Re: Oberon and Wirthian languages

[Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP>]

On 2014-04-23, Simon Wright <simon@pushface.org> wrote:
> Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP> writes:
>
>> I wonder if we need something like that for Ada proper so we don't
>> have to read the register into a temporary variable first in order to
>> guarantee a single register read and a single register write while
>> updating a subset of bitfields in the register.
>>
>> Having to use a temporary variable to guarantee a update of, say, a
>> couple of bitfields (out of a possible large number of bitfields) at
>> the same time just feels as if it isn't elegant.
>
> But hardware isn't (usually) elegant! If what the hardware designer
> wants is for you to read the register, change the values you need to in
> the local copy, then write the modified value back, that's what the code
> should do, surely, without relying on compiler magic with obscure
> pragmas to make it happen? (Ada's pragma Atomic is quite obscure enough
> already!)

The problem is the compiler does this automatically when you use C style
masks, but you have to do the work manually when you try to write clean
code by using record bitfields (in Ada) or bitfield structs (in C) and
need to do the read/write exactly once and want to update more than one
of the bitfields at the same time.

This whole discussion started when someone suggested that in a C
replacement language we should drop the use of masks when possible
in favour of a bitfield approach. I _strongly_ agree with this, but
neither C or Ada, as they stand, make it easy to do this.

I proposed the following syntax for a C replacement language:

        atomic using uart0.config1
                .flag1  := 0;
                .enable := 1;
                .txdis  := 0;
        end atomic;

(I'm not too sure about the leading dots however.)

which would be a direct replacement for the use of C style masks when
modifying registers and would update the bitfields listed while
preserving the other unlisted bitfields and would guarantee only one
read and one write to the register.

The code above is a _lot_ safer and cleaner than using masks but to do
that in C or Ada at the moment requires the use of temporary variables
which is not clean as the above approach (or an approach like it).

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
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