This article throws light upon the six commonly used storage devices in a computer. The storage devices are: 1. Magnetic Core Memory 2. Thin Film Memory 3. Plated Wire Memory 4. Semi-Conductor Memories 5. Magnetic Bubble Memory 6. Holographic Memory.

Storage Device # 1. Magnetic Core Memory:

A magnetic core is molded from a ferrite (iron) powder into a doughnut shape about the size of the head of a pin. The individual cores are stung on wires to form core planes. Several core planes stacked one on top of another form a core stack (Fig. 36.24).

Core Stack

According to elementary physics, an iron bar can be magnetised in one of two directions: north- south or south-north. Similarly a ferrite core can be polarized in two directions. These two directions of polarity form the two states of the core. A core can be placed in either state by the application of a magnetising force. The direction of polarity will depend on the direction of magnetising current.


The computer designer selects one direction to represent a 0-bit and the other to represent a 1-bit. Once, polarized in one direction, a core will retain that polarity until changed. Fig 36.25 shows a single wire being used to apply current to the core.

The typical arrangement uses two wires strung through the core. In Fig. 36.25 (a) current is applied in one direction to polarize ‘1′, (b) current is reversed and polarity flips to the opposite direction to represent ‘0’ (zero).

Two-State Representation with Ferrite Cores

Storage Device # 2. Thin Film Memory:

Thin film memory is produced by depositing very thin spots of metallic (Ni-Fe) alloy on a ceramic or metal plate. This spot performs in the same manner as the core, except that only two wires are required. A typical thin film element consists of a rectangle about 0.625 mm by 1.25 mm with a thickness of about 0.00001 mm.


This rectangle can be thought of as a bar magnet. Applying a current which causes the polarity to rotate but not to flip, induces a current which indicates whether a 1 (one) or 0 (zero) was stored (Fig. 36.26). After the sense field is removed, polarity is returned to its prior state by a digit pulse which steers the polarity back to a 1 or 0 state.

Thin Film Memory

An overlay of etched copper wires provides the circuitry necessary to connect the individual elements with the circuits which read and write. The magnetic properties of film elements allow much faster switching times than can be achieved with cores. This provides faster memory cycle speeds, but the technical difficulties of thin films have, up to now, discouraged their use.

1024 Bit Mated Film Memory Array

Storage Device # 3. Plated Wire Memory:

Plated wire memory or woven wire memory is a thin film which is deposited around a fine wire. This wire carries the write current during a write operation and is the sense line in a read operation. Insulated wires are woven across the plated wire in a fashion similar to weaving cloth. The area where the insulated wires loop around the plate wire (Fig. 36.28) forms a bit storage location. Reading is non-destructive.

Plated-Wire Memory

Storage Device # 4. Semi-Conductor Memories:

Semiconductor memories consist of electronic devices etched on monolithic integrated circuit chips. A single silicon chip will contain many memory circuits plus support circuits. The advantages are increased speed of memory access and reduced memory size (reduction of 50% or more).

Storage Device # 5. Magnetic Bubble Memory:

A bubble memory is a magnetic device in which magnetic bubbles can be manipulated in such a way as to store data. The devices comprise a base plate upon which a pattern of electrical plates is arranged. An, alternating current can be applied to each of the plates in turn so as to cause the movement of magnetic material from one place to the next. Upon the base plate a magnetic fluid (Fig. 36.29) is placed and kept in contact with the base plate by being encapsulated.

Structure of Bubble Memory

The complete assembly is then held in a magnetic field which ensures that the fluid particles are all aligned in the same direction. Within the assembly, there is a means of causing a particle of the fluid to be magnetically reversed, thus forming a bubble of reverse magnetization to the majority of the fluid. There is also a mechanism by which it is possible to detect the magnetization of a particular bubble.


The operation of the bubble memory is like this. Data is written to the memory by sending the bit pattern to the memory, whenever there is a one (1) bit the write assembly causes the current bubble to be inverted relative to the rest of the fluid. If there is a zero (0), then the bubble has the same orientation as the rest of the fluid. The applied alternating current then causes the bubble to move to the next plate and so on until the whole of the data has been encoded. This memory device, although cheap and small, is relatively slow.

Storage Device # 6. Holographic Memory:

a. In the mechanical type, data bits are stored by a laser beam which permanently records the information by burning the coating on tape. By this way about two million bits can be stored per sq. cm. These spots can be read by a laser beam of lesser power.

b. In the optical holographic memory, a laser beam is passed through an array of light valves to produce binary pattern of light and darkness. This is stored on light-sensitive material. The data thus stored is read by another laser beam.