Filter
Top Products
Vol. 3 7-9
TASK MANAGEMENT
7.2.4 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 7-5 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:
Figure 7-4. Format of TSS and LDT Descriptors in 64-bit Mode
31
24
23
22
212019 16
15
13
14 12
11
8
7
0
P
Base 31:24
G
D
P
L
Type
0
0
31
16
15
0
Base Address 15:00
Segment Limit 15:00
Base 23:16
A
V
L
Limit
19:16
0
TSS (or LDT) Descriptor
AVL
B
BASE
DPL
G
Available for use by system software
Busy flag
Segment Base Address
Descriptor Privilege Level
Granularity
LIMIT
P
TYPE
Segment Limit
Segment Present
Segment Type
0
4
31
13
12
8
7
0
Reserved
31
0
Base Address 63:32
Reserved
0
8
12