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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: How do I write directly to a memory address?
Date: Wed, 9 Feb 2011 21:29:51 -0600
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2011-02-09T21:29:51-06:00
List-Id: <comp.lang.ada>

"Hyman Rosen" <hyrosen@mail.com> wrote in message 
news:4d52b82f$0$19486$882e7ee2@usenet-news.net...
> On 2/8/2011 9:33 PM, Adam Beneschan wrote:
>>  Dmitry asks, "How do you set the third
>> bit?", and you say, "Well, if the third bit is a Ready bit in a
>> hardware I/O register, I'd define a struct with bit components [yes, I
>> think C has those] and just assign 1 to the component, which would be
>> more readable than using a mask."
>
> This can be considerably more complicated depending on
> the hardware and the compiler; you are trying to assign
> to a single bit, but the compiler may do this by reading
> an entire word, setting the bit in the result, and
> assigning the entire word back. That could have dire
> effects if writing adjacent bits does something.

Quite true, there have been devices where you can't read the adjacent bits. 
Those are going to be problematical to deal with in any case (even in 
assembler). But of course the Ada solution handles that just fine (presuming 
you've appropriately used the Volatile and/or Atomic pragmas) -- you just 
have to assign an entire aggregate in that case. You still don't do any bit 
twiddling yourself.

                               Randy.