Intel IA-32 - Manual (Page 258)

6-16 Vol. 3A
TASK MANAGEMENT
The TS (task switched) flag in the control register CR0 is set every time a task switch occurs.
System software uses the TS flag to coordinate the actions of floating-point unit when gener-
ating floating-point exceptions with the rest of the processor. The TS flag indicates that the
context of the floating-point unit may be different from that of the current task. See Section 2.5,
“Control Registers”, for a detailed description of the function and use of the TS flag.
6.4 TASK LINKING
The previous task link field of the TSS (sometimes called the “backlink”) and the NT flag in the
EFLAGS register are used to return execution to the previous task. EFLAGS.NT = 1 indicates
that the currently executing task is nested within the execution of another task.
When a CALL instruction, an interrupt, or an exception causes a task switch: the processor
copies the segment selector for the current TSS to the previous task link field of the TSS for the
new task; it then sets EFLAGS.NT = 1. If software uses an IRET instruction to suspend the new
task, the processor checks for EFLAGS.NT = 1; it then uses the value in the previous task link
field to return to the previous task. See Figures 6-8.
When a JMP instruction causes a task switch, the new task is not nested. The previous task link
field is not used and EFLAGS.NT = 0. Use a JMP instruction to dispatch a new task when
nesting is not desired.
DS, ES, FS, and GS segments are present in memory. #NP New Data Segment
DS, ES, FS, and GS segment DPL greater than or
equal to CPL (unless these are conforming segments). #TS New Data Segment
NOTES:
1. #NP is segment-not-present exception, #GP is general-protection exception, #TS is invalid-TSS excep-
tion, and #SF is stack-fault exception.
2. The error code contains an index to the segment descriptor referenced in this column.
3. A segment selector is valid if it is in a compatible type of table (GDT or LDT), occupies an address within
the table's segment limit, and refers to a compatible type of descriptor (for example, a segment selector
in the CS register only is valid when it points to a code-segment descriptor).
Table 6-1. Exception Conditions Checked During a Task Switch (Contd.)
Condition Checked Exception 1
Error Code
Reference
2