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34 CHAPTER 2 / COMPUTER EVOLUTION AND PERFORMANCE

(DEC).At a time when the average computer required an air-conditioned room, the
PDP-8 (dubbed a minicomputer by the industry, after the miniskirt of the day) was
small enough that it could be placed on top of a lab bench or be built into other
equipment. It could not do everything the mainframe could, but at $16,000, it was
cheap enough for each lab technician to have one. In contrast, the System/360 series
of mainframe computers introduced just a few months before cost hundreds of
thousands of dollars.
The low cost and small size of the PDP-8 enabled another manufacturer to
purchase a PDP-8 and integrate it into a total system for resale. These other manu-
facturers came to be known as original equipment manufacturers (OEMs), and the
OEM market became and remains a major segment of the computer marketplace.
The PDP-8 was an immediate hit and made DEC’s fortune. This machine and
other members of the PDP-8 family that followed it (see Table 2.5) achieved a pro-
duction status formerly reserved for IBM computers, with about 50,000 machines
sold over the next dozen years. As DEC’s official history puts it, the PDP-8 “estab-
lished the concept of minicomputers, leading the way to a multibillion dollar indus-
try.” It also established DEC as the number one minicomputer vendor, and, by the
time the PDP-8 had reached the end of its useful life, DEC was the number two
computer manufacturer, behind IBM.
In contrast to the central-switched architecture (Figure 2.5) used by IBM on
its 700/7000 and 360 systems, later models of the PDP-8 used a structure that is now
virtually universal for microcomputers: the bus structure. This is illustrated in
Figure 2.9. The PDP-8 bus, called the Omnibus, consists of 96 separate signal paths,
used to carry control, address, and data signals. Because all system components
share a common set of signal paths, their use must be controlled by the CPU.This ar-
chitecture is highly flexible, allowing modules to be plugged into the bus to create
various configurations.

Later Generations
Beyond the third generation there is less general agreement on defining generations
of computers.Table 2.2 suggests that there have been a number of later generations,
based on advances in integrated circuit technology. With the introduction of large-
scale integration (LSI), more than 1000 components can be placed on a single inte-
grated circuit chip. Very-large-scale integration (VLSI) achieved more than 10,000
components per chip, while current ultra-large-scale integration (ULSI) chips can
contain more than one million components.
With the rapid pace of technology, the high rate of introduction of new prod-
ucts, and the importance of software and communications as well as hardware, the
classification by generation becomes less clear and less meaningful. It could be said
that the commercial application of new developments resulted in a major change in
the early 1970s and that the results of these changes are still being worked out. In
this section, we mention two of the most important of these results.

SEMICONDUCTOR MEMORY The first application of integrated circuit technology
to computers was construction of the processor (the control unit and the arithmetic
and logic unit) out of integrated circuit chips. But it was also found that this same
technology could be used to construct memories.