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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 02:18:22 +0200
Organization: Tidorum Ltd
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Date: 2014-07-05T02:18:22+02:00
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On 14-07-05 01:51 , Niklas Holsti wrote:
> 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
                                                               ^^^
(Delete the word "not", above -- it is quite possible that some reserved
bits read as 1, although they should be written with zero.)

> 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
      .      @       .