferrite core memory
cells for the same amount of power.
Core memory is used as one of the primary storage
media of digital computers. It is used primarily on large
mainframes and minicomputers. Depending on the
mainframe or minicomputer, core memory is contained
in memory modules; usually two to four large memory
modules to a mainframe computer set or one to four
small modules in a minicomputer.
Magnetic core storage is composed of hundreds of
thousands of very small doughnut-shaped ferrite cores
(fig. 6-8). The ferrite cores are strung together on grids
of very thin wires known as core planes. Each core can
store one binary bit (0 or 1) of data. A core is
magnetized by current flow through the wires on which
the core is strung. A core magnetized in one direction
represents a binary zero, and when magnetized in the
opposite direction, a binary one. The direction the core
is magnetized is dependent on the direction of current
flow through the wires on which it is strung. Figure 6-8
shows the magnetization of a core based on the direction
of current flow.
CORE WINDINGS (FOUR-WIRE). Magnetic
cores are strung on several fine wires to allow for the
reading and writing of data in core. Two basic methods
are used to string cores, the four-wire method and the
three-wire method. Core windings strung through
each core using the four-wire method consist of 2
drive lines (X and Y), 1 sense line, and 1 inhibit line.
Figure 6-8.Magnetizing a ferrite core.