18 Macintosh
User's
Handbook
The system unit
is
surprisingly compact. It only requires a
10
inch by
10
inch desk
or
table top area and
is
only
13'l2
inches
high. With a weight
of
less
than
23
pounds, the system
is
light as
well as compact.
INTERNAL CIRCUITRY
Two circuit boards are installed inside the system unit.
One board contains all
of
the Mac's analog circuitry while the
other board contains all
of
its digital circuitry.
Most personal computers contain four or more boards.
By
using only two boards, the Mac's manufacturing cost was
reduced and its reliability increased.
THE MAC MICROPROCESSOR
The heart
of
the Mac
is
the 68000 microprocessor. A
microprocessor can be defined as a single chip which contains
the logic
of
a central processing unit as well as
any
additional
logic that must complement the
CPU. The central processing
unit,
or
CPU,
is
the heart
of
any computer. The
CPU
controls
all
of
the other computer components.
Microprocessor logic
is
based upon the bit. A bit (or
binary digit)
is
the basis
of
all information storage within the
computer. A bit consists
of
a simple switch that can lie in either
of
the two binary states, on
or
off. Bits are often separated into
groups
of
eight. These groups
of
eight bits are known as bytes.
A byte
is
required to represent a single character (i.e. letter,
number,
or
symbol). Generally, bytes are processed by the
computer in groups
of
two.
The Motorola
68000 can process
32
bits internally
at
one
time. This makes it more powerful than other microprocessors
generally used in personal computers which, as a rule, can only
process
16
or 8 bits internally.
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