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iAPX
432
Interface
Processor
Architecture
Reference
Manual
Direct
vs.
Indirect
Accessibility
If
a copy
of
an
access
descriptor
for
an
object
is
in
one
of
t.he
four
entry
access
segments,
the
object
it
references
is
dir.ectly
accessible.
To
reference
such
an
object,
two
values
must be
specified:
o The
number
(0
to
3)
of
the
entry
access
segment
in
which
the
access
descr
iptor
is
located,
and
o The index
(0
to
16383)
of
the
access
descriptor
within
the
specified
entry
access
segment
When
viewed from
the
standpoint
of
the
432 system and
the
Peripheral
Subsystem,
there
are
actually
several
perspectives
on
accessibility
as
srown
in
Table
2-3.
A
processor
(GOP
or
IP)
in
the
432 system
can
directly
reference
any
object
for
which
it
holds
an
access
descriptor
in
one
of
its
entry
access
lists.
In
addition,
by
traversing
access
paths,
the
432
processor
can
manipulate
obj
ects
which
are
indirectly
accessible.
If
a copy
of
the
access
descri1;>tor
is
not
currently
in
one
of
the
four
entry
access
segments,
the
desired
object
may
be
indirectly
accessible.
The
target
object
may
be
part
of
a complex
object
structure
which must
be
traversed
by
following
the
appropr
iate
access
path.
Once
the
particular
access
descriptor
for
the
object
has
been
located,
the
object
may
be
made
directly
accessible
by
enter
ing
the
access
segment
into
one
of
the
reuseable
entry
access
lists
(1-3).
Entry
access
segment 0
is
always
reserved
for
the
original
context
access
segment. An
access
segment
of
process
globals
may
be
entered
into
one
of
the
other
three
access
lists
by
the
"enter
global
access
segment"
function.
Together,
these
two
access
segments
provide
access
to
all
the
objects
which a
context
can
reference.
An
AP
has
a
different
view
of
accessibility.
The
AP
can
only
access
432
data
through
IP windows which
are
opened
onto
432
data
segments.
When
a window
is
open,
the
AP
can
use
its
native
data
manipulation
operators
to
modify
the
information
through
the
winnow.
When
the
AP
must
reference
data
in
a segment which
is
indirectly
accessible,
it
issues
a
function
request
to
the
IP
to
traverse
an
access
path
to
the
segment.
When
the
data
segment
has
been
made
direct]~
accessible
for
the
AP,
the
IP
interrupts
the
AP.
2-10