Describe memory hierarchy of a personal computer system

For a practical system, the cost of memory must be reasonable in relationship to other components.

As might be expected, there is a trade-off among the three key characteristics of memory namely capacity, access time and cost. A variety of technologies are used to implement memory systems and access this spectrum of technologies, the following relationships hold:

•  Faster access time, greater cost per bit.
• Greater capacity, smaller cost per bit.
• Greater capacity, slower access time.

The designer would like to use memory technologies that provide for larger-capacity memory, both because the capacity is needed and because the cost per bit is low. However, to meet performance requirements, the designer needs to use expensive, relatively lower capacity memories with short access times.

Thus, smaller, more expensive, faster memories are supplemented by larger, cheaper, slower memories. The key to the success of this organization is item (d): decreasing frequency of access.

A typical hierarchy is illustrated in fig-1, As one goes down the hierarchy, the following occur:

(a) Decreasing cost per bit

(b) Increasing capacity

(c) Increasing access time

(d) Decreasing frequency of access of the memory by the processor.