[Tutor] what is 'doubleword alignment'?

[Kushal Kumaran]

On Sun, Jul 17, 2011 at 9:15 PM, Walter Prins <wprins at gmail.com> wrote:
>
>
> On 17 July 2011 15:26, Lisi <lisi.reisz at gmail.com> wrote:
>>
>> Sorry to be slow.  Blame virtually no sleep last night ;-(  But even were
>> the
>> power of two bit correct (and I see subsequently that it is not), how is
>> 18 a
>> power of two?
>>
>
> The 18 bytes is a bit of an irrelevance.  The point is that if the start of
> the buffer falls on a dword (double word) alligned memory location then in
> theory the access should be faster.  The term is a little bit ambiguous
> because strictly speaking different processors have different word sizes.
> Even so, usually when people speak of double-word alignment, it's often the
> case that the term word in such a context has its original meaning, e.g. 16
> bits.  A dword is then 32bits or 4 bytes.   A doubleword aligned memory
> address is, using these assumptions, therefore an address that is divisible
> by 4.  Obviously if the word size is 32bits, then a double word would be
> 64bits and a doubleword aligned address would need to be divisible by 8.  As
> an aside, this type of optimization is often taken care of by compilers
> under the hood, and in any case it's generally not something that you'll
> really be considering as a Python programmer.  (If however you were working
> on one of the Python runtimes or implementations, then you might well be
> sometimes considering this type of thing, depending on exactly how
> performance critical what you are working might be and what the runtime was
> being implemented in.)
>

It's not just about performance.  Some hardware simply cannot access
data that is not correctly aligned.  C programs that indiscriminately
cast among pointers to types of different sizes are a pain to port off
lenient architectures like x86.  If you're writing C code that deals
with pointers, you *always* need to keep alignment in mind.

-- 
regards,
kushal