Intel MCS48 User Manual page 23

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-
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
enable
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
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
registers
1-5