7-8 Vol. 3A
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 7-5. Task Register
Figure 7-6. Task-Gate Descriptor
Segment Limit
Selector
+
GDT
TSS Descriptor
0
Base Address
Task
Invisible Part
Visible Part
TSS
Register
31
16 15
13
14
12 11
8 7
0
P
D
P
L
Type
0
31
16 15
0
TSS Segment Selector
1
0
1
0
DPL
P
TYPE
Descriptor Privilege Level
Segment Present
Segment Type
4
0
Reserved
Reserved
Reserved