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Фрагмент инструкции
The
link
editor
places
all
co de
and
literals
in
the
rst
quadran
t
of
the
co de
space,
and
all
data
in
the
second
quadran
t
of
the
data
space.
Th
us,
ev
ery
executable
mo
dule,
whether
in
a
executable
program
le
or
in
an
executable
library
,
consists
of
one
co de
space
and
one
data
space.
When
the
program
is
loaded,
a
new
co de
space
is
created
for
the
co de
in
the
executable
program
le
and
for
eac
h
executable
library
that
is
loaded
to
satisfy
external
references.
Only
one
data
space
is
created.
The
data
spaces
in
eac
h
executable
mo
dule
are
loaded
one
after
another
in
to
a
single
data
space.
If
y
ou
run
a
program
that
is
already
loaded
(someone
else
is
also
running
it),
the
co de
spaces
already
loaded
can
b e
reused.
Only
a
data
space
m
ust
b e
created
for
the
second
program
(pro
cess).
The
co de
and
literals
can
b e
shared
b ecause
they
cannot
b e
mo
died
b
y
a
pro cess.
An
external
call
is
a
pro cedure
call
that
transfers
con
trol
from
one
External
Calls
executable
mo
dule
to
another.
These
calls
are
not
resolved
during
the
link
op eration
for
t
w
o
reasons.
First,
the
link
editor
builds
(using
LINK
or
ADDXL )
one
executable
mo
dule
at
a
time.
Therefore,
it
do es
not
kno
w
where
the
called
pro cedure
is
lo
cated.
Second,
since
space
iden
tiers
are
assigned
at
run
time,
there
is
no
w
a
y
to
predict
what
the
space
iden
tier
for
either
co de
space
will
b e.
Because
the
link
editor
cannot
resolve
external
calls,
it
builds
an
imp
ort
stub
during
linking
for
eac
h
pro cedure
that
is
called
but
not
dened
in
the
executable
mo
dule.
It
also
allo cates
an
en
try
in
the
External
Reference
T
able
(XR
T)
for
the
unresolv
ed
pro cedure.
The
imp ort
stub
con
tains
a
short
sequence
of
co de
that
is
used
at
run
time
to
transfer
to
the
pro cedure's
en
try
p oint.
The
imp ort
stub
uses
the
XR
T
en
try
to
nd
b oth
the
space
iden
tier
and
the
space
oset
of
the
target
pro cedure.
The
imp ort
stub
then
sa
v
es
the
curren
t
v
alue
of
space
register
4
(which
corresp onds
to
the
rst
quadran
t
of
the
co de
space),
and
copies
the
new
space
iden
tier
in
to
space
register
4.
This
ensures
that
the
space
register
alw
a
ys
con
tains
the
space
iden
tier
of
the
curren
t
co de
space.
The
loader
lo
cates
eac
h
pro cedure
referenced
in
the
XR
T
and
initializes
eac
h
XR
T
en
try
with
the
appropriate
v
alues.
When
a
pro cedure
is
called
externally
,
it
m
ust
restore
the
space
iden
tier
of
the
calling
pro cedure
b efore
returning
to
it.
T
o
do
this,
the
link
editor
builds
an
exp
ort
stub
for
ev
ery
pro cedure
that
can
b e
called
externally
.
The
exp ort
stub
gives
an
alternate
en
try
p oint
to
the
pro cedure
that
is
executed
b
y
an
external
call.
All
in
ternal
calls
(that
is,
calls
to
pro cedures
in
the
same
executable
mo
dule)
use
the
ordinary
pro cedure
en
try
p oint.
External
calls
use
the
exp ort
stub
as
the
en
try
p oint.
On
LISTPROG
and
LISTXL
sym
b ol
listings,
exp ort
stubs
are
sho
wn
as
entry
sym
b ols,
while
the
ordinary
en
try
p oints
are
sho
wn
as
code
sym
b ols.
DRAFT
Adv
anced
T
opics
7-3
11/7/97
02:46
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