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68000 Motherboard User's Manual
Figure 1: 68000 Die Function Blocks
Source- BYTE Publications Inc.,
Design Philosophy Behind Motorola's MC68000,
April 1983
within the encoding of different instructions. A picture of the processor die is broken
down in Figure 1, indicating the function of each circuit section. Among the sections are
microcode and nanocode ROMs, control logic and, situated along the bottom, the data
Arithmetic Logic Unit and two address Arithmetic Logic Units.
The power and grace of the 68000 leaves a deep impression on the evolution of computer
and microprocessor technology. And in doing so, it leaves an impression on those with
interest in detailed hardware and software design. As both a historical milestone and a
strong example of approachable modern computing, the 68000 drives continued authority
decades after its inception.
4
What is a com put er ?
Computers are programmable data processing devices. Central to their function is their
ability to move data internally, and much of the computer's design is dedicated to this
purpose. Data in a digital computer is represented by binary digits, abbreviated as "bits."
These bits can possess two possible states, either 0 or 1. This differs from the familiar,
base 10 numeral system, where each digit may be one of ten possibilities. The binary
Many earlier microprocessor
designs
instructions
defined directly in digital logic.
The 68000 offers a much richer
functionality
instructions through the use of
microcoding.
defines the internal sequence to
carry out the instructions. It
does this through the use of
low-level
operating
microprocessor logic. Like the
drum pegs scrolling through a
music box, this microcoded
program controls the timing
and sequencing of various
parts
microprocessor
execute the instruction.
this idea extends further into
the use of nanocoding since
many
common
Nanocoding
deeper level of execution for
common sequences that appear
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Rev. A
implemented
their
as
sequences
within
its
Microcoding
programming
within
the
of
the
internal
logic
to
And
instructions
share
functions.
implements
a
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