Intel 253668-032US Benutzerhandbuch

Vol. 3 7-17

TASK MANAGEMENT

Table 7-2 shows the busy flag (in the TSS segment descriptor), the NT flag, the
previous task link field, and TS flag (in control register CR0) during a task switch.
The NT flag may be modified by software executing at any privilege level. It is
possible for a program to set the NT flag and execute an IRET instruction. This might
randomly invoke the task specified in the previous link field of the current task's TSS.
To keep such spurious task switches from succeeding, the operating system should
initialize the previous task link field in every TSS that it creates to 0.

Figure 7-8. Nested Tasks

Table 7-2. Effect of a Task Switch on Busy Flag, NT Flag,

Previous Task Link Field, and TS Flag

Flag or Field

Effect of JMP

instruction

Effect of CALL

Instruction or

Interrupt

Effect of IRET

Instruction

Busy (B) flag of new

task.

Flag is set. Must have

been clear before.

Flag is set. Must have

been clear before.

No change. Must have

been set.

Busy flag of old task.

Flag is cleared.

No change. Flag is

currently set.

Flag is cleared.

NT flag of new task.

Set to value from TSS

of new task.

Flag is set.

Set to value from TSS

of new task.

NT flag of old task.

No change.

Flag is cleared.

Previous task link field

of new task.

No change.

Loaded with selector

for old task’s TSS.

No change.

Previous task link field

of old task.

No change.

TS flag in control

Flag is set.

Top Level

Task

NT=0

TSS

Nested

Task

NT=1

TSS

More Deeply

Nested Task

NT=1

TSS

Currently Executing

Task

NT=1

EFLAGS

Task Register

Task Link