Intel 253668-032US Benutzerhandbuch
Vol. 3 7-11
TASK MANAGEMENT
7.2.5 Task-Gate
Descriptor
A task-gate descriptor provides an indirect, protected reference to a task (see
Figure 7-6). It can be placed in the GDT, an LDT, or the IDT. The TSS segment
selector field in a task-gate descriptor points to a TSS descriptor in the GDT. The RPL
in this segment selector is not used.
The DPL of a task-gate descriptor controls access to the TSS descriptor during a task
switch. When a program or procedure makes a call or jump to a task through a task
gate, the CPL and the RPL field of the gate selector pointing to the task gate must be
less than or equal to the DPL of the task-gate descriptor. Note that when a task gate
is used, the DPL of the destination TSS descriptor is not used.
Figure 7-6). It can be placed in the GDT, an LDT, or the IDT. The TSS segment
selector field in a task-gate descriptor points to a TSS descriptor in the GDT. The RPL
in this segment selector is not used.
The DPL of a task-gate descriptor controls access to the TSS descriptor during a task
switch. When a program or procedure makes a call or jump to a task through a task
gate, the CPL and the RPL field of the gate selector pointing to the task gate must be
less than or equal to the DPL of the task-gate descriptor. Note that when a task gate
is used, the DPL of the destination TSS descriptor is not used.
A task can be accessed either through a task-gate descriptor or a TSS descriptor.
Both of these structures satisfy the following needs:
Both of these structures satisfy the following needs:
•
Need for a task to have only one busy flag — Because the busy flag for a task
is stored in the TSS descriptor, each task should have only one TSS descriptor.
There may, however, be several task gates that reference the same TSS
descriptor.
is stored in the TSS descriptor, each task should have only one TSS descriptor.
There may, however, be several task gates that reference the same TSS
descriptor.
•
Need to provide selective access to tasks — Task gates fill this need, because
they can reside in an LDT and can have a DPL that is different from the TSS
descriptor's DPL. A program or procedure that does not have sufficient privilege
to access the TSS descriptor for a task in the GDT (which usually has a DPL of 0)
may be allowed access to the task through a task gate with a higher DPL. Task
gates give the operating system greater latitude for limiting access to specific
tasks.
they can reside in an LDT and can have a DPL that is different from the TSS
descriptor's DPL. A program or procedure that does not have sufficient privilege
to access the TSS descriptor for a task in the GDT (which usually has a DPL of 0)
may be allowed access to the task through a task gate with a higher DPL. Task
gates give the operating system greater latitude for limiting access to specific
tasks.
•
Need for an interrupt or exception to be handled by an independent task
— Task gates may also reside in the IDT, which allows interrupts and exceptions
— Task gates may also reside in the IDT, which allows interrupts and exceptions
Figure 7-6. Task-Gate Descriptor
31
16 15
13
14
12 11
8 7
0
P
D
P
P
L
Type
0
31
16 15
0
TSS Segment Selector
1
0
1
0
DPL
P
TYPE
P
TYPE
Descriptor Privilege Level
Segment Present
Segment Type
Segment Present
Segment Type
4
0
Reserved
Reserved
Reserved