Linkage Modification

Refer to Figure 11-6 on page 203. Assume that task A called task B, and task B called task C. If an IRET instruction is executed at the end of task C, a task switch back to task B will result (using the TSS selector stored in task C's Link field). Now assume that the programmer wants the IRET instruction at the end of task C to cause a task switch to task A, rather than to task B. This should be accomplished in the following manner:

  1. Disable interrupts by executing a CLI instruction.

  2. Replace the task C Link field (which currently contains the GDT selector for the task B TSS descriptor) with the selector for task A's TSS descriptor.

  3. Clear the Busy bit in task B's TSS descriptor.

  4. Reenable interrupt recognition with an STI instruction. ...

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