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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Forcing GNAT to use 32-bit load/store instructions on ARM?
Date: Sat, 05 Jul 2014 01:51:09 +0200
Organization: Tidorum Ltd
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Date: 2014-07-05T01:51:09+02:00
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On 14-07-02 09:30 , Simon Wright wrote:
> daniel.dmk@googlemail.com writes:
> 
>> Thanks for this suggestion, I'll define the reserved bits in the
>> register, and use static checks to enforce that they are not modified
>> from their reset value in the code.
> 
> You could use something like
> 
>    type Reserved_Bit is range 0 .. 0 with Size => 1;
>    type Reserved is array (Natural range <>) of Reserved_Bit
>      with Component_Size => 1;
> 
>    ...
> 
>    type T is record
>       A : Boolean;            -- at bit 0
>       Res : Reserved (1 .. 6);
>       B : Boolean;            -- at bit 7
>    end record with Size => 8;
> 
> (with representation clauses as indicated)

That has two potential problems...

- First, even if Reserved_Bit is constrained to a single value (0), that
does not mean that an uninitialized variable or component of this type
will have that value. It will still have to be assigned the value zero,
to be sure of having that value.

- Second, if you read the value of the control register into a variable
of type T, and some of the reserved bits read as 1 (which is not quite
possible), these Reserved_Bit components will now, very probably, have
an invalid representation, which is a nasty state.

For these reasons, I would define Reserved_Bit as range 0 .. 1 and
perhaps provide the default value (others => 0) for the Res component.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
      .      @       .