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Question about prvInitialiseNewTask() function

Posted by ctc8631 on May 23, 2017

Hello, I'm new learner for freeRTOS, I was confused about code in prvInitialiseNewTask(), when I call xTaskCreate()


if( portSTACK_GROWTH < 0 )

{ pxTopOfStack = pxNewTCB->pxStack + ( ulStackDepth - ( uint32t ) 1 ); pxTopOfStack = ( StackTypet * ) ( ( ( portPOINTERSIZETYPE ) pxTopOfStack ) & ( ~( ( portPOINTERSIZETYPE ) portBYTEALIGNMENTMASK ) ) ); } ```

I'm not sure why calculate pxTopOfStack need to minus 1, I read the later code in pxPortInitialiseStack(), when it need stack space, it will minus first and use the space, So I'm not really understand why need -1 here, then it will do alignment, did it waste space? Why not do alignment without -1?


Question about prvInitialiseNewTask() function

Posted by rtel on May 23, 2017

This sets the top of stack to be within the stack space, not off the end of it. pxPortInitialiseStack() is port specific, and not all ports will align the top of the stack, but even the ones that do may find the top of stack to coincidentally already be aligned and therefore not make any adjustment.

Question about prvInitialiseNewTask() function

Posted by ctc8631 on May 23, 2017

Mm, I know a little but still a little confused.

I'm using FreeRTOS on cortex-m4. So, in this platform, I do not actually need to make pxTopOfStack-1, right? Because in one situation, the original pxTopOfStack is aligned to 8, if pxTopOfStack-1 then be aligned, it will waste 8bytes, I thought it's a little pity.

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