Re: Pragma Volatile

[Jeffrey Carter <jrcarter@acm.org>]

minyard@acm.org wrote:
> 
> Tucker Taft <stt@avercom.net> writes:
> 
> > There shouldn't be any problem if you are using this in a mono-processor.
>   (talking about volatiles not being important on uniprocessors).

From my reading of Taft's post, he did not say that pragma Volatile was
unimportant on uniprocessors. He said that achieving the desired
volatility is not a problem on uniprocessors; that you can count on the
compiler locating an object at an appropriate location if pragma
Volatile applies to it.

> Also, consider a register to reset a device, where you write a 0 then
> a 1 to reset it.
> 
>   B : Register_Access;
> 
>   B.all := 0;
>   B.all := 1;
> 
> Again, a compiler can say, "Hey, we can just ignore the first
> operation, because it doesn't have any effect", from the compiler's
> point of view.  But setting B volatile guarantees that both operations
> get done.  Actually, I'm not sure of the syntax here, because what B
> references is volatile, not B itself.  So I'm guessing you would have
> 
>   type Register_Access is access all Integer;
>   pragma Volatile(Register_Access);

If a register is an Integer, I would generally do

B : Integer;
for B'Address use Register_Location;
pragma Volatile (B);
...
B := 0;
B := 1;

But then, I don't have the C pointer fixation.

-- 
Jeff Carter
"Go and boil your bottoms."
Monty Python & the Holy Grail