Memory Hierarchy A memory unit is an essential component in any digital computer since it is needed for storing programs and data. Typically, a memory unit can be classified into two categories: The memory unit that establishes direct communication with the CPU is called Main Memory . The main memory is often referred to as RAM (Random Access Memory). The memory units that provide backup storage are called Auxiliary Memory . For instance, magnetic disks and magnetic tapes are the most commonly used auxiliary memories. Apart from the basic classifications of a memory unit, the memory hierarchy consists all of the storage devices available in a computer system ranging from the slow but high-capacity auxiliary memory to relatively faster main memory. The following image illustrates the components in a typical memory hierarchy. Auxiliary Memory Auxiliary memory is known as the lowest-cost, highest-capacity and slowest-access storage in a computer system. Auxiliary memory provides stora