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30 CHAPTER 2 / COMPUTER EVOLUTION AND PERFORMANCE

• Data movement: The paths among components are used to move data from
memory to memory and from memory through gates to memory.
• Control: The paths among components can carry control signals. For example,
a gate will have one or two data inputs plus a control signal input that activates
the gate. When the control signal is ON, the gate performs its function on the
data inputs and produces a data output. Similarly, the memory cell will store
the bit that is on its input lead when the WRITE control signal is ON and will
place the bit that is in the cell on its output lead when the READ control sig-
nal is ON.
Thus, a computer consists of gates, memory cells, and interconnections among
these elements. The gates and memory cells are, in turn, constructed of simple digi-
tal electronic components.
The integrated circuit exploits the fact that such components as transistors, re-
sistors, and conductors can be fabricated from a semiconductor such as silicon. It is
merely an extension of the solid-state art to fabricate an entire circuit in a tiny piece
of silicon rather than assemble discrete components made from separate pieces of
silicon into the same circuit. Many transistors can be produced at the same time on
a single wafer of silicon. Equally important, these transistors can be connected with
a process of metallization to form circuits.
Figure 2.7 depicts the key concepts in an integrated circuit. A thin wafer of
silicon is divided into a matrix of small areas, each a few millimeters square. The
identical circuit pattern is fabricated in each area, and the wafer is broken up into
chips. Each chip consists of many gates and/or memory cells plus a number of input
and output attachment points. This chip is then packaged in housing that protects it
and provides pins for attachment to devices beyond the chip. A number of these
packages can then be interconnected on a printed circuit board to produce larger
and more complex circuits.
Initially, only a few gates or memory cells could be reliably manufactured and
packaged together. These early integrated circuits are referred to as small-scale in-
tegration (SSI). As time went on, it became possible to pack more and more com-
ponents on the same chip.This growth in density is illustrated in Figure 2.8; it is one
5
of the most remarkable technological trends ever recorded. This figure reflects the
famous Moore’s law, which was propounded by Gordon Moore, cofounder of Intel,
in 1965 [MOOR65]. Moore observed that the number of transistors that could be
put on a single chip was doubling every year and correctly predicted that this pace
would continue into the near future. To the surprise of many, including Moore,
the pace continued year after year and decade after decade. The pace slowed to a
doubling every 18 months in the 1970s but has sustained that rate ever since.
The consequences of Moore’s law are profound:

1. The cost of a chip has remained virtually unchanged during this period of
rapid growth in density. This means that the cost of computer logic and mem-
ory circuitry has fallen at a dramatic rate.

5 Note that the vertical axis uses a log scale.A basic review of log scales is in the math refresher document
at the Computer Science Student Support Site at WilliamStallings.com/StudentSupport.html.