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...