Re: [Contiki-developers] Information about aligned memory

[Moritz S. <Mor...@in...>]

 Hi Sarah,

whether memory needs to be aligned really depends on what you want to
do. As memory can be accessed byte-wise you only need word-alignment
when accessing the memory word-wise.
As long as you have an byte-array and don't cast it, you _should_ be
safe (The C compiler should use byte-access instead of word-access).
Once you start casting you must make sure there your cast is aligned.
E.g: "char buf[10]" is word aligned. If you do "(int *)&buf[1]" the
result will have an unpredictable high byte (as far as I understood the
manual - but it might as well just cause a reset).
As you see this is rather a hardware then a software issue. And, as the
msp430 hasn't got an interrupt to handle unaligned access, the OS hasn't
even got a chance to do something.
There are two ways to get something word aligned. Either you allocate an
uint16_t array and cast it to char* or, if you use attributes:
http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html

Cheers
Morty
 

Am 12.10.2010 08:34, schrieb Sarah:
> Hi all,
>
> Could someone tell me how memory access works in Contiki (with telosB/tmote sky platform)? Because this platform needs aligned memory.
> How be sure that the memory is aligned... What is the reaction of the OS if the memory is not aligned (exception...)?
> If I declare a table like this:    buffer[256]  , it can be unaligned?
>
>
> Thanks for your help.
>
> Best regards,
>
> --
> Sarah
>
>
>
>
>
>


-- 
Dipl.-Ing. Moritz 'Morty' Struebe (Wissenschaftlicher Mitarbeiter)
Lehrstuhl für Informatik 4 (Verteilte Systeme und Betriebssysteme)
Friedrich-Alexander-Universität Erlangen-Nürnberg
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