Question about prvInitialiseNewTask() function

Kernel
1

ctc8631 wrote on Tuesday, 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 - ( uint32_t ) 1 );
    pxTopOfStack = ( StackType_t * ) ( ( ( portPOINTER_SIZE_TYPE ) pxTopOfStack ) & ( ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) ) );
}

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?

Thanks

2

rtel wrote on Tuesday, 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.

3

ctc8631 wrote on Tuesday, 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.