The Best of Creative Computing Volume 2 (published 1977)

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Microprocessors and Microcomputers - The State of The Art (What is a microcomputer?, History, Evolution)
by Brian L. J. Callahan

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Brian L. J. Callahan . Managing Editor, DataPro Minicomputer Reports

The time of the microcomputer has arrived, forcefully and
almost without advance warning. This latest evolution in
technology has resulted from the efforts of the ajor semiconductor 
suppliers to gain a share of the EDP market.

This article defines the microcomputer from several
viewpoints - from its innate design, from its uses, and from
its impact on the EDP industry.



The distinguishing characteristic or component of a microcomputer 
is the microprocessor, one or more large-scale
integration (LSI) chips that perform the basic functions of
a processing unit. Contained within a typical 0.16-inch
square package (thus the "micro" designation) are the usual
elements of any processor -the arithmetic logic unit, I/O
control logic and general-purpose registers. When memory
and a complement of I/O devices accompany or work
jointly with a microprocessor, a microcomputer is formed.

Present microcomputers incorporate devices fabricated
by metal oxide semiconductor (MOS) techniques. MOS
offers extremely high densities of transistors-per-unit-area,
but is inherently slower than bipolar devices. Current MOS
speeds for a logic element or chip range from 40 nanosec
for fast, n-channel silicon gate devices, to 200 nanosec for
p-channel metal units. Architectural attributes which
exploit MOS technology have been added to increase the
speed of microcomputers vis-a-vis bipolar units. They
consist of hardware index registers, parallel bus structures,
register stacks with programmable stack pointers, and
decimal arithmetic.



Since the microcomputer is the apparent successor to the
minicomputer as the latest and most advanced evolutionary
step in EDP, its lineage will be briefly discussed.

At their commercial introduction in 1965, minicomputers 
constituted a revolution in data processing.

Their compact size and low cost permitted the development 
of dedicated systems to meet specialized needs in
communications, control, data acquisition, and small
business data processing.

The potentials of minicomputers were at first not
recognized or appreciated by system designers weaned on
larger computers who viewed minis in terms of the features
and programming languages offered by the larger machines.

Program loading was awkward and time consuming, and the
shorter word lengths and limited instruction sets made
minicomputer programming tedious. Today, systems
designers are more familiar with the vagaries and capabilities 
of minicomputers, and are implementing minis in a
myriad of applications.

Microcomputers and microprocessors are following a
similar course. ln many existing minicomputer applications
they offer improved price-performance, compactness and
reliability over the mini. Moreover, the characteristics of
the LSI microprocessor lend themselves to new
applications and system concepts that are impractical with



As the minicomputer evolved upwards into the high end of
small scale systems, electronic technology was advancing in
circuit miniaturization and the use of MOS as a low cost
altemative to bipolar logic. This steady advance in MOS
technology has increased the large scale integration of
digital circuits from 100 MOS transistors per chip to over
14,000 per chip during the last five years. This increase in
chip density has caused a revolution in digital hardware
applications. Among the more publicized are the pocket
calculator and the digital watch.

A microcomputer uses no more than 10 MOS/LSI
packages, each holding more than 500 transistor circuits. A
minicomputer would typically require about 100 TTL
packages. This simple comparison reflects the prime
difference between a minicomputer and a microcomputerits physical size and the
complexity of its

A concurrent development which has contributed to the
evolution of microprocessors, and thus microcomputers, is
microprogramming, where each machine instruction
initiates a sequence of more elementary instructions (microinstructions). 
A microprogramming approach allows,
replacing fixed, conventional, CPU control logic with a
control memory. Addresses in control memory represent
unique states in conventional control logic, and each
memory output represents control lines from conventional
logic. Stored in this memory are basic microinstructions,
including the fundamental control, testing, branching and
moving operations.

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