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114 CHAPTER 4 / CACHE MEMORY

For non-random-access memory, the following relationship holds:
n
T = T + (4.1)
N
A
R
where
T = Average time to read or write N bits
N
T = Average access time
A
n = Number of bits
R = Transfer rate, in bits per second (bps)

A variety of physical types of memory have been employed. The most com-
mon today are semiconductor memory, magnetic surface memory, used for disk and
tape, and optical and magneto-optical.
Several physical characteristics of data storage are important. In a volatile
memory, information decays naturally or is lost when electrical power is switched off.
In a nonvolatile memory, information once recorded remains without deterioration
until deliberately changed; no electrical power is needed to retain information.
Magnetic-surface memories are nonvolatile. Semiconductor memory may be either
volatile or nonvolatile. Nonerasable memory cannot be altered, except by destroying
the storage unit. Semiconductor memory of this type is known as read-only memory
(ROM). Of necessity, a practical nonerasable memory must also be nonvolatile.
For random-access memory, the organization is a key design issue. By organi-
zation is meant the physical arrangement of bits to form words. The obvious
arrangement is not always used, as is explained in Chapter 5.

The Memory Hierarchy
The design constraints on a computer’s memory can be summed up by three ques-
tions: How much? How fast? How expensive?
The question of how much is somewhat open ended. If the capacity is there,
applications will likely be developed to use it.The question of how fast is, in a sense,
easier to answer.To achieve greatest performance, the memory must be able to keep
up with the processor. That is, as the processor is executing instructions, we would
not want it to have to pause waiting for instructions or operands.The final question
must also be considered. For a practical system, the cost of memory must be reason-
able 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 across 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 dilemma facing the designer is clear.The designer would like to use mem-
ory technologies that provide for large-capacity memory, both because the capacity
is needed and because the cost per bit is low. However, to meet performance