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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.

Figure 7-1.  80386 32-Bit Task State Segment

      31              23              15              7             0
     +---------------+---------------+---------------+-------------+-+
     |          I/O MAP BASE         | 0 0 0 0 0 0 0   0 0 0 0 0 0 |T|64
     |---------------+---------------+---------------+-------------+-|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              LDT              |60
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              GS               |5C
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              FS               |58
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              DS               |54
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              SS               |50
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              CS               |4C
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              ES               |48
     |---------------+---------------+---------------+---------------|
     |                              EDI                              |44
     |---------------+---------------+---------------+---------------|
     |                              ESI                              |40
     |---------------+---------------+---------------+---------------|
     |                              EBP                              |3C
     |---------------+---------------+---------------+---------------|
     |                              ESP                              |38
     |---------------+---------------+---------------+---------------|
     |                              EBX                              |34
     |---------------+---------------+---------------+---------------|
     |                              EDX                              |30
     |---------------+---------------+---------------+---------------|
     |                              ECX                              |2C
     |---------------+---------------+---------------+---------------|
     |                              EAX                              |28
     |---------------+---------------+---------------+---------------|
     |                            EFLAGS                             |24
     |---------------+---------------+---------------+---------------|
     |                    INSTRUCTION POINTER (EIP)                  |20
     |---------------+---------------+---------------+---------------|
     |                          CR3  (PDPR)                          |1C
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              SS2              |18
     |---------------+---------------+---------------+---------------|
     |                             ESP2                              |14
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              SS1              |10
     |---------------+---------------+---------------+---------------|
     |                             ESP1                              |0C
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|              SS0              |8
     |---------------+---------------+---------------+---------------|
     |                             ESP0                              |4
     |---------------+---------------+---------------+---------------|
     |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|   BACK LINK TO PREVIOUS TSS   |0
     +---------------+---------------+---------------+---------------+

----------------------------------------------------------------------------
NOTE
      0 MEANS INTEL RESERVED. DO NOT DEFINE.
----------------------------------------------------------------------------

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