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7.1 Task State Segment
All the information the processor needs in order to manage a task is stored
in a special type of segment, a task state segment (TSS).
Figure 7-1
shows
the format of a TSS for executing 80386 tasks. (Another format is used for
executing 80286 tasks; refer to
Chapter 13.)
The fields of a TSS belong to two classes:
- A dynamic set that the processor updates with each switch from the
task. This set includes the fields that store:
- The general registers (EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI).
- The segment registers (ES, CS, SS, DS, FS, GS).
- The flags register (EFLAGS).
- The instruction pointer (EIP).
- The selector of the TSS of the previously executing task (updated
only when a return is expected).
- A static set that the processor reads but does not change. This set
includes the fields that store:
- The selector of the task's LDT.
- The register (PDBR) that contains the base address of the task's
page directory (read only when paging is enabled).
- Pointers to the stacks for privilege levels 0-2.
- The T-bit (debug trap bit) which causes the processor to raise a
debug exception when a task switch occurs . (Refer to
Chapter 12
for more information on debugging.)
- The I/O map base (refer to
Chapter 8 for more information on the
use of the I/O map).
Task state segments may reside anywhere in the linear space. The only case
that requires caution is when the TSS spans a page boundary and the
higher-addressed page is not present. In this case, the processor raises an
exception if it encounters the not-present page while reading the TSS during
a task switch. Such an exception can be avoided by either of two strategies:
- By allocating the TSS so that it does not cross a page boundary.
- By ensuring that both pages are either both present or both
not-present at the time of a task switch. If both pages are
not-present, then the page-fault handler must make both pages present
before restarting the instruction that caused the task switch.
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