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 chip. 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 you'll 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.

Figure 1-5.\Semiconductor memory chip exposed.

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, a drawback to this type of storage is that it must have a constant power source. The term for this is volatile storage. An example of volatile storage is

RAM. When you turn the power to the computer off, all the stored data is lost. Also, when there is a power failure and you do not have a backup power supply, all the stored data is lost. As mentioned, this is not the case with magnetic core storage. With core storage, the data is retained even when there is a power failure or breakdown, since data is stored in cores in the form of magnetic charges, not electric current.

BUBBLE STORAGE.\ Bubble memory is one of the newer storage technologies, generally used in laptops. It consists of a very thin crystal made of semiconductor material. The molecules of the crystal act as tiny magnets. Data is stored by changing the polarity of these molecules, called magnetic domains. The magnetic domains can be switched in an opposite direction by passing a current through a control circuit imprinted on top of the crystal. Like magnetic core storage, bubble memory is nonvolatile. The data is retained even when the power is turned off or there is a power failure. Unlike magnetic storage, reading from bubble memory is nondestructive. The data does not have to be regenerated; it is still present after being read.

If we were to view these magnetic domains under a microscope, they would look like tiny bubbles; hence, the name, bubble memory. (See figure 1-6.)

Memory Types by Function

Functionally, we can classify memory by its operational features: random-access memory (RAM),

Figure 1-6.\Bubble memory.

read-only memory (ROM), programmable read-only memory (PROM), and erasable programmable read-only memory (EPROM).