Memory

Random Access Memory (RAM)

When most people think of computer memory, random access memory (RAM) is what they mean. RAM is composed of chips. These chips can hold:

1. data for processing;
2. instructions, or programs, for processing the data;
3. data that has been processed and is waiting to be sent to an output, secondary storage, or communications device;
4. operating system instructions that control the basic functions of the computer system.

All data and instructions held in RAM are temporary. The contents can and do change as data are processed, programs are run, and instructions are carried out by the computer. RAM is a reusable computer resource.

Most RAM is said to be volatile. This means that when the power to the computer is turned off, or the power goes out, all contents of RAM instantaneously disappear and are permanently lost. Because RAM is temporary and volatile, other forms of more permanent storage were developed. Secondary storage is long term, non-volatile storage of data or programs outside the central processing unit (CPU) and RAM. Some of the more common types of secondary storage include magnetic tape, magnetic disk, and optical disk.

The storage capacity of RAM varies in different types of computers. Capacity is important because it determines how much data can be processed at once and how large and complex a program may be. The computer's operating system manages RAM so that programs run properly. To understand the capacity of RAM, the following terms are used:

• Bit—a binary digit representing the smallest unit of data in the computer system. A bit can be only a 1 or a 0. In the computer, a 0 means that an electronic or magnetic signal is absent, while a 1 signifies its presence;
• Byte—a group of eight bits. A byte represents one character, one digit, or one value. The capacity of the computer's memory, RAM, is expressed in bytes or in multiples of bytes.

Data, instructions, and programs stored in RAM are really stored as bits that represent those data, instructions, and programs. These bits are stored in microscopic electronic parts called capacitors.

Read Only Memory

Read Only Memory (ROM) is a set of chips that contain portions of the operating system that are needed to start the computer. ROM is also known as firmware. ROM cannot be written to or altered by a user. It is nonvolatile memory. ROM chips come from the manufacturer with programs or instructions already stored and the only way to change their contents is to remove them from the computer and replace them with another set. ROM

chips can contain frequently used programs, such as computing routines for calculating the square root of numbers.

The most common use for ROM chips is the storage of manufacturer-specific programming such as the Basic Input Output System (BIOS). The BIOS is a critical part of the operating system that tells the computer how to access the disk drives. When the computer is started, RAM is empty and the instructions in the ROM BIOS are used by the CPU to search the disk drives for the main operating system files. The computer then loads these files into RAM and uses them.

There are three variations of ROM.

1. PROM, or programmable read only memory. PROM chips are blank chips on which programs can be written using special equipment. PROM chips can be programmed once and are usually used by manufacturers as control devices in their products.
2. EPROM, or erasable programmable read only memory. EPROM is similar to PROM, but the program can be erased and a new program written by using special equipment that uses ultraviolet light. EPROM is used for controlling devices such as robots.
3. EEPROM, electronic erasable programmable read only memory. EEPROM chips can be reprogrammed using special electric impulses. They do not need to be removed to be changed.

CMOS

CMOS (pronounced SEE MOSS) stands for "complementary metal oxide semiconductor." It is a specialized memory that contains semi-permanent vital data about the computer system's configuration. Without this data, the computer would not be able to start. CMOS is more permanent than RAM and less permanent than ROM. CMOS requires very little power to retain its contents; the chip is powered by a battery. When a change is needed in the computer system's configuration (i.e., a new hard drive is installed, more RAM is added, or the number of floppy disk drives is changed), CMOS can be updated by running a special utility program available through the operating system.

Virtual Memory

Virtual memory is a storage method where portions of a program or data are stored on magnetic disk rather than in RAM until needed, giving the illusion that main memory is unlimited. Virtual memory simulates RAM. It allows the computer to run more than one program at a time, manipulate large data files, and run large programs without having sufficient RAM. Virtual storage is slower than RAM, and is nonvolatile.

How Data and Programs Are Stored in Memory

Computer main memory can be thought of as a two-dimensional table where each cell has a unique address. See Figure 1. Each cell can store one byte of data by using eight capacitors to represent the eight bits in a byte.

SEE ALSO BINARY NUMBER SYSTEM; INTEGRATED CIRCUITS; MICROCHIP; TRANSISTORS; VACUUM TUBES.

Charles R. Woratschek

Bibliography

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Long, Larry, and Nancy Long. Computers: Brief Edition, 3rd ed. Upper Saddle River, NJ: Prentice Hall, Inc., 1999.

Parsons, June Jamrich, and Dan Oja. New Perspectives on Computer Concepts, 3rd ed. Cambridge: Course Technology, 1998.