User's Manual

TASKS AND STATE TRANSITIONS
The linking order of tasks may need to be changed to restart an interrupted task before the task that
interrupted it completes. To remove a task from the list, trusted operating system software must change
the backlink field in the
TSS of the interrupting task first, then clear the busy bit in the TSS descriptor
of the task removed from the list.
When trusted software deletes the link from one task to another, it should place a value in the backlink
field, which
will pass control to that trusted software when the task attempts to resume execution of
another task via IRET.
8.5 TASK GATES
A task may be invoked by several different events. Task gates are provided to support this need. Task
gates are used
in the same way as call and interrupt gates. The ultimate effect of jumping to or calling
a task gate
is the same as jumping to or calling directly to the TSS in the task gate.
Figure
8-3 depicts the layout of a task gate.
A task gate
is identified by the access byte field in bits 0 through 4 being 00101. The gate provides an
extra level of indirection between the destination address and
theTSS selector value. The offset portion
of the
JMP or CALL destination address is ignored.
Gate use provides flexibility
in controlling access to tasks. Task gates can appear in the GDT, IDT, or
LDT. The
TSS descriptors for all tasks must be kept in the GDT. They are normally placed at level 0
to prevent any task from improperly invoking another task. Task gates placed in the LDT allow private
access to selected tasks with full privilege control.
The data segment access rules apply to accessing a task gate via JMP, CALL, or
INT instructions.
The effective privilege level (EPL) of the destination selector must be numerically
less than or equal
to the
DPL of the task gate descriptor. Any violation of this requirement causes a general protection
fault with an error code identifying the task gate involved.
+7
+5
,:1
15
PIDPLlolo
0
7
0
INTEL RESERVED·
+6
1
01
1
1
UNUSED
+4
:~5 5=!,,=CT0~
+2
UNUSED
1
0
0
·MUST BE SET TO 0 FOR
COMPATIBILITY
WITH THE 80386
Figure 8-3. Task Gate Descriptor
8-8
G30108