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Figure
1-8
shows
hOil
the
message
sent
by
the
GOP
process
might
be
organized.
It
oonsists
of
two
parts,
an
object
reference
part
and a
text
part.
The
object
references
are
for
the
text
part
of
the
object,
the
432
port
at
which
the
process
will
wait
for
the
message
to
be
returned,
arrl a
reference
for
the
process
itself
(GOP
or
IP).
This
last
reference
is
not
strictly
necessary
in
the
example,
but
is
provided
to
shOil one
way
in
which a message
may
identify
its
originator.
The
text
part
of
the
message
oontains
a
command
field
which
specifies
what
is
to
be
done
(e.g.,
print
one
page),
a
status
field
which
reflects
the
disposition
of
the
print
request,
and
the
data
to
be
printed.
With
the
exception
of
the
status
information,
all
data
in
the
message
is
provided
by
the
GOP
process
i
the
status
field
is
updated
by
the
printer
task.
The
next
three
sections
descr
ibe
the
operation
of
the
example
system
as
seen
by
the
GOP
process,
the
printer
task,
and
the
IP
controller.
These
descriptions
present
an
overview
of
the
operations.
For
nnre
detail
on
hOil
these
acti
vi
ties·
relate
to
IP
facilities,
please
refer
to
Appendix
F,
(Interprocess
Cbmmunication Example), which
refines
the
printer
example.
GOP
Process
Perspective
To
direct
outp.1t
to
the
line
printer,
a
GOP
process
builds
a
print
object
and
serrls
it
as
a message
to
the
print_request..J?Ort.
The
port
is
the
process's
"oonnection"
to
the
line
pr
inter.
After
it
has
sent
the
message,
the
process
is
free
to
continue
running.
When
it
cannot
proceed
further
without
acknowledgement
of
the
print
operation,
the
process
attempts
to
receive
a message
fran
the
print_replY'-port
it
specified
in
the
print_object.
When
the
operation
has
been
oompleted,
the
process
will
receive
the
message.
It
then
inspects
the
status
field
and
takes
appropriate
action,
perhaps
writing
new
data
into
the
print_object
and
serrling
it
off
again.
1-19