A computer has two kinds of memory. The first is called primary storage, and is used to hold program instructions and data while the computer is running. The other kind, called secondary storage, is used for storage of data even while the computer is not running. The primary storage is memory of a kind called Random Access Memory, RAM for short. It is volatile, i.e., susceptible to loss easily upon power outage. The secondary storage can be of many kinds--hard drives, disk drives, tape drives, and so on. The secondary storage is persistent, i.e., retains its contents even when the power is switched off.

RAM is said to be "random access" because any memory cell in RAM can be accessed directly, if the physical address of that cell is known. The counterpart of RAM is SAM, Sequential Access Memory. This kind of...