5.11 Tasks 2.

5.11 Tasks 2.

1. Task State Segments TSS

   • Processor state information used to store a task is saved in a TSS
     • TSS has 2 main catagories
       • Static fields - Initiated when the task is created
       • Dynamic fields - processor updates with each task

2. Task State Descriptor

   • A TSS is defined like all other Segments by a Descriptor
     • Hence a TSS has a DPL and Type
       • Must be stored in the GDT

3. Task Register

   • Used to locate the current TSS
     • Contains a visible part that can be modified by software
       • Stores a selector for the TSS descriptor
     • Invisible part that can only be modified by the processor
       • Stores the base limit in the descriptor

4. Task gate desscriptors

   • Provides an indirect protected reference to a taks
   • Need not reside in the GDT
   • Used to allow selective access by less privileged tasks to more privileged tasks

5. Controlling task swithces

   • A tasks CPL is determined by the DPL of the code segment
     • Its RPL is stored in the selector for the code segment
   • A task cannot access a task segment descriptor unless the tasks RPL and CPL are numerically less than or equal to the DPL of the TSS descriptor
   • A task cannot access a task gate descriptor unless the tasks RPL and CPL are numerically less than or equal to the DPL of the TSS descriptor

6. Controlled invocation

   • Task gates are necessary to support controlled invocation
     • A task that does not have sufficient privilege to access a task in the GDT can still call another task if it has access to a task gate in its LDT
   • Task Gates enable interrupts and exceptions to cause a task switch
   • Several task gates can reference the same TSS descriptor