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From: Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP>
Newsgroups: comp.lang.ada
Subject: Re: Oberon and Wirthian languages
Date: Tue, 22 Apr 2014 00:34:09 +0000 (UTC)
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Date: 2014-04-22T00:34:09+00:00
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On 2014-04-21, Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> wrote:
> On 04/21/2014 06:06 AM, Simon Clubley wrote:
>>
>> What I have not checked however is if a record assignment in Ada to a bit
>> field record variable marked as Atomic would result in all the bit fields
>> listed in the assignment being _guaranteed_ to be updated at the same time
>> or whether there would just be a atomic update of each component in
>> isolation.
>
> If you have a record:
>
> type Register_Info is record
>     A : Boolean;
>     ...
> end record;
>
> for Register_Info use record
>     A at 0 range 0 .. 0;
>     ...
> end record;
>
> and an atomic object of the type:
>
> Register : Register_Info;
> pragma Atomic (Register);
>
> If the compiler accepts the pragma, then assignment to Register (as opposed to 
> its components) is atomic:
>
> Register := (A => True, ...);
>

Thank you. It was not at all 100% clear from the RM if Ada offered the
_guarantee_ that if you had, say, 3 components in a record assignment
for a atomic bitfield record, that the record assignment would be treated
as one atomic operation instead of 3 atomic operations (one for each
component).

> Though the best bet is to do it and check the generated code.
>

Unfortunately, that only tells you how one compiler implemented it and
doesn't say anything about if such behaviour is mandatory in the standard.

Thanks for the clarification above,

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
Microsoft: Bringing you 1980s technology to a 21st century world