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From: Simon Wright <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Re: Robert Dewar's great article about the Strengths of Ada over
  other langauges in multiprocessing!
Date: Fri, 14 Mar 2008 21:54:26 +0000
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Date: 2008-03-14T21:54:26+00:00
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gpriv@axonx.com writes:

> On Mar 13, 6:25 pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
>> On 13 Mar, 17:16, gp...@axonx.com wrote:
>>
>> > So if you write to IO through volatile and later read from it, you may
>> > get the same value fetched back by "smart" CPU.
>>
>> I would expect that I/O is realized within the range of memory
>> addresses that is excluded from the "smart" handling (your BIOS setup
>
> I would expect that too, but not sure that "compatitive" compiler
> vendors will share my/your beliefs.

I think that 'volatile' is only a way to tell the compiler that this
object must be read/written directly each time; don't copy it to a
register and manipulate values there.

Likewise 'atomic' says to read/write the whole thing with one machine
operation. Probably because that's what the IO hardware requires; eg,
you must write all 32 bits at once, not just the only byte you've
changed..

The only time this makes sense is with memory-mapped IO where the
system must be designed so that this is sensible.

If you try it with memory with varying levels of hardware cache and
with multiple cores you are going to be disappointed, I think.