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Vol. 3 7-7
TASK MANAGEMENT
• Task switches are carried out faster if the pages containing these structures are
present in memory before the task switch is initiated.
7.2.2 TSS Descriptor
The TSS, like all other segments, is defined by a segment descriptor. Figure 7-3
shows the format of a TSS descriptor. TSS descriptors may only be placed in the GDT;
they cannot be placed in an LDT or the IDT.
An attempt to access a TSS using a segment selector with its TI flag set (which indi-
cates the current LDT) causes a general-protection exception (#GP) to be generated
during CALLs and JMPs; it causes an invalid TSS exception (#TS) during IRETs. A
general-protection exception is also generated if an attempt is made to load a
segment selector for a TSS into a segment register.
The busy flag (B) in the type field indicates whether the task is busy. A busy task is
currently running or suspended. A type field with a value of 1001B indicates an inac-
tive task; a value of 1011B indicates a busy task. Tasks are not recursive. The
processor uses the busy flag to detect an attempt to call a task whose execution has
been interrupted. To insure that there is only one busy flag is associated with a task,
each TSS should have only one TSS descriptor that points to it.
The base, limit, and DPL fields and the granularity and present flags have functions
similar to their use in data-segment descriptors (see
Section 3.4.5, “Segment
Descriptors”). When the G flag is 0 in a TSS descriptor for a 32-bit TSS, the limit field
must have a value equal to or greater than 67H, one byte less than the minimum size
Figure 7-3. TSS Descriptor
31
2423 22
2120 19 16
15
13
14 12
11
87
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
1B01
TSS 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