Intel IA-32 Manuale Utente
Vol. 3A 6-9
TASK MANAGEMENT
6.2.4
Task Register
The task register holds the 16-bit segment selector and the entire segment descriptor (32-bit base
address, 16-bit segment limit, and descriptor attributes) for the TSS of the current task (see
Figure 2-5). This information is copied from the TSS descriptor in the GDT for the current task.
Figure 6-5 shows the path the processor uses to access the TSS (using the information in the task
register).
address, 16-bit segment limit, and descriptor attributes) for the TSS of the current task (see
Figure 2-5). This information is copied from the TSS descriptor in the GDT for the current task.
Figure 6-5 shows the path the processor uses to access the TSS (using the information in the task
register).
The task register has a visible part (that can be read and changed by software) and an invisible
part (maintained by the processor and is inaccessible by software). The segment selector in the
visible portion points to a TSS descriptor in the GDT. The processor uses the invisible portion
of the task register to cache the segment descriptor for the TSS. Caching these values in a
register makes execution of the task more efficient. The LTR (load task register) and STR (store
task register) instructions load and read the visible portion of the task register:
part (maintained by the processor and is inaccessible by software). The segment selector in the
visible portion points to a TSS descriptor in the GDT. The processor uses the invisible portion
of the task register to cache the segment descriptor for the TSS. Caching these values in a
register makes execution of the task more efficient. The LTR (load task register) and STR (store
task register) instructions load and read the visible portion of the task register:
Figure 6-4. Format of TSS and LDT Descriptors in 64-bit Mode
31
24 23 22 21 20 19
16 15
13
14
12 11
8 7
0
P
Base 31:24
G
D
P
P
L
Type
0
0
31
16 15
0
Base Address 15:00
Segment Limit 15:00
Base 23:16
A
V
V
L
Limit
19:16
0
TSS (or LDT) Descriptor
AVL
B
BASE
DPL
G
B
BASE
DPL
G
Available for use by system software
Busy flag
Segment Base Address
Descriptor Privilege Level
Granularity
Busy flag
Segment Base Address
Descriptor Privilege Level
Granularity
LIMIT
P
TYPE
P
TYPE
Segment Limit
Segment Present
Segment Type
Segment Present
Segment Type
0
4
31
13 12
8 7
0
Reserved
31
0
Base Address 63:32
Reserved
0
8
12