INTRODUCTION
1.1
The Function of a Computer
This chapter introduces certain basic com-
puter concepts. 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.
1.1.1 A Typical Computer System
A typical digital computer consists of:
A central processor unit (CPU)
Program Memory
Data Memory
Input/output (I/O) ports
The processor memory
serves
as
a place to
store Instructions, the coded pieces
of
information that direct the activities of the
CPU,
while Memory stores the
Data,
the
coded pieces of information that
are
processed by the
CPU.
A group of logically
related instructions stored
in
memory
is
referred to
as
a Program. The
CPU
"reads"
each instruction from memory
in
a logically
determined 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 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 application. The problem
can
be
resolved by providing the computer with
one or more Input Ports. The
CPU
can
address these ports and 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 processing to the outside
world. The output
may
go to a display, for
use
by a human operator, to a peripheral device
that produces "hard-copy", such
as
a line-
1·5
printer, to a 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 compon-
ents. 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
instructions.
In
addition, the
CPU
should
be
able to recognize
and
respond to certain
external control signals, such
as
INTER-
RUPT
requests. The functional units within a
CPU
that
enabl~
it to perform these functions
are
described below.
1.1.2 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 accumulator,
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
reg
ister 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 registers