Every modern personal computer utilizes electronic circuits to hold data and instructions. These devices are categorized as RAM (random access memory) or ROM (read-only memory), and are compact, reliable, and efficient. RAM is volatile, and is primarily used for the temporary storage of programs that are running.
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ROM is non-volatile, and usually holds the basic instruction sets a computer needs to operate. Electronic medium is susceptible to static electricity damage and has a limited life expectancy, but in the modern personal computer, electronic hardware usually becomes obsolete before it fails. Optical storage medium, on the other hand, will last indefinitely.
Optical storage is an increasingly popular method of storing data. Optical disk drives use lasers to read and write to their medium. When writing to an optical disk, a laser creates pits on its surface to represent data. Areas not burned into pits by the laser are called lands. The laser reads back the data on the optical disk by scanning for pits and lands. There are three primary optical disk mediums available for storage: CD-ROM (compact disc read-only memory), WORM (write once read many), and rewritable optical disks.
The CD-ROM is, by far, the most popular form of optical disk storage; however, CD-ROMs are read-only. At the factory, lasers are used to create a master CD-ROM, and a mold is made from the master and used to create copies. WORM drives are used almost exclusively for archival storage where it is important that the data cannot be changed or erased after it is written, for example, financial record storage. Rewritable optical disks are typically used for data backup and archiving massive amounts of data, such as image databases.
Although there are many manufacturers of the data storage devices used in the modern personal computer, each fits into one of four technological classes according to the material and methods it uses to record information. Hardcopy medium existed before the invention of the computer, and magnetic medium is predominantly used today. Electronic medium is used by every computer system, and is used to store instructions or temporarily hold data. Finally, optical storage medium utilizes lasers to read and write information to a disk that lasts indefinitely. Each type of medium is suitable for certain functions that computer users require. Although they use differing technologies, they all have equal importance in the modern personal computer system.