Intel IA-32 Manuale Utente

Vol. 3A 6-7

The TSS, like all other segments, is defined by a segment descriptor. Figure 6-3 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 indicates 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 inactive 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.

31

24 23 22 21 20 19

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

1

B

0

1

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