Filter
Top Products
SYSTEM ARCHITECTURE
operating system must insert a descriptor for the
segment into the G
DT
or
into the task's LDT. In
protected systems, the G
DT
and
LDT
can
be
made privileged structures so that only the
operating system can modify them.
As its name implies, all tasks share the Global
Descriptor Table; operating systems normally
place descriptors for segments that are shared
DESCRIPTOR TABLES
GOT
system-wide
in
the GDT. The operating system's
code segment (or segments)
is
a good example of
a segment that should be accessible to all tasks
and whose descriptor
is
therefore normally
located
in
the GDT. In contrast, each task can
have its own Local Descriptor Table. The
80386
maintains the current task's
LDT
address in its
Local Descriptor Table Register (LDTR),
but
it
reloads this register (just as
it
reloads its general
SEGMENTS
:~~~'
[~~.I
I}~:~~'
LOT
,~'"''
[
TASK 1
AND
TASK 2
I----------+-
....
L-
__
...... ]
~t:~::
,~.,
LOT
PRIVATE TO
[
TASK 3
Ii;'7ii77JiI
= ALIASES OF SAME
~
SEGMENT
'----
...
L-
__
...... ]
I:l'tl'ro
Figure 3-5. Sharing Segments
3-6