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Unformatted text preview: r a limit greater than 67H on a task switch; however, it does when accessing the I/O permission bit map or interrupt redirection bit map. Any program or procedure with access to a TSS descriptor (that is, whose CPL is numerically equal to or less than the DPL of the TSS descriptor) can dispatch the task with a call or a jump. In most systems, the DPLs of TSS descriptors should be set to values less than 3, so that only privileged software can perform task switching. However, in multitasking applications, DPLs for some TSS descriptors can be set to 3 to allow task switching at the application (or user) privilege level. 6-7 TASK MANAGEMENT 6.2.3. Task Register The task register holds the 16-bit segment selector and the entire segment descriptor (32-bit base address, 16-bit segment limit, and descriptor attributes) for the TSS of the current task (refer to Figure 2-4 in Chapter 2, System Architecture Overview). This information is copied from the TSS descriptor in the GDT for the current task. Figure 6-4 shows the path the processor uses to accesses the TSS, using the information in the task register. The task register has both a visible part (that can be read and changed by software) and an invisible part (that is maintained by the processor and is inaccessible by software). The segment selector in the visible portion points to a TSS descriptor in the GDT. The processor uses the invisible portion of the task register to cache the segment descriptor for the TSS. Caching these values in a register makes execution of the task more efficient, because the processor does not need to fetch these values from memory to reference the TSS of the current task. The LTR (load task register) and STR (store task register) instructions load and read the visible portion of the task register. The LTR instruction loads a segment selector (source operand) into the task register that points to a TSS descriptor in the GDT, and then loads the invisible portion of the task register with information from the TSS descriptor. This instruction is a privileged instruction that may be executed only when the CPL is 0. The LTR instruction generally is used during system initialization to put an initial value in the task register. Afterwards, the contents of the task register are changed implicitly when a task switch occurs. The STR (store task register) instruction stores the visible portion of the task register in a general-purpose register or memory. This instruction can be executed by code running at any privilege level, to identify the currently running task; however, it is normally used only by operating system software. On power up or reset of the processor, the segment selector and base address are set to the default value of 0 and the limit is set to FFFFH. 6.2.4. Task-Gate Descriptor A task-gate descriptor provides an indirect, protected reference to a task. Figure 6-5 shows the format of a task-gate descriptor. A task-gate descriptor can be placed in the GDT, an LDT, or the IDT. The TSS segment selector field in a task-gate descriptor points to a TSS descriptor in the GDT. The RPL in this segment selector is not used. The DPL of a task-gate descriptor controls access to the TSS descriptor during a task switch. When a program or procedure makes a call or jump to a task through a task gate, the CPL and the RPL field of the gate selector pointing to the task gate must be less than or equal to the DPL of the task-gate descriptor. (Note that when a task gate is used, the DPL of the destination TSS descriptor is not used.) 6-8 TASK MANAGEMENT TSS + Visible Part Task Register Selector Invisible Part Base Address Segment Limit GDT TSS Descriptor 0 Figure 6-4. Task Register 31 16 15 14 13 12 11 P D P L 87 0 Type 00101 0 4 31 16 15 TSS Segment Selector 0 DPL P TYPE Descriptor Privilege Level Segment Present Segment Type Reserved Figure 6-5. Task-Gate Descriptor 6-9 TASK MANAGEMENT A task can be accessed either through a task-gate descriptor or a TSS descriptor. Both of these structures are provided to satisfy the following needs: • • The need for a task to have only one busy flag. Because the busy flag for a task is stored in the TSS descriptor, each task should have only one TSS descriptor. There may, however, be several task gates that reference the same TSS descriptor. The need to provide selective access to tasks. Task gates fill this need, because they can reside in an LDT and can have a DPL that is different from the TSS descriptor’s DPL. A program or procedure that does not have sufficient privilege to access the TSS descriptor for a task in the GDT (which usually has a DPL of 0) may be allowed access to the task through a task gate with a higher DPL. Task gates give the operating system greater latitude for limiting access to specific tasks. The need for an interrupt or exception to be handled by an independent task. Task gates may also reside in the IDT, which allows interrupts and exceptions to be handled by handler tasks. When an interrupt or exception vector p...
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This note was uploaded on 06/07/2013 for the course ECE 1234 taught by Professor Kwhon during the Spring '10 term at University of California, Berkeley.

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