The techniques, equipment, and organization for providing the memory capability required by computers in order to store instructions and data for processing at high electronic speeds. In early computer systems, memory technology was very limited in speed and high in cost. Since the mid-1970s, the advent of high-density, high-speed random-access memory (RAM) chips has reduced the cost of computer main memory by more than two orders of magnitude. Chips are no larger than ¼ in. (6 mm) square and contain all the essential electronics to store tens to hundreds of thousands of bits of data or instructions. An analogous increase in magnetic recording density has increased the capacity and reduced the cost per bit of secondary memory. Traditionally, computer storage has consisted of a hierarchy of three or more types of memory storage devices (for example, RAM chips, disks, and magnetic tape units).