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The reason a kernel's queue send from a handler may request a context switch on return is that

  • A.the woken task may be of higher priority than the interrupted one
  • B.the queue must be compacted before the handler returns
  • C.the tick would otherwise be lost while the handler ran
  • D.the handler cannot return while a task is ready to run

Answer: A

Waking a high priority task is pointless if the system then resumes the low priority task it interrupted and waits for the next tick. The from-ISR calls therefore report whether a switch is wanted, and the handler yields on its way out, which is what keeps the response time short.

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