Primary Memory and the Memory Hierarchy

A computer stores data in many different places, and those places differ enormously in speed, cost, capacity, and whether they keep data when the power is removed. Arranging these stores by their characteristics produces the memory hierarchy, a model that explains why computers use several kinds of memory rather than one.

Primary memory: RAM and ROM

Primary memory is the memory the CPU can address directly and read from or write to quickly. It comes in two main forms.

Random access memory (RAM) holds the programs and data the computer is currently using. It is volatile, meaning its contents are lost when power is switched off. RAM is called "random access" because the CPU can reach any location in roughly the same amount of time, regardless of order. When you open an application, it is loa