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Why SVC is used in FreeRTOS

Posted by niekiran on January 21, 2017

Hello All, I went through the xTaskStartSechudler() function, which ends up triggering the SVC instruction. and i came to know that it used in only in this function. If i am not wrong , is SVC used to launch the first Task ? So, in my understanding SVC is used only one time, and subsequent task switching in and out is carried out in the PendSV handler. am i right ?

Why can cant we use pendsv itself instead of SVC ? Thanks


Why SVC is used in FreeRTOS

Posted by rtel on January 21, 2017

Yes you are correct, unless you are using the version that supports the MPU the SVC instruction is only used once.

Old versions don't use the SVC, and instead use the PendSV, and there was a really good reason for changing that I can't recall the exact details of but it was something to do with having to add extra code into the PendSV instruction so it knew whether it had to save a context or not, and also loosing a bit of stack space. If you use SVC to start the first task then the SVC handler resets the stack to the start of the stack and just has to restore the context of the first task to run. If you are using the PendSV handler then you can't reset the stack (because you are using it?), and you have to test whether or not a task was running when the PendSV handler was called (so the context is not saved if it is the first call because there is no task context to save).


Why SVC is used in FreeRTOS

Posted by niekiran on January 21, 2017

Thanks @Real Time Eng. for clarifying the doubts .


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