7.5 Task Switching

The 80386 switches execution to another task in any of four cases:

  1.  The current task executes a JMP or CALL that refers to a TSS
      descriptor.

  2.  The current task executes a JMP or CALL that refers to a task gate.

  3.  An interrupt or exception vectors to a task gate in the IDT.

  4.  The current task executes an IRET when the NT flag is set.

To cause a task switch, a JMP or CALL instruction can refer either to a TSS descriptor or to a task gate. The effect is the same in either case: the 80386 switches to the indicated task.

An exception or interrupt causes a task switch when it vectors to a task gate in the IDT. If it vectors to an interrupt or trap gate in the IDT, a task switch does not occur. Refer to Chapter 9 for more information on the interrupt mechanism.

Whether invoked as a task or as a procedure of the interrupted task, an interrupt handler always returns control to the interrupted procedure in the interrupted task. If the NT flag is set, however, the handler is an interrupt task, and the IRET switches back to the interrupted task.

A task switching operation involves these steps:

1. Checking that the current task is allowed to switch to the designated task. Data-access privilege rules apply in the case of JMP or CALL instructions. The DPL of the TSS descriptor or task gate must be less than or equal to the maximum of CPL and the RPL of the gate selector. Exceptions, interrupts, and IRETs are permitted to switch tasks regardless of the DPL of the target task gate or TSS descriptor.
2. Checking that the TSS descriptor of the new task is marked present and has a valid limit. Any errors up to this point occur in the context of the outgoing task. Errors are restartable and can be handled in a way that is transparent to applications procedures.
3. Saving the state of the current task. The processor finds the base address of the current TSS cached in the task register. It copies the registers into the current TSS (EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI, ES, CS, SS, DS, FS, GS, and the flag register). The EIP field of the TSS points to the instruction after the one that caused the task switch.
4. Loading the task register with the selector of the incoming task's TSS descriptor, marking the incoming task's TSS descriptor as busy, and setting the TS (task switched) bit of the MSW. The selector is either the operand of a control transfer instruction or is taken from a task gate.
5. Loading the incoming task's state from its TSS and resuming execution. The registers loaded are the LDT register; the flag register; the general registers EIP, EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI; the segment registers ES, CS, SS, DS, FS, and GS; and PDBR. Any errors detected in this step occur in the context of the incoming task. To an exception handler, it appears that the first instruction of the new task has not yet executed.

Every task switch sets the TS (task switched) bit in the MSW (machine status word). The TS flag is useful to systems software when a coprocessor (such as a numerics coprocessor) is present. The TS bit signals that the context of the coprocessor may not correspond to the current 80386 task. Chapter 11 discusses the TS bit and coprocessors in more detail.

Exception handlers that field task-switch exceptions in the incoming task (exceptions due to tests 4 thru 16 of Table 7-1) should be cautious about taking any action that might load the selector that caused the exception. Such an action will probably cause another exception, unless the exception handler first examines the selector and fixes any potential problem.

The privilege level at which execution resumes in the incoming task is neither restricted nor affected by the privilege level at which the outgoing task was executing. Because the tasks are isolated by their separate address spaces and TSSs and because privilege rules can be used to prevent improper access to a TSS, no privilege rules are needed to constrain the relation between the CPLs of the tasks. The new task begins executing at the privilege level indicated by the RPL of the CS selector value that is loaded from the TSS.

Table 7-1. Checks Made during a Task Switch

Test     Test Description                   Exception
NP = Segment-not-present exception, GP = General protection fault, TS =
Invalid TSS, SF = Stack fault   Error Code Selects

  1      Incoming TSS descriptor is         NP           Incoming TSS
         present
  2      Incoming TSS descriptor is         GP           Incoming TSS
         marked not-busy
  3      Limit of incoming TSS is           TS           Incoming TSS
         greater than or equal to 103

             -- All register and selector values are loaded --

  4      LDT selector of incoming           TS           Incoming TSS
         task is valid
  5      LDT of incoming task is            TS           Incoming TSS
         present
  6      CS selector is valid

                                            TS           Code segment
  7      Code segment is present            NP           Code segment
  8      Code segment DPL matches           TS           Code segment
         CS RPL
  9      Stack segment is valid

                                            GP           Stack segment
 10      Stack segment is present           SF           Stack segment
 11      Stack segment DPL = CPL            SF           Stack segment
 12      Stack-selector RPL = CPL           GP           Stack segment
 13      DS, ES, FS, GS selectors are       GP           Segment
         valid

 14      DS, ES, FS, GS segments            GP           Segment
         are readable
 15      DS, ES, FS, GS segments            NP           Segment
         are present
 16      DS, ES, FS, GS segment DPL         GP           Segment
         . CPL (unless these are
         conforming segments)