Intel ARCHITECTURE IA-32 User Manual download pdf (Page 258)

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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

Error Code

Reference

1 2 ... 253 254 255 256 257 258 259 260 261 262 263 ... 635 636

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