(2) Storage Devices
Computer memory is a physical device that is used to store such information as data or programs on a temporary or permanent basis. Most digital computer systems have two levels of memory – the main (or internal) memory and secondary (or external) memory.
Internal Memory
The main memory is the systems component of the computer, which holds data and instructions required for ongoing process. It receives this information from an input device or a secondary storage unit. The more memory you have in your computer, the more operations you can perform, i.e. the faster it works.
There are two types of main memory: RAM and ROM.
Read-only memory (ROM) is built into the computer and cannot be changed. It contains information and software (such as the operating system) that controls the computer's actions from start up to shut down. ROM is called nonvolatile memory because the memory chips do not lose their information when power to the computer is turned off.
Random-access memory (RAM) is used to store the information and instructions temporarily while data is being processing. RAM is also known as volatile memory because the information within the computer chips is lost when power to the computer is turned off. This kind of internal memory is also called read/write memory. Today's memory chips can each store up to 512 MB of data; a set of 16 chips on a RAM module can store up to 1 GB of data.
External Memory
Secondary storage units are used for long-term storing data and instructions. They can store considerably more information than a main memory can but operate at slower speeds.
The most common form of secondary storage is magnetic disk. Magnetic disks are flat, circular plates coated with a magnetic material that contains all the data stored on the disk. There are two types: hard disks, which are made of aluminum, glass or ceramic and are physically rigid; and floppy disks, which are made of plastic and are flexible. Hard disk is a permanent part of the computer. It can store from 20 megabytes to 2 gigabytes of information and retrieve that information very quickly. Floppy disks have a much smaller capacity of only 2 to 3 megabytes.
Another form of external memory is the optical disk, which uses optical rather than electrical means for reading and writing. The standard compact disc was introduced in 1982 for digital audio reproduction. But, because any type of information can be represented digitally, the computer industry began to use the standard CD as an inexpensive storage-and-distribution medium for large computer programs, graphics and databases.
CDs, whose data can be read but not changed, are called CD-ROMs. They are used to distribute relatively static data, for example in encyclopedias and other reference works, and their large capacity makes them ideal for combinations of text with audio and graphics or other multimedia formats. Each disc can usually hold about 700 megabytes of data; the largest ones can store 1.5 gigabytes of information, which is equal to about 700,000 pages of printed material.
I
n
1991, the CD-R (recordable) format was introduced. This variation of
CD-ROM allows a user to write information on each disk only once,
with subsequent erasure impossible.
Such
CDs are also known as WORM discs, for “Write Once Read Many”.
A rewritable version, CD-RW, was introduced in 1997. (See Figure 4)
Figure 4
Compact Discs
A new type of discs – digital videodisc (DVD) – was developed by Philips and Sony in 1995. It looks and works like a CD-ROM but can store more than 15 times as much information. Single-sided disc2 can store up to 4.7 gigabytes of data such as high-definition digital video files. A DVD has the same dimensions as a standard CD but cannot be read by a standard CD player, though a DVD player can read standard CDs. Beginning in the late 1990s, various writable and rewritable DVD formats were introduced.
Flash memory is another electronic storage unit. Since its invention in the late 1980s (by Intel and Toshiba), it has become standard for portable devices such as digital cameras, cellular telephones, PDAs, MP3 players, and video-game machines. In the early 21st century, flash memory devices that could fit on a key ring and had storage capacities of up to 1 GB (and later more) began to serve as portable hard drives.