Primary and Secondary Memory

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(Last Updated On: February 19, 2013)

Modern electronic computers generally possess several distinct types of memory, each of which “holds” or stores information for subsequent use. The vast majority of computer memory can be placed into one of two categories: primary memory and secondary memory.

Primary memory, often called main memory, constitutes that device, or group of devices, that holds instructions and data for rapid and direct access by the computer’s central processing unit (CPU). Primary memory is synonymous with random-access memory (RAM). As a computer performs its calculations, it is continuously reading and writing (i.e., storing and retrieving) information to and from RAM. For instance, instructions and data are retrieved from RAM for processing by the CPU, and the results are returned to RAM. Modern RAM is made of semiconductor circuitry, which replaced the magnetic core memory widely used in computers in the 1960s. RAM is a volatile form of information storage, meaning that when electrical power is terminated any data that it contains is lost. There are other semiconductor memory devices accessed by the CPU that are generally considered as being distinct from primary memory (i.e., different from RAM). These memory units include cache memory, read-only memory (ROM), and Programmable Read Only Memory (PROM).

Secondary memory, also called auxiliary memory or mass storage, consists of devices not directly accessible by the CPU. Hard drives, floppy disks, tapes, and optical disks are widely used for secondary storage. The input and output of these devices is much slower than for the semiconductor devices that provide the computer’s primary memory. Although access times (i.e., the time to read or write information) are slow as compared to that of primary memory, secondary memory devices have important features that are unmatched by primary memory. First, most secondary storage devices are capable of containing much more information than is feasible for primary memory (hence the use of the term “mass storage” as a synonym for secondary memory). A second, and essential, feature of secondary memory is that it is non-volatile. This means that data is stored with or without electrical power being supplied to the device, as opposed to RAM, which can retain its data only so long as electrical power is present.

Like primary memory, many secondary memory devices are capable of storing information, as well as retrieving it. Magnetic technology devices (such as hard drives, floppy disks, and tape) have this read-write capability, as do magneto-optical drives. However, some mass storage devices can only read data, as in the case of CD-ROM (Compact Disk-Read Only Memory) drives. CD-ROMs utilize optical technology; however, newer optical technologies, such as CD-RW (compact disk-rewriteable), can both read and write information like magnetic storage devices.

 

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