Re: Fixed point constants issue

["Vinzent Hoefler" <nntp-2010-09@t-domaingrabbing.de>]

On Wed, 15 Sep 2010 10:35:22 +0200, Dmitry A. Kazakov  
<mailbox@dmitry-kazakov.de> wrote:

> On Tue, 14 Sep 2010 19:42:52 +0200, Vinzent Hoefler wrote:
>
>> -- 8< -- snip --
>>     type BASE_TYPE is delta 1.0 / 64.0 range 0.0 .. 1.0;
>>     for BASE_TYPE'Small use 1.0 / 128.0;
>
> But use suggests rather:
>
>    type Base_Type is delta 0.01 digits 3 range 0.0..1.0;
>
> or
>
>    type Base_Type is delta 0.01 digits 3 range -1.0..1.0;

No. These are bus-protocol types (read: hardware types) where MSB values,  
bit sizes
and (valid) range are given, so we definitely want to make sure that the  
compiler
uses the correct representation. Thus, a "simple" digits constraint is  
ruled out
here.

Usually such a type definition looks similar to that:

       type Foo is delta 1.638_400 / 2**13 range -0.819_200 .. 0.819_200;
       for Foo'Small use 1.638_400 / 2**13;
       for Foo'Size use 13;

For reading from the bus we use the (full-range) base type and checking  
its range
before assigning it to the appropriate object of the constrained subtype.


Vinzent.

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