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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:

  1. 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).
  2. 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:
  1. By allocating the TSS so that it does not cross a page boundary.
  2. 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.

up: Chapter 7 -- Multitasking
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