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Re: Thread stack alignment requirements for ARM


On 23/06/11 03:37, Michael Bergandi wrote:
> Hi all,
> 
> I am hoping that someone can share some knowledge on thread stack
> alignment requirements for ARM targets and how eCos is handling it.
> 
> According to the ARM site, they say that stacks should be 16 byte
> aligned. Then, they go on to say that there are a couple ways that
> stack alignment requirement can be managed. One of which was if you
> are running on an OS and the OS has taken steps to ensure the
> requirement is met, then the application need not worry about it. I
> don't think I fully understand what this means exactly.
> 
> Our particular target is the mx27 (ARM9). Out of habit, we make all
> the memory for the thread stacks in our applications 4 byte aligned.
> Is this enough? Is it really necessary?
> 
> I have found some packages in the kernel (specifically, bsd_tcpip)
> that has thread stack memory allocated with no alignment attribute
> set. This got me wondering how this all works.
> 
> I would love to here from someone with a much better grasp on this.
> 

The alignment of the pieces of memory that are used as stacks does not
really matter. It is the alignment of the stack pointer that matters.
This is handled in HAL_THREAD_INIT_CONTEXT() which ensures that the
stack pointer is 16 byte aligned regardless of the alignment of the
stack pointer passed to it.


-- 
Nick Garnett                                       eCos Kernel Architect
eCosCentric Limited    http://www.eCosCentric.com       The eCos experts
Barnwell House, Barnwell Drive, Cambridge, UK.      Tel: +44 1223 245571
Registered in England and Wales:                         Reg No: 4422071


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