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INTRODUCTION
Memory
Write
A
memory
write
operation
is
similar to
a read
except
for
the
direction of
data
flow.
The
CPU
issues
a
write
signal,
sends
the
proper
memory
address,
then
sends
the data
word
to
be
written
into
the
addressed
data
memory
location.
Input/Output
Input
and Output
operations are
similar to
memory
read
and
write
operations with the
exception
that
an
I/O
port
is
addressed
instead
of
a
memory
location.
The
CPU
issues
the
appropriate
input
or
output
control
signal,
sends
the
proper address
and
either
receives the
data being
input or
sends
the data
to
be
output.
Data can be
input/output
in
either
parallel
or
serial
form.
All
data
within a
digital
computer
is
represented
in
binary
coded
form.
A
binary
data
word
consists
of
a
group
of
bits;
each
bit
is
either
a
one
or a
zero.
Parallel
I/O consists
of transferring
all
bits
in
the
word
at
the
same
time,
one
bit
per
line.
Serial
I/O consists
of
transferring
one
bit
at
a time
on
a single
line.
Naturally
serial
I/O
is
much
slower, but
it
requires
considerable
less
hardware
than
does
parallel I/O.
Interrupts
Interrupt
provisions are included
on
many
central
processors, as a
means
of
improving
the processor's
efficiency.
Consider
the
case
of
a
computer
that
is
processing a
large
volume
of data,
portions
of
which
are
to
be
output
to
a
printer.
The
CPU
can
output
a
byte
of
data
within
a
single
machine
cycle but
it
may
take the
printer
the equivalent
of
many
machine
cycles
to actually
print
the char-
acter specified
by
the data
byte.
The
CPU
could then remain
idle
waiting
until
the
printer
can accept
the next data
byte.
If
an
interrupt capability
is
implemented on
the
computer,
the
CPU
can
output a data byte
then
return to
data processing.
When
the
printer
is
ready
to
accept
the next data
byte,
it
can
request
an
interrupt.
When
the
CPU
acknowledges
the
interrupt,
it
suspends
main program
execution
and
automatically
branches
to
a
routine that
will
output the next
data
byte.
After
the byte
is
output,
the
CPU
continues
with
main program
execution.
Note
that
this
is,
in
principle,
quite similarto a
subroutine
call,
except
that
the
jump
is
initiated
externally
rather
than
by
the
program.
More
complex
interrupt
structures
are
possible,
in
which
several
interrupting
devices share the
same
processor
but
have
different priority
levels.
Interruptive
proces-
sing
is
an important
feature that
enables
maximum
utilization
of
a processor's capa-
city
for
high
system
throughput.
1-9
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