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This chapter introduces certain basic
computer
con-
cepts. It provides background information and definitions
which will be useful
in
later chapters
of
this manual. Those
already familiar with
computers
may skip this material,
at
their option.
A TYPICAL COMPUTER SYSTEM
A typical digital
computer
consists of:
a)
A central processor
unit
(CPU)
b)
A memory
c)
Input/output
(I/O)
ports
The memory serves as a place
to
store Instructions,
the coded pieces of information
that
direct
the
activities
of
the CPU, and Data, the coded pieces
of
information
that
are
processed by
the
CPU. A group
of
logically related instruc-
tions stored
in
memory
is
referred
to
as a Program.
The
CPU
"reads"
each instruction from memory
in
a logically deter-
mined sequence, and uses it
to
initiate processing actions.
If
the
program sequence
is
coherent
and logical, processing
the
program will produce intelligible and useful results.
The memory
is
also used
to
store
the
data
to
be manip-
ulated, as well as
the
instructions
that
direct
that
manipu-
lation. The program
must
be organized such
that
the
CPU
does
not
read a non-instruction word when it expects
to
see an instruction.
The
CPU can rapidly access any data
stored
in
memory;
but
often
the
memory
is
not
large enough
to
store
the
entire
data
bank required for a particular appli-
cation.
The
problem can be resolved by providing
the
com-
puter with one or more
Input
Ports.
The
CPU can address
these portsand
input
the
data
contained there.
The
addition
of
input
ports enables
the
computer
to
receive information
from external
equipment
(such as a paper
tape
reader
or
floppy disk)
at
high rates
of
speed and in large volumes.
A
computer
also requires
one
or
more
Output
Ports
that
permit
the
CPU
to
communicate
the
result
of
its pro-
cessing
to
the
outside world.
The
output
may go
to
a dis-
play,
for
use by a human operator,
to
a peripheral device
that
produces
"hard-copy,"
such as a line-printer,
to
a
1-1
peripheral storage device, such as a floppy disk unit,
or
the
output
may
constitute
process control signals
that
direct
the
operations
of
another
system, such as an
automated
assembly
line. Like
input
ports,
output
ports
are addressable. The
input and
output
ports
together
permit
the
processor
to
communicate
with
the
outside world.
The CPU unifies the system. It controls
the
functions
performed by
the
other
components.
The
CPU
must
be able
to
fetch instructions from
memory,
decode
their
binary
contents
and execute them.
It
must
also be able
to
reference
memory and I/O
ports
as necessary
in
the
execution
of
in-
structions.
In
addition,
the
CPU should be able
to
recognize
and respond
to
certain external control signals, such as
INTERRUPT and WAIT requests.
The
functional units
with
in
a CPU
that
enable it
to
perform these functions are
described below.
THE ARCHITECTURE OF A
CPU
A typical central processor
unit
(CPU) consists
of
the
following interconnected functional units:
• Registers
• Arithmetic/Logic Unit (ALU)
• Control Circuitry
Registers are
temporary
storage units within
the
CPU.
Some registers, such as
the
program
counter
and instruction
register, have dedicated uses.
Other
registers, such as
the
ac-
cumulator, are for more general purpose use.
Accumulator:
The
accumulator
usually stores
one
of
the
operands
to
be manipulated by
the
ALU. A typical instruction might
direct
the
ALU
to
add
the
contents
of
some
other
register
to
the
contents
of
the
accumulator and store
the
result
in
the
accumulator itself.
In
general,
the
accumulator
is
both a
source (operand) and a destination (result) register.
Often a CPU will include a number
of
additional
general purpose registers
that
can be used
to
store operands
or intermediate data.
The
availability
of
general purpose