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4.1 / COMPUTER MEMORY SYSTEM OVERVIEW 115

requirements, the designer needs to use expensive, relatively lower-capacity memo-
ries with short access times.
The way out of this dilemma is not to rely on a single memory component or
technology, but to employ a memory hierarchy. A typical hierarchy is illustrated in
Figure 4.1.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
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. We examine this concept in greater detail when we
discuss the cache, later in this chapter, and virtual memory in Chapter 8. A brief
explanation is provided at this point.

Reg-
isters
Cache
Main
Inboard
memory
memory

Magnetic disk
CD-ROM
CD-RW
Outboard
DVD-RW
storage
DVD-RAM

Magnetic tape
Off-line
storage

Figure 4.1 The Memory Hierarchy