Question on Alignment requiremnet

[Greg Freemyer]

On Aug 6, 2011 1:57 PM, "Mulyadi Santosa" <mulyadi.santosa at gmail.com> wrote:
>
> On Sat, Aug 6, 2011 at 23:59, subin gangadharan
> <subingangadharan at gmail.com> wrote:
> > Hi All,
> > I have some doubts on the alignment requirement.It would be really
> > helpful,if someone can shed some light on this.
> > Why there are so many different types of alignment like 4 byte, 8
byte,16
> > byte ?.My exact question is, in a 32 bit machine(I assume processor
reads
> > data in 4 bytes),how 16 byte alignment makes different from 4 byte
alignment
> > ?.How this will influence the processor performance ?.
>
> personal guess: cache line alignment?
>
> ... so the data all can be read in one read swipe.....
>
> or in other hand, if several data has different access type (some are
> read only, the rest are read/write), then by aligning them to
> different cache line, they won't interfere to each other...since AFAIK
> a write to  even one bit in a cache line will update the whole cache
> line. Other data in the same cache line will stay, they will be just
> rewritten AFAIK.
>
> I hope my guess is right :)

Too complex I think.   Don't try to think so hard. ;-)

Most cpu's offer an atomic test_and_set instruction.

It is used to write busy loop semaphore code like.

     While (test_and_set(flag) == false);
# coded in assembly

Normally that busy loops until flag initially has a value of zero during the
test and the test changes it to a one.

But if test_and_set always fails due lack of alignment, that loop will never
finish.

Greg
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