What is computer memory?

As defined earlier, memory refers to the temporary internal storage areas within a computer. The term memory is usually used as shorthand for ‘physical memory’, which refers to the actual chips capable of holding data. Some computers also use ‘virtual memory’, which expands physical memory onto a hard drive.

The main type of memory and the most familiar to users is random access memory (RAM). RAM is the same as main memory. A computer can both write data into RAM and read data from RAM.

Every time a user turns on his or her computer, a set of operating instructions is copied from the hard disk into RAM. These instructions, which help control basic computer functions, remain in RAM until the computer is turned off. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM disappears. The contents of RAM are necessary for the computer to process data. The results of the processing are kept temporarily in RAM until they are needed again or until they are saved onto the hard disk or other storage device.

Today the storage capacity of RAM is measured in megabytes (MB). PCs (microcomputers) typically have between 16 and 64 MB of RAM, which means they can hold between 16 and 64 million bytes of data (a standard A4 page of text typically holds about 2,000 bytes or characters of text).

Other types of memory include

 ROM (read only memory): unlike RAM, ROM is non-volatile and only permits the user to read data. Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer.

 PROM (programmable read-only memory): a PROM is a memory chip on which you can store a program. Once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.

• EPROM (erasable programmable read-only memory): an EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light.

• EEPROM (electrically erasable programmable read-only memory): an EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge.

How are data stored?

It is very important to distinguish between memory, which refers to the circuitry that has a direct link to the computer processor, and the storage, which refers to media such as disks that are not directly linked to the processor. Remember, a storage device is anything that is used to store computer data. Floppy disks, hard disks, optical disks, CDs and magnetic tapes are all types of storage device.

Physical storage refers to how data are actually kept on the storage disk. The most commonly used medium for storage is magnetic storage. With magnetic storage the computer stores data on disks and tape by magnetising selected particles of an oxide-based surface coating. The particles retain their magnetic orientation until that orientation is changed. Thus magnetic disks and tapes are modifiable storage media. The two most popular types of magnetic storage media are hard disks and diskettes. Magnetic tape provides a third type of magnetic storage, and optical disk is a new storage medium. Following is a discussion of each; the definitions included earlier are repeated.

Hard drive or hard disk storage provides faster access to files than a diskette.

Hard drive: The storage area within the computer itself, where megabytes of space are available to store bits of information. Also known as a hard disk.

A hard disk platter is a flat, rigid disk made of aluminium or glass and coated with a magnetic oxide. A hard disk consists of one or more platters and their read-write heads. A read-write head is the device that reads the data from the disk platter into the computer. It also records (or ‘writes’) data onto the platters. Hard disk platters in

microcomputers are typically 3½ inches (about 10 centimetres) in diameter: the same size as the circular mylar disk in a diskette. However, the storage capacity of a hard disk far exceeds that of a floppy disk. Also, the access time of a hard disk is significantly faster than a diskette. Unlike diskettes, which begin to rotate when one requests data, hard disks are continually in motion, so there is no delay as the disk spins up to speed. Like diskettes, hard disks provide random access to files by positioning the read-write head over the sector that contains the requested data.

Diskette. A small, removable, flexible mylar plastic disk covered with a thin layer of a magnetisable substance, onto which digital data can be recorded and stored. Also known as a floppy disk.

Diskettes get another name – floppy disk – from the thin mylar disk. If one cuts open the disk casing (not recommended because the disk will be ruined) one would see that the mylar disk inside is thin and “floppy”. Diskettes are also called floppies. Although today’s microcomputers typically use 3½ inch (about 10 centimetre) disks, you may still find 5¼ inch (about 15 centimetre) disks that were popular many years ago. Finding a disk drive to read 5¼ inch disks may be very difficult.

Diskettes are generally used for transporting or shipping data files or for making duplicate copies of data files for back-up purposes. The storage capacity of a diskette varies but is considerably smaller than that of a hard drive.

Since the 1960s, magnetic tape has been a popular form of mainframe computer storage.

Magnetic tape: A continuous plastic strip covered with magnetic oxide; the tape is divided into parallel tracks onto which data may be recorded by selectively magnetising parts of the surface, or spots, in each of the tracks. The data can then be stored and reused.

When IBM introduced its first microcomputer in 1981, the legacy of tape storage continued in the form of a cassette tape drive, similar to those used for audio recording and playback. Using tape as a primary storage device, however, instead of a hard disk is slow and inconvenient because tape requires sequential access rather than random access.

Sequential access means that data is stored and read as a sequence of bytes along the length of the tape. To find a file stored on a microcomputer tape storage device, one has to advance the tape to the appropriate location of the file, then wait for the computer to slowly read each byte until it finds the beginning of the file. Like an audio cassette, for example, a user must go through the tape in sequence to find the part he or she wants.

Microcomputer users quickly abandoned tape storage for the convenience and speed of random access disk drives. Recently, however, tape storage for microcomputers has experienced a revival, not as a principal storage device but for making backup copies of the data stored on hard disks. The data on magnetic storage can be easil

destroyed, erased, or otherwise lost. Protecting the data on the hard disk is of particular concern to users because it contains so much data, which could be difficult and time consuming to reconstruct. This is why backups are so important.

Back up: To copy a computer file or collection of files to a second medium, usually on a diskette or magnetic tape, so that the data are safe in case the original file is damaged or lost. Backups are usually copied to storage devices that can be removed from the computer and kept separately from the original.

A tape backup is a copy of the data from a hard disk, stored on magnetic tape and used to recover lost data. A tape backup is relatively inexpensive and can rescue an organisation from the overwhelming task of trying to reconstruct lost data.

Backing up electronic data is critical to protecting it from loss or damage.

For more information on backing up data and protecting information, see Automating Records Services and Emergency Planning for Records and Archives Services.

The most popular types of tape drives for microcomputers also use tape cartridges, but there are several tape specifications and cartridge sizes. A tape cartridge is a removable magnetic tape module similar to a sound or video cassette tape. Quarter inch tape, called QIC, is a tape cartridge that contains ¼ inch (approximately ½ centimetre) wide tape. Depending on tape length, QIC tape capacities range from 340 MB to 2 gigabytes. Digital audio tape, called DAT, was originally an audio recording format, but is now also used for data storage. The 4mm wide DAT tape format storage capacity ranges from 2 gigabytes to 12 gigabytes.