Context Switching: Difference between revisions
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===Other Possibilities=== |
===Other Possibilities=== |
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During a context switch the operating system can do additional work that isn't strictly part of the context switch. One common thing is calculating the amount of time the last thread/task/process used so that software (and the end user) can determine where all the CPU time is going. Another possibility would be dynamically changing thread/task/process |
During a context switch the operating system can do additional work that isn't strictly part of the context switch. One common thing is calculating the amount of time the last thread/task/process used so that software (and the end user) can determine where all the CPU time is going. Another possibility would be dynamically changing thread/task/process priorities. |
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==Hardware Context Switching== |
==Hardware Context Switching== |
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In addition to the CALL and JMP instructions, a context switch can be triggered by a using a Task-Gate Descriptor. Unlike TSS Descriptors, task-gate descriptors can be in the GDT, LDT or IDT. Normally, task-gate descriptors are used in the IDT, so that an exception (or IRQ) can cause a context switch, which is the only way of handling a double fault exception with complete reliability. |
In addition to the CALL and JMP instructions, a context switch can be triggered by a using a Task-Gate Descriptor. Unlike TSS Descriptors, task-gate descriptors can be in the GDT, LDT or IDT. Normally, task-gate descriptors are used in the IDT, so that an exception (or IRQ) can cause a context switch, which is the only way of handling a double fault exception with complete reliability. |
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The design of the basic hardware mechanism is limited by the number of usable entries in the GDT because TSS descriptors can be in the GDT only (theoretical limit is 8190 tasks). However, it is possible to avoid this restriction by dynamically changing TSS descriptor/s, by setting the TSS descriptor's base before each context switch. Care must be taken when using this approach when task-gate descriptors in the IDT are also used (the TSS descriptors |
The design of the basic hardware mechanism is limited by the number of usable entries in the GDT because TSS descriptors can be in the GDT only (theoretical limit is 8190 tasks). However, it is possible to avoid this restriction by dynamically changing TSS descriptor/s, by setting the TSS descriptor's base before each context switch. Care must be taken when using this approach when task-gate descriptors in the IDT are also used (the TSS descriptors referred to by each task-gate descriptor would have to be constant). Also context switches can't be initiated with a CALL instruction, because the CPU saves the GDT entry to use for the return in the TSS's "backlink" field. |
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If the FPU/MMX and SSE state also needs to be changed during a context switch there are a few options. The data could be explicitly saved by any code that causes a context switch, or the CPU can generate an exception the first time an FPU/MMX or SSE instruction is used. With the second option, the exception handlers would save the old FPU/MMX/SSE state and reload the new state. This option may prevent this data from being changed when it's not necessary (for e.g. when no tasks or only one task is using them), but fails to work correctly in a multiprocessor environment without additional synchronization which may be more expensive than using the first option. |
If the FPU/MMX and SSE state also needs to be changed during a context switch there are a few options. The data could be explicitly saved by any code that causes a context switch, or the CPU can generate an exception the first time an FPU/MMX or SSE instruction is used. With the second option, the exception handlers would save the old FPU/MMX/SSE state and reload the new state. This option may prevent this data from being changed when it's not necessary (for e.g. when no tasks or only one task is using them), but fails to work correctly in a multiprocessor environment without additional synchronization which may be more expensive than using the first option. |
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